Recommended Methods for Monitoring Bird Populations by Counting
and Capture of Migrants
Written by David J. T. Hussell, Ontario Ministry of Natural Resources,
P.O. Box 5000, Maple, Ontario L6A 1S9, Canada, and Environment
Canada (Ontario Region), 49 Camelot Drive, Nepean, Ontario K1A 0H3, Canada and
C. John Ralph, USDA Forest Service, Redwood Sciences Laboratory,
1700 Bayview Drive, Arcata, California 95521 U.S.A.
for the Intensive Sites Technical Committee of the Migration Monitoring Council,
November 18th, 1998.
Table of Contents
Introduction and Objectives
The Migration Count
Methods to be Considered
The Problem of Residents and Stopovers
Factors in Site Selection
Field Protocol and Manual
Summary of Recommendations
Appendix 1: Migration Monitoring Council and Technical Committees
Appendix 2: Migration Monitoring Manuals
Appendix 3: A Preliminary Assessment of Species Coverage Priorities For Migration Monitoring Programs
Appendix 4: Daily Estimated Totals: Additional Information
In September 1993, a workshop was held to evaluate the potential of migration monitoring as a means of assessing population changes in
migrant landbirds (organized by the Canadian wildlife Service and a branch of the U.S. Fish and wildlife Service, the latter now the Biological
Resources Division, a branch of the U.S. Geological Survey). The workshop recommended establishment of a continental migration monitoring
program, including "a series of intensive long-term monitoring sites to monitor the maximum number of species with emphasis on populations
breeding in northern Canada and Alaska" (Blancher et al. 1994).
A "Migration Monitoring Council" (Appendix 1), appointed to implement
the recommendations of the workshop, met in March 1994. It appointed two technical committees charged with establishing standards and
guidelines for the operation of monitoring programs. One committee dealt with extensive monitoring (such as checklist programs). The second
technical committee was responsible for the requirements of intensively operated sites, such as bird observatories and bird-banding capture
stations. This document was drafted in February 1996 and was widely circulated; it is a report of the Intensive Sites Technical Committee.
Below we detail the current recommendations for field methods for population monitoring of landbird migrants at intensively operated sites.
Introduction and Objectives
We describe options and recommendations for field methods for monitoring population changes of small landbirds during migration. The
recommendations are designed for intensively operated sites such as bird observatories and bird-banding capture stations, and they apply equally
to spring and fall migrations. The target audience is any individual or group who wishes to collect high quality data on birds during migration
in a scientifically rigorous manner. These data would be suitable for analyses of both long-term trends in populations and for comparisons
among sites throughout North America. These methods can also be used to good advantage during other seasons, including breeding and winter,
with minor changes.
The goals of a migration monitoring station are to contribute data to an international effort to determine what
changes are taking place in populations of migratory birds, as well as to document migration at the station itself. The primary method of
attaining these goals is to generate what we term a "Migration Count," a count of migrant birds that can be used for long-term population
Migration monitoring stations can also gather data relevant to a wide variety of other important population parameters on
migrating birds. These parameters, such as condition, timing of movements, age and sex ratios, are derived from capture of individual birds.
They may indicate the basis of changes in migrant populations and generate hypotheses of specific causes of changes, including declines.
Our recommendations for monitoring changes in population size are based on methods that have been shown in at least one study to generate
population trends which correspond well with trends from the Breeding Bird Survey (BBS). Other approaches may also prove to be valid as
migration monitoring is an active field of research.
The recommendations are primarily aimed at a single station working daily through the migration season, because such stations have been
shown to produce trends that correspond to BBS trends. Therefore, our current emphasis is on stations that can sustain near daily coverage of
one or both migration seasons over at least 10 years. However, it has been suggested that population trends and composition could also be
monitored from a network of stations each operated on one or two days per week. The efficacy of this approach has not been fully tested.
Ideally, they would be coordinated such that most or all days are covered by at least one station. We encourage stations that can only manage
intermittent coverage to consider following our guidelines in all other respects, and to coordinate their efforts with other, nearby stations to
form a network.
This is not intended to be a detailed field manual, as several exist (see list of existing manuals and sources in Appendix 2). The key
recommendation we make is that each station develop its own field protocol of standard operating procedures, which will be used in the same
manner from day to day and from year to year. We do NOT recommend that every station use identical methods; the guidelines presented here are
meant to establish the methodological bounds within which we suggest that data useful for monitoring migrating birds can be collected. Within
these bounds, techniques should be modified to meet constraints imposed by local geographical conditions and the availability of skilled
personnel. We also offer suggestions for choosing new sites and provide guidance on which species are of highest priority for migration
Many of our recommendations are designed to maximize the usefulness of migration counts in the calculation of population
changes. Standardization of counting methods can make a major contribution to removing extraneous variation derived from variable observer
effort and sampling procedures. Nevertheless, migration counts will still be subject to uncontrollable variation from weather, observer
differences, and unavoidable changes in the level of effort. Some of these problems can be addressed by the use of appropriate analytical
procedures. We do not directly address these statistical issues in this document, although we do allude to them in support of a given
recommendation. Those interested in details should see Hussell (1981), Hagan et al. (1992), Hussell et al. (1992), Pyle et al. (1994), Dunn and
The Mirgration Count
A "migration count" is any tally or count of birds, usually directed towards those on spring or fall passage (Dunn and Hussell 1995). This
definition could include birds counted at a site, observed flying past a fixed point in diurnal migration, captured in nets or traps, seen on
radar screens, or whose calls are recorded in nocturnal migration. For monitoring small landbirds, particularly nocturnal migrants, we will be
concerned mainly with birds observed or captured at short-term stopover sites immediately following a migratory flight.
the situation, a migration count may be derived exclusively from observations (as is the case with most hawk migration counts), exclusively from
standardized netting of birds, or from a combination of observations and captures. Incidental observations (unstandardized or incompletely
standardized) can be a useful component of a "daily estimated total" (ET). Therefore, incidental observations are discussed as an option below,
although we do not recommend that they be used alone.
Obviously, the more standardized the method, the more consistent and useful the counts will be. Regardless of the method used, the
migration count can never be a complete tally of every bird present at or passing over a site. Instead, observers record a sample of the
population. Therefore, standardization helps ensure the proportion of the population counted remains similar from day to day and year to
The ideal that we should strive for is a separate tally each day of three groups: (1) newly-arrived migrants, (2) resident individuals, and
(3) stopovers that arrived on earlier days. Inclusion of the latter two groups in successive counts are inevitable in most methods. The number
of newly-arrived migrants is required for some analyses of the data. Recommendations for this separation are detailed later (p. 6).
Methods to Be Considered
There are several options for producing a useful migration count of small landbirds. More than one type of data can be collected
simultaneously, which can be useful to facilitate assessment of each method. These options include: visible migration count; area search or
route census counts; banding captures by net or trap; incidental observations; and daily estimated totals.
Visible Migration Counts
A count of migrants can be conducted at sites where an important feature of migration is diurnal movement of birds (including day-time
movement of primarily nocturnal migrants, and/or movements of strictly diurnal migrants such as swallows). This count is usually from a fixed
point, and tallies those species that can be identified in flight. This method is widely used for recording migrations of hawks in North
America (Fuller and Titus 1990), as well as waterbirds, and has also been used for recording diurnal flights of landbirds (some normally
nocturnal migrants) in Europe, and at a site on the shore of Lake Superior at Duluth (Eckert 1990).
A standardized visible migration
count may be adopted in addition, or as an alternative, to a census (see below), and as a component of an estimated total. If the count
includes individuals that may remain in, or return to, the count area from day to day, then a procedure for estimating such individuals should
Advantages of visible migration counts are:
- Birds involved in unidirectional flights are clearly
in migration, are new arrivals, and are unlikely
to be counted more than once.
- Counts are unlikely to be affected by year-to-year
changes in food abundance or other local site changes.
- Procedures can be highly standardized.
- Counts are relatively unselective with respect to species,
in that potentially all birds seen can be counted
(but see Disadvantage #1).
- No special skills for handling birds are needed, and birders
like to participate in these counts.
- Although count accuracy may be influenced by weather,
visible migration counts can be conducted in any weather,
including conditions when it is impossible to use traps or nets.
Disadvantages of visible migration counts are:
Area Search or Route Census Counts
- Many small birds may be hard to identify at the
range and speed that they pass the observation point,
and successful counting may require
personnel with exceptional abilities
to identify birds in flight.
- If large numbers of birds are involved,
it may be difficult to count or estimate
their numbers accurately and consistently.
- Because birds are not handled, age and sex
composition of the migrants will be unavailable
for most species.
- Variation in skill level of observers will be
reflected in the counts more than with some of
the other methods of data collection.
A census is an attempt to identify and count all of the birds in a specified area within a specific time period (Ralph 1981b), and is most
effective if taken at the same time each day. It may involve an "area search" of the entire study site, or of selected plots within the area
(Ralph et al. 1993:35), usually in multiples of 20 minutes. A survey along a predetermined linear route within the entire study area can also
be used as a census. A census route should be 1-2 km, so that it can be walked at a slow pace in about 1 hour. A census of migrants may be
most effective in relatively open sites, where the vegetation is not too dense for birds to be seen easily. For population monitoring, a census
can be used independently or as a component of an estimated total. If it is to be used independently, a procedure for estimating stopovers and
residents should be included, if possible.
A census has the following advantages:
- It is relatively unselective with regard to species
(except that secretive species are likely to be missed).
- Although census accuracy may be influenced by weather,
counts can be conducted in any weather,
including conditions when it is impossible to
use traps or nets.
- Procedures can be highly standardized.
- It can be completed in a relatively short time,
by one person.
- No special skills for handling birds are needed,
and birders like to participate in censuses.
Disadvantages of a census are:
- Some individual birds may be missed,
especially in heavy cover, and others may be counted twice.
- Observers must be skilled in identification
and detection of birds. Training necessary to bring
observers up to acceptable standards can
- The ability of observers to detect birds accurately varies.
- The census sample may represent a relatively small
proportion of the total birds present in or passing
through an area on a particular day, especially
when the census duration is short.
- Small birds moving rapidly through an area may
be difficult to detect and identify.
- Secretive species are likely to be missed.
- Residents and migrants may be difficult to
separate and turnover hard to assess
(i.e., the same individuals may be counted on more than one day).
Captures of birds during migration have been used as a count method for monitoring changes in population size (Berthold and Schlenker 1975;
Berthold et al. 1986; Hagan et al. 1992; Dunn et al. MS; Dunn and Hussell 1995) and composition (Dunn and Nol 1980, Ralph 1981a). Most
standardized capture procedures will involve only the use of mist nets. However, Heligoland traps or other traps (including baited traps) could
be used for the same purposes. The count of newly-arrived birds should be the capture rate (e.g. birds/net-hour) of newly captured individuals.
Standardized capture can also be a component of an estimated total.
While standardization of effort is very important, at some sites, such as
some exposed coastal locations, it may be difficult to maintain an adequately standardized trapping or netting procedure, due to frequent
adverse weather, wide fluctuations in bird numbers, and/or habitat changes. In these circumstances, we do not recommend banding captures as the
only, independent count method (Dunn et al., in press), but they can still be used as a component of a daily estimated total and to determine
age and sex composition of the population.
To further standardization, net and trap sites should be clearly identified and marked and
set in exactly the same positions each year, if at all possible. The array of nets and traps should allow the participants to visit them within
10-15 minutes when no birds are present. The number and types of nets and traps should remain constant from hour to hour and year to year, as
these variables can affect capture totals (Pardieck and Waide 1992). We suggest that bait not be used with a standardized Heligoland trap or
mist-netting program, because it is difficult to use consistently and may influence stopover behavior of migrants. Unavoidable changes in
position, number or type of nets or traps (e.g. dimensions, mesh size) should be documented. The nets and traps should be operated during the
same number of hours during the same standard time period each day, starting at a constant clock time or at a constant time relative to sunrise.
For monitoring nocturnal migrants, an early morning start at the same time relative to sunrise is preferred. Nonetheless, even during the
standard period, the trapping or netting operation should be stopped if conditions arise that endanger the safety of the birds (e.g., severe
weather, more birds captured than can be safely handled, predator problems). Non-standard opening and closure times of nets and traps should be
recorded. Partial closure and opening of the array of nets and traps should be avoided whenever possible.
Attention should be given to the possible need to manage the vegetation in the vicinity of net or trap sites to mitigate potential
long-term changes in the habitat (e.g. Berthold and Schlenker 1975, Berthold et al. 1986).
As discussed in Ralph et al. (1993), the minimum data taken at a capture station are: species; band number; age and sex of the birds; and a
measure of the effort expended (e.g. net hours). How aged and sexed, wing chord, body mass and/or fat condition, and molt condition are highly
recommended, but are at the discretion of the individual stations. It is essential that knowledge and experience of principal banders be at a
very high level to ensure accurate identification, use of plumage criteria for age and sex, and especially use of skull ossification for ageing.
We suggest that a permanent and fully-equipped banding laboratory or shelter should be conveniently located within the trapping/netting area of
all major stations to enable banders to process birds accurately and efficiently.
A standardized netting or trapping procedure has the following advantages:
- It is objective (all birds caught are identified and counted).
- It is relatively unaffected by differences among the
- It will produce a random
sample within each species, and will do this more
effectively than other methods at sites with
heavy cover or in other situations where birds
are not easily observed or counted.
- Previously-banded stopover migrants or residents
can be identified as individuals and separated.
- It provides detailed demographic information on age
and sex classes which can be used to formulate hypotheses
about causes of changes of population levels.
- Condition of brood patch, cloacal protuberance,
subcutaneous fat, and molt can be used to separate
at least some residents from migrants and
generate further hypotheses about causation
of changes in populations.
Disadvantages of trapping or netting are:
- It is relatively selective with regard to species,
and many individuals present in the area that might
be included in a visual count will not be captured.
- Its efficiency is likely to be adversely affected by
severe weather conditions, and weather will dictate
complete closure of the operation on some days.
- Since habitat change, such as height of the canopy,
can affect capture totals more than in other methods,
habitat maintenance at the sites may be necessary.
- Large day-to-day variation in numbers of birds at
some sites may make it difficult to operate nets in
consistent numbers or locations.
- Netting and banding requires special skills,
training, and permits.
Incidental observations of birds seen or heard by observers or banders in the course of their work at a site can be tallied. This method
is particularly important in determining the presence or absence of rare and unusual species not normally observed, not captured by nets or
traps, or missed during regular censuses or visible migration counts. This is inherently an unstandardized method. Nevertheless
standardization can be improved if consistent procedures or rules are adopted that result in a similar amount of effort being devoted to
incidental observation each day and we recommend that this be done as much as possible because this is the least standardized method that we
describe and we do not recommend it except when used in combination with other methods. It can be an important component of the Daily Estimated
Total discussed below, especially if data are collected in a reasonably consistent way from day to day.
Incidental observations have the following advantages:
- They are relatively unselective as to species
and can be made in any weather.
- They provide a measure of species which are
rare at the site or at a particular time of year.
- They can detect presence and/or numbers of birds at a time of
day or in a part of the site not covered by
- They add interest for volunteers and keep them focused on birds
throughout the day (or shorter observation period),
as they are aware that all observations will
become part of the permanent record.
- No special skills for handling birds are needed.
Disadvantages of incidental observations:
Daily Estimated Totals
- Many individual birds may be missed and others
may be counted twice or more.
- Observers must be skilled in identification and their
abilities to detect and identify birds will vary.
- Incidental observations are not suitable for
obtaining consistent estimates or samples of total numbers,
but only for detecting presence and
absence and for providing supplementary information
on levels of abundance.
- Because they are not well standardized,
they cannot be used as a sole migration count.
The "Daily Estimated Total" (or ET) approach to deriving a daily count has been used at many European and some North American observatories
and has been validated as a population monitoring method with data from Long Point, Ontario (Hussell 1981, Hussell et al. 1992), and Southeast
Farallon Island, California (Pyle et al. 1994). The ET method should combine data from the at least two of the four methods described above
Regardless of the components used as input, the objective is to integrate all available information to estimate the
numbers of each species in or passing through a defined count area during the count period. Because the procedure may be unfamiliar to many
readers (and can involve several variations), we include some additional details in Appendix 4.
ETs are likely to be most successful in
small areas with relatively open habitat, where personnel housed on-site are making more or less continuous observations. New stations planning
to use ETs should develop procedures that are standardized to the maximum practicable extent. Nevertheless, when incompletely standardized
procedures are used to collect data that form a component of the ET input (e.g. unstandardized banding captures, incidental observations), then
the ET procedure might be helpful in overcoming those deficiencies in standardization.
At sites that experience fairly wide fluctuations in numbers of migrants (particularly at exposed coastal sites), the ET procedure might be
preferable to visual counts or banding captures alone. We recommend that each component of the ET (census, visible migration count, banding,
other observations) be standardized to the maximum extent feasible and be recorded separately (in addition to the ET), so that each can be
analyzed separately. A procedure should be included for estimating the numbers of stopovers and residents.
The ET method has the following advantages:
- It is based on 2 or more separate sampling procedures
and attempts to take advantage of the strengths of
each while mitigating their
weaknesses, by compensating for variation in effort
and day-to-day variation in the effectiveness of
- It usually covers a
longer period of the day than is allocated to
a census, visible migration count, or banding
alone and is therefore likely to detect more species
- It attempts to include all individuals of all species
that were detected in the count area in a
specified time; a procedure
that is intuitively appealing and a challenge to
many birders and volunteers.
- Because an ET is often larger and never smaller
than counts derived by other methods, ETs may
conform more closely to the assumptions
required by analysis methods used to estimate
population indices and trends.
Disadvantages of the ET:
- Preparation of the daily totals is comparatively subjective,
rather than strictly objective and standardized. Rules
for preparation of
daily totals must be devised and followed. Nevertheless,
such rules will be open to subjective interpretation,
making it difficult to ensure a
fully consistent estimation procedure.
- Variation in effort devoted to any of the component
procedures may introduce a bias into the estimates.
- When birds are moving quickly around or through the site,
ETs will be particularly subject to error unless effort
devoted to incidental
observations is standardized, or standardized
visible migration counts are part of the regular procedure.
- In heavily vegetated sites, the ET for many species is
likely to be based mainly on capture samples, and will
be affected by any variation
in trapping or netting effort.
The site should include or encompass a defined Count Area that has features that are compatible with the chosen count procedures. A Count
Area of 5-20 ha is likely to be appropriate, but a larger area may be manageable in very open habitats. If capture is to occur there must be
suitable sites for nets or traps. If visual censuses are to be conducted, the vegetation should not be too dense for birds to be easily seen.
An area with natural edges, such that birds moving in and out can be readily detected, may be preferable to an area without such boundaries.
Depending on method used, the area should include census routes for one or more 20-minute area searches, a census count route of up to 2 km
in length, and/or a suitable observation point for conducting a visible migration count. If netting or trapping is the exclusive method for
obtaining a count, then the count area is defined by the array of nets and traps. If parts of the count area are not regularly covered by the
netting and trapping operation, they should be included in a route census or a standardized area search.
The count area and key features (such as observation points or net lanes) should be clearly defined and identified on a map included in the
Normally, any birds seen or heard in or over the count area may be included in any census or other 'visual' count.
However, special rules may be made for including or excluding birds seen from the count area that are beyond its boundaries or are very high
above it. Any techniques involving nocturnal counts should be separated from the more usual diurnal counts.
The size and configuration
of the area selected as the Count Area should be such that it can be adequately covered by available personnel (usually 2-3 people) to
consistently generate an acceptable Migration Count using the chosen methods. A pilot project should be used to experiment with the size of
area that can be thoroughly covered and with locations of nets, traps, census route, etc.
No matter the type of method, organizers should define the total daily "count period", as well as the standard daily time periods during
which the various component activities of bird sampling procedures occur. At some monitoring sites, the count period for ETs is all daylight
hours. In other situations, it may be desirable to limit the count to a specific, standardized period, e.g. the first 6 hours after sunrise; or
from dawn to 2 p.m. Length of count period should take into account the normal numbers of personnel and the hours that they are available.
The early morning hours after dawn must always be included in the count period to ensure that newly-arriving nocturnal migrants are included,
and because bird activity is generally greater at that time of day.
If the daily count period for the ET or for banding captures is less than all daylight hours, then records of birds seen or captured
outside the count period should be identified as such in the records, and excluded from the standard total used in population trend analyses.
In general trapping or netting should be limited to the standard count period, especially if significant numbers of day-to-day stopover
migrants occur regularly and trap or net avoidance is suspected or known to occur, as discussed below.
The Problem of Residents and Stopovers
Here and elsewhere in this document, the resident birds that we wish to separate from migrants are individuals of targeted migrant species
that are present at the site on summer or winter territories, or are otherwise present for more than a few days (e.g. post-breeding adults or
young of the year). Year-round residents of non-migratory species, or of non-targeted migrants, are of less concern because these will not be
included in an analysis that concerns only migrants. We would prefer to not include multiple counts of the same individuals in population trend
analyses because most analysis methods assume that daily counts are independent of each other. In addition, variability attributable to the
effects of weather on counts can be identified most effectively if counts include only newly-arrived migrants.
In practice, it will
often be impossible to exclude all earlier arriving migrants and residents from the count, particularly at sites that have many individuals that
stop over for more than a day. Various techniques are possible for mitigating the problem of stopovers and residents, but the major factor is
choice of site. Exposed coastal sites that have few residents and tend not to hold stopover migrants for more than a day or two would minimize
this factor. Finally, it must be emphasized that the residents and non-target species or individuals all may have importance for other
management or research considerations, and should always be recorded.
At sites with few stopovers and residents, failure to distinguish
these individuals from newly-arrived migrants will not create serious problems in the analysis. However, the daily counts should separate
likely stopovers and residents whenever possible. We do not recommend modifying the basic count procedure to accomplish this. Rather, we
recommend recording additional information that will allow stopovers and residents to be deducted from the total counts at the analysis
Retraps of birds banded on previous days are obviously stopovers, and can later be separated from the ET or capture totals. The
retrap rate, each day's recapture rate for each species, can also be used to estimate the proportion of the total daily count consisting of
In addition to these data derived directly from banding and retrapping, other individuals can often be identified with a
high level of certainty as stopovers or residents even though they were not captured on the day in question. Included here are
previously-banded birds that were seen but not captured, birds of rare or scarce species that are highly unlikely to be represented by new
individuals each day, other birds that can be identified as individuals, and birds of known resident species regularly present in specific
Observers in an area soon become aware of these regular residents and also of the known or presumed stopovers. From this, they are then
able to estimate their numbers as readily as they can the ETs. This is done routinely at Thunder Cape, Ontario, which has an additional column
on its ET sheets for "PKS" (Probable and Known Stopovers; see McCracken et al. 1993). Only individuals judged to be residents or stopovers with
at least 80% certainty are recorded as such. A similar procedure is used on Southeast Farallon Island, where separate daily estimates are made
of total present and number of arrivals (P. Pyle, personal communication).
Operation of traps or nets outside the standard count period
could introduce a bias if birds captured in extra hours subsequently tend to avoid traps or nets, as has been demonstrated for some breeding
birds. If this occurs at sites where significant numbers of stopovers occur regularly, netting and trapping should be limited to the standard
period. Some good migration monitoring sites do not hold large numbers of stopovers, and newly-arrived migrants may not react to nets and traps
in the same way as breeding birds on territory. If so, capture of migrants in extra hours may not seriously bias subsequent data and might be
permitted. Extra trapping and netting may then help to show how much (or little) stopping over occurs and to distinguish newly-arrived migrants
from stopovers on subsequent days. We would suggest, however, that operation of nets and traps outside of the standard hours be given careful
consideration before being undertaken. Netting and trapping effort and the birds captured in non-standard hours should be recorded as such, and
excluded from data used for trend analysis.
Factors in Site Selection
Coverage of Species
A key recommendation of the Migration Monitoring Workshop was that a series of intensive monitoring sites be established to monitor the
maximum number of species, with emphasis on species and populations breeding in northern Canada and Alaska that are not currently adequately
covered by the Breeding Bird Survey. Therefore, newly-established migration monitoring stations will contribute most to monitoring North
American bird populations if they monitor populations or species that are not currently well-monitored by the Breeding Bird Survey (Appendix 3),
particularly those species breeding in northern Canada or Alaska, and are at geographic locations that enhance development of a continental
network of stations that effectively intercepts all populations of northern-breeding migrants.
Ideally, a site should be visited several times during the migration seasons before being selected as a potential migration monitoring
site. Before making a final selection, a pilot monitoring program should be undertaken, with the objective of determining whether target
species are likely to be adequately monitored. In order to monitor a species at a migration station, it has to be present in numbers that are
adequate for analysis. We think it desirable that an average of at least 10, and preferably more than 20, individuals of a species are recorded
per season. In addition, we suggest that coverage of at least 75% of the days in the species' migratory period is desirable (i.e., the period
when the middle 95% of the individuals normally occur). For the purpose of assessing whether or not a species can be monitored at a site,
species recorded at a lower rate, or over a lower percentage of its migratory period, should be regarded as unmonitored.
Number of Sites
Most migration monitoring station operators aiming for daily coverage will probably work at only one site, while others work at a cluster
of two or more sites, each operated daily, in the same local area. Addition of sites will add extra counts and will increase the reliability of
population monitoring, but will add to the costs, as well as the task of maintaining consistent, long-term daily coverage. Sites operated in
tandem should be far enough apart to provide independent samples of migrants (i.e. the same individual birds should not normally be available
for counting or capture at more than one site). We suggest that such sites should normally be within 5-30 km of others, but they could be
closer if there is evidence that the same individuals are unlikely to be encountered at more than one site.
An alternative strategy is a network of cooperating sites, each operated on a regular, but not daily, basis. This strategy may be more
effective at monitoring a range of species than a single site. Each site in the network may observe or capture only a few individuals of rarer
species, but the combined counts from sites within a region may provide an adequate sample of these species. The region covered should be a
reasonably homogeneous bioregion, perhaps covering an area of a few to several hundred kilometers on a side.
Other site criteria
Assuming that targeted species are recorded in adequate numbers at a site or a network, there are several other technical and practical
factors that should influence the choice of site before a long-term commitment is made. None of these is an absolute requirement, but each
contributes strategically to the long-term viability of a migration monitoring station.
- The site should be a concentration area for
migrants. Coastal (especially peninsular) sites are
apt to "funnel" or attract migrants, but attention
should also be paid to concentration
sites such as river valleys, off-shore islands,
and "islands" of habitat (e.g. a brushy area in the middle of
a larger prairie or an oasis of
greenery in a desert). Among other considerations,
it will be difficult to attract volunteer personnel
over long periods of time to sites that
consistently have low numbers of migrants.
- There are advantages to a site that does not
regularly "hold" large numbers of individuals
stopping over for more than a day, and that
has relatively few resident species.
- Working and inexpensive living quarters for participants
(especially for volunteers) should be
on-site. One reason for this is that the first hours
of daylight can be readily covered, when most
nocturnal migrants have not yet dispersed.
Another is that on-site facilities provide a focal
point for the program and tend to lead to a stronger
- The area must be reasonably accessible. However, a degree
of isolation is an attractive feature for many volunteer
participants. If stations are
to be operated in tandem, then they should be no
more than 30 km apart. Ideally, major stations
should be reasonably close (within 150 km) to a
major population centre that can provide a supply
of volunteers to assist in the operation and
administration of the station. Stations that are
remote from population centres may be best operated
as a satellite of, or in partnership with, an
existing major station.
- Preferably, the site should not be frequented
regularly by the public and it should be well
protected from any sources of development and
- In order for the monitoring program to yield
useful results, some kind of secure tenure
is virtually essential.
Ideally, the area should be owned outright
by the organization. Alternatively,
a long-term commitment should be available from the owner
(preferably a government or conservation agency).
- Migration counts have less chance of long-term bias
from change in vegetation if the site has climax
vegetation or is naturally
maintained at an early successional stage.
If vegetation is going to grow up, the site
should be one where there are minimal restrictions on
cutting, trimming and mowing as necessary to maintain
the habitat at about the same stage
(especially where netting is to be the sole means of
- We suggest that stations should plan on continuing
monitoring for a minimum of 5, and preferably beyond
10 years. Such a commitment
normally requires institutional sponsorship, and
site selection should take into account whether a
suitable institution exists to take on the
migration monitoring as a project. A monitoring
program with such an organizational sponsorship
is more likely to continue despite turnover in
personnel, and may provide a route to financial support.
Vegetation changes may affect the numbers present in an area, as well as their detectability. We recommend that sites with reasonably
stable habitat conditions be selected whenever possible. Coastal areas that are held in an early or mid-stage of succession by local conditions
are likely to be ideal for this and other reasons. Alternatively, the habitat should be quantitatively monitored, and, if necessary, managed to
prevent major changes from occurring. A habitat management plan should be included in the Field Protocol.
One experienced person should be in charge of the operation of the station at all times. If the regular station manager is absent, another
experienced person should be designated as a replacement for the duration of the regular manager's absence. The stations should not be operated
if there are not enough experienced personnel on hand to make complete, accurate counts, or to capture and handle birds safely. Volunteers
should be encouraged to participate. Qualified volunteers can be responsible for station management. An effective way to ensure consistent
coverage at major stations, however, is to place a paid employee in charge as station manager.
The skills required of participants in a migration monitoring program will vary with the methods selected. We do not give direction on
specific skill levels or training regimes here, but we do recommend that acceptable levels of knowledge and experience be specified in the
station manual or field protocol.
Between-year changes in personnel can lead to between year bias in population monitoring results, because people differ in their abilities
to observe and count birds. It is better to involve a variety of people (even if their abilities differ somewhat) than to create between-year
bias by relying heavily on single observers who may have superior skills but who change from year to year.
Three important recommendations follow from this:
- All observers who are responsible for counts
(especially censuses and visible
migration counts) should meet some high,
but reasonably attainable, level of ability
to identify and count birds.
- Do not assign sole responsibility for all counts
in a year to a single individual. Wherever possible
use several qualified people within the season.
Estimated totals should always be a joint responsibility
of all qualified participants.
- Rotate qualified personnel whenever possible.
If a census or visible migration count is part
of the routine, different observers should do
it on different days. If you operate more than one site,
rotate observers among sites within each season.
Recommendations 2 and 3
contrast with those of some multi-site monitoring programs, including the Breeding Bird Survey, which prefer continuity of observers at the same
site between years. Migration count requirements differ because usually only one or a few sites are sampled in a region, making it more
difficult to distinguish observer effects from year to year population changes in the statistical analysis of trends.
Field Protocol and Manual
Once a site has been chosen and pilot studies conducted, each migration monitoring station should write a protocol or manual describing its
procedures for deriving daily counts and collecting other field data. This is a mandatory requirement for any long-term migration monitoring
station or network. The methods adopted should follow the recommendations proposed here. The manual should enable people who are unfamiliar
with the station to collect data in a manner that is consistent with procedures followed previously. In addition it should form a guide and
reference for current field personnel. As a minimum, the manual should include the following:
- A brief statement of the goals and objectives
of the program, including the main target
species or groups, such as all landbirds,
diurnal migrants, neotropical migrants, species
breeding in the forests of northern Canada,
etc. A list of high priority species that can be
monitored should be included (Appendix 3).
- A definition of the area being monitored,
including a map or maps showing (if applicable)
the boundaries of the area, census route(s),
visible migration observation points, and sites
of nets and traps.
- Definition of the daily time period(s) during
which the count(s) are to be conducted
(see Count Period).
- A description of the methods used to produce a
migration count, with particular attention
to any procedures that are specific to the site
(e.g. site-specific rules for deriving ETs,
as in McCracken et al. 1993:16-18). Also,
for visual counts, include rules about which birds may be
counted that are flying over or beyond the
boundaries of the count area.
- A description of procedures for recording stopover and resident
individuals. It should include site-specific
rules for determining which individuals are
to be recorded as stopovers and residents.
- A description of record-keeping procedures.
- Statements specifying the knowledge, skills and
experience required of participants and descriptions
of training programs to bring
inexperienced personnel up to the required levels.
- A discussion of potential changes in the habitat
at the site and, if necessary, a
management plan designed to maintain a stable
Basic field and data management protocols are available from established stations (Appendix 2) and many of these procedures will be directly
applicable to other stations. Nevertheless, each station should adapt these procedures to its needs and provide detailed descriptions of
site-specific field procedures, bearing in mind the main objective of obtaining consistent daily counts. Before final adoption, the draft
protocol should be reviewed by at least two persons experienced in field procedures and analysis of migration data for population monitoring
purposes. At least one such person with experience elsewhere should visit the site for several days to see the proposed protocol in operation
and to advise on possible modifications.
Some changes to the field protocol may be unavoidable. If so, the change and its timing should
be fully recorded, so that its possible impacts can be assessed at the analysis stage. The potential effects on the value of the data for
population monitoring should be carefully considered before optional changes are adopted. In general, the best way to mitigate negative effects
is to phase in the changes over 2 or 3 years. Ideally, the new and the old protocols should be run simultaneously (i.e. on the same day) or on
alternate days during the phase-in period. This will enable the effects of the two protocols to be detected (and corrected for) in the
analysis. Avoid making major changes in the protocol between years.
Summary of Recommendations
To contribute useful information to the North American Migration Monitoring Program, stations or networks should monitor several priority
species (Appendix 3) and should attempt to be in operation for an indefinite period beyond a minimum of 10 years. Single stations should
sustain daily or near daily coverage through one or both migration seasons. Coordinated regional networks, as discussed above (p. 2), may
involve less intense coverage at individual sites, but should aim to maintain daily coverage across the network by alternating operation of
individual sites. A summary of our recommendations for selection of sites and operation of migration monitoring stations follow.
- Migration Count.--The choice of method should be
appropriate to the site characteristics.
Standardization from day-to-day and year-to-year
is the key to obtaining a consistent and reliable count.
A census or area search, visible migration count,
banding captures, or a daily
estimated total are all acceptable procedures.
Incidental observations can contribute to a
daily estimated total. More than one method can,
and where possible should, be used at a site.
- Count Area.--The count area should be clearly
defined and identified on a map in the field
- Count Period.--The total daily count period
must be clearly defined, together with time
periods in which various component
activities should be carried out.
- Site Selection.--The site(s) should be capable
of monitoring several of the priority species listed in
Appendix 3. Habitat and other site conditions
should be reasonably stable. Sites should meet
as many as possible of the other site selection
- Habitat Management.--Major changes in the
vegetation and other aspects of habitat at the
sites should be avoided or managed to
maintain a reasonably stable environment.
- Personnel.--Personnel should have the training,
experience and skills necessary to conduct the
counts. Attention should be given to the potential
impact of personnel changes on the count.
In general, avoid heavy reliance on single
observers and avoid between-year changes.
- Field Protocol and Manual.--This is a mandatory
requirement for all long-term migration monitoring
stations and networks. Changes in the field
protocol should be avoided, especially between years.
Unavoidable changes and their timing should
We are grateful to the following people for their valuable comments on drafts of this document: Peter Blancher, Michael Bradstreet,
André Cyr, Brenda Dale, Sam Droege, John M. Hagan III, Keith Hobson, Jon McCracken, Doug McNair, Borja Milá, Bill Murphy, Chris
Otahal, Dan Petit, Peter Pyle, and Mark Shieldcastle. Erica Dunn contributed substantially to the editing process.
Berthold, P. and R. Schlenker. 1975. [The "Mettnau-Reit-Illnitz-Programm"-- a long-term bird trapping program of the Vogelwarte
Radolfzell with multiple goals]. (German, English summary). Vogelwarte 28: 97-123.
Berthold, P. G. Fliege, U. Querner and H. Winkler. 1986. [Change in songbird populations in central Europe: analysis of trapping
data]. (German, English summary). J. Ornithol. 127: 397-437.
Blancher, P., A. Cyr, S. Droege, D. Hussell and L. Thomas [compilers]. 1994. Results of a U.S./Canada Workshop on monitoring
landbirds during migration and recommendations towards a North American Migration Monitoring Program (MMP). 27 pp. [Available from P. Blancher,
Canadian wildlife Service, National wildlife Research Centre, Hull, P.Q. K1A 0H3; or S. Droege, National Biological Survey 1849 C St. NW,
Washington, D.C. 20240].
DeSante, D. F. 1983. Annual variability in the abundance of migrant landbirds on Southeast Farallon
Island, California. Auk 100: 826-852.
Dunn, E.H., and E. Nol. 1980. Age-related migratory behavior of warblers. Journal of Field Ornithology 51:254-269.
Dunn, E.H., and D.J.T. Hussell. 1995. Using migration counts to monitor landbird populations: review and evaluation of current
status. In, Power, D.M. [ed.], Current Ornithology, Vol. 12, Pp 43-88. Plenum Press, New York.
Dunn, E.H., D.J.T. Hussell and R.J. Adams. MS. Monitoring songbird population change with autumn mist-netting. (Submitted to Journal
of wildlife Management).
Dunn, E.H., D.J.T. Hussell and J.D. McCraken. In press. A comparison of 3 count
methods for monitoring songbird abundance during spring migration: banding, census and estimated totals. U.S.D.A. Forest Service publication.
Eckert, K.R. 1990. Lakewood Pumping Station census of fall migration. Loon 62: 99-105.
Fuller, M.R. and K. Titus. 1990. Sources of migrant hawk counts for monitoring raptor populations. Pp. 41-46 In J.R. Sauer and S.
Droege (Eds.). Survey designs and statistical methods for the estimation of avian population trends. USFWS Biol. Rept. 90.
J.M. III, T.L. Lloyd-Evans, J.L. Atwood and D.S. Wood. 1992. Long-term changes in migratory landbirds in the northeastern United States:
evidence from migration capture data. Pp. 115-130 in J.M. Hagan III and D. Johnston [Eds.] Ecology and Conservation of Neotropical Migrant
Landbirds. Washington: Smithsonian Institution Press.
Hussell, D.J.T. 1981. The use of migration counts for detecting population
levels. Pp. 92-102. In C.J. Ralph and J.M. Scott [Eds.], Estimating Numbers of Terrestrial Birds, Studies in Avian Biology No. 6.
Hussell, D.J.T., M.H. Mather and P.H. Sinclair. 1992. Trends in numbers of tropical- and temperate-wintering migrant landbirds in migration at
Long Point, Ontario, 1961-1988. Pp. 101-114 in J.M. Hagan III and D. Johnston [Eds.] Ecology and Conservation of Neotropical Migrant
Landbirds. Washington: Smithsonian Institution Press.
McCracken, J.D., D.J.T. Hussell and E.H. Dunn. 1993. A manual for
monitoring bird migration. Long Point Bird Observatory, Port Rowan, Ontario. 65 pp.
Pardieck, K. and R.W. Waide. 1992. Mesh size
as a factor in avian community studies using mist nets. J. Field Ornithology 63: 250-255.
Pyle, P., N. Nur and D.F. DeSante. 1994.
Trends in nocturnal migrant landbird populations at Southeast Farallon Island, California, 1968-1992. Pp. 58-74 in J.R. Jehl, Jr. and N.K.
Johnson [Eds], A Century of Avifaunal Change in Western North America, Studies in Avian Biology No. 15.
Ralph, C.J. 1981a. Age ratios and their possible use in determining routes of passerine migrants. Wilson Bulletin 93:164-188.
Ralph, C.J. 1981b. Terminology used in estimating numbers of birds. Pp. 577-578. In C.J. Ralph and J.M. Scott [Eds.], Estimating
Numbers of Terrestrial Birds. Studies in Avian Biology No. 6.
Ralph, C.J., G.R. Geupel, P. Pyle, T.E. Martin and D.F. DeSante. 1993. Handbook of Field Methods for Monitoring Landbirds. USDA
Forest Service, Pacific Southwest Research Station, Albany, California. USDA-PSW-Gen. Tech. Rept. 141.
Migration Monitoring Council and Technical Committees
Migration Monitoring Council:
*Peter Blancher, CWS, National wildlife Research Centre
Michael Bradstreet, Long Point Bird Observatory
Greg Butcher, American Birding Association
André Cyr, University of Sherbrooke
Loney Dickson, CWS, Edmonton
*Sam Droege, U.S. National Biological Service
Bill Murphy, Ottawa Banding Group
Nadav Nur, Point Reyes Bird Observatory
C. John Ralph, U.S. Forest Service
Stan Temple, University of Wisconsin
Intensive Sites Technical Committee:
John Hagan III, Manomet Bird Observatory
Keith Hobson, CWS, Saskatoon
*David Hussell, Ontario Ministry of Natural Resources
Nadav Nur, Point Reyes Bird Observatory
*C. John Ralph, U.S. Forest Service
Extensive Sites Technical Committee:
*Greg Butcher, American Birding Association
Jim Cox, Florida Fish and Game
Brenda Dale, CWS, Edmonton
*Erica Dunn, CWS, National wildlife Research Centre
Jeff Price, U.S. National Biological Service, North Dakota
Ken Rosenberg, Cornell Laboratory of Ornithology
Rick West, Delaware Spring Bird Count
* = Co-chairs of Council or Committee
CWS = Canadian wildlife Service, Environment Canada.
Appendix 2: Migration Monitoring Manuals
A manual for monitoring bird migration, by J.D. McCracken, D.J. T. Hussell and E.H. Dunn (1993). Source: Long Point Bird Observatory,
P.O. Box 160, Port Rowan, Ontario N0E 1M0.
A manual for monitoring bird migration at Last Mountain Lake, by Alan R. Smith (1994). Source: Alan R. Smith, Canadian wildlife
Service, 115 Perimeter Road, Saskatoon, Saskatchewan, Canada S7N 0X4.
A manual for monitoring bird migration, Beaverhill Bird Observatory (1994). Source: Beaverhill Bird Observatory, P.O. Box 1418,
Edmonton, Alberta, Canada T5J 2N5.
A revised field protocol for monitoring bird migration at Thunder Cape Bird Observatory, by Jul
K. Wojnocoski and David J.T. Hussell (1995). Source: Long Point Bird Observatory, P.O. Box 160, Port Rowan, Ontario N0E 1M0.
A manual for monitoring bird migration at the Delta Marsh Bird Observatory, by Heidi den Haan and Paula Grieef (revised 1998). Source:
Delta Marsh Bird Observatory, R.R.#1, Box 1, Portage la Prairie, Manitoba, Canada R1N 3A1.
Coyote Creek Riparian Station Bird Bander's Handbook. Source: Coyote Creek Riparian Station, P.O. Box 1027, Alviso, California
The Lesser Slave Lake Bird Observatory Operation Manual (1995). Source: Beaverhill Bird Observatory, P.O. Box 1418,
Edmonton, Alberta, Canada T5J 2N5.
Appendix 3: A Preliminary Assessment of Species Coverage Priorities for Migration Monitoring Programs
prepared by Erica H. Dunn [From Appendix D in Blancher et al. (1994)]
Note: Priorities may be lower if breeding density in inaccessible northern
breeding populations is very low. Regional lists might include more species. Recent analyses indicate that Christmas Bird Counts may provide
good trends for those species that winter largely in the United States (lists 2. and 4. below); if so, those species may be lower priority than
- 50+% of North American breeding range is north of Breeding
Bird Survey coverage and 50+% of winter range is south of U.S.
Cape May Warbler
- 50+% of North American breeding range is north of
Breeding Bird Survey coverage but much of winter range is in U.S.
American Tree Sparrow
- 50+% of Canadian (but not North American)
breeding range is north of Breeding Bird Survey
coverage and 50+% of winter range is south of U.S.
Western Wood Pewee
Black-throated Green Warbler
- 50+% of Canadian (but not North American) breeding
range is north of Breeding Bird Survey coverage but
much of winter range is in U.S.
Appendix 4: Daily Estimated Totals & Additional Information
The "daily estimated total" (or ET) method for deriving a daily
count for population monitoring combines data from incidental observations and from one or more other completely or partially standardized
procedures. It can be regarded as a special case of an area search census conducted over an extended count period (part or all of a day) and
using more than one procedure (e.g. netting, route census, incidental observations) to make the search. The ET method is useful for obtaining a
census (total count) of birds present or passing through an area that reflects observed abundance levels and is as complete as possible
(including species that are rare at a particular site or time of year). Depending on the site characteristics, we recommend a combination of
either a census or visible migration count and a netting/trapping procedure that is standardized to the maximum extent possible at the
particular site, together with incidental observations. At sites where trapping or netting is not possible, an ET based on a census and/or a
visible migration count, together with incidental observations, is acceptable; but inclusion of banding is preferred because of the additional
useful information it yields. Moreover, the ET procedure is particularly well-suited for incorporating incidental observations made while
operating nets and traps.
The method adopted by Long Point Bird Observatory (LPBO) (McCracken et al. 1993) is based on the "daily census" conducted at British bird
observatories. The "daily census" attempts to estimate and record the actual number of birds of each species present in or passing through a
specified area and detected by observers on a given day. The LPBO procedure includes a standardized hour-long "census" of birds observed along
a pre-determined route covering essentially the entire count area. LPBO has adopted the more realistic term "daily estimated total" (or "ET")
to replace the British "daily census" and to avoid confusion with the formal hour-long census. The daily census of landbird migrants conducted
regularly since 1968 on Southeast Farallon Island, California by Point Reyes Bird Observatory follows a procedure similar to that at British
bird observatories and LPBO, but at that site the ET closely approaches a true census because all or nearly all individuals are detected
(DeSante 1983). Input to the daily estimated total at Long Point typically consists of conducting the morning census, at least six hours of
intensive trapping and netting starting at dawn (weather permitting) and more or less continuous incidental observation by banders and other
observers throughout the day. In other situations, other input to the daily estimated total may be more appropriate. At Thunder Cape, Ontario,
for example, it was found that continuous observation of diurnal migration in the morning was essential to coming up with realistic ETs, because
the area under observation was relatively small, and there was much movement of birds into and out of the site. These birds were not covered
either by the banding totals or by a census of relatively short duration, nor could they be tallied adequately by casual or intermittent
observation. Therefore, the morning census was replaced by a standard 6-hour visible migration count at Thunder Cape.
Regardless of the components used as input, all observers and banders present at the site should participate in arriving at the consensus
ET for each species each day. The objective is to use all available information to estimate the numbers of each species present in or passing
through the count area during the count period (including incidental observations, e.g. of fly-bys seen while nets were being checked).
Some subjectivity is involved in making decisions about which birds have been double-counted (by 2 procedures or by 2 persons making
incidental observations) and in estimating overall abundance. Inclusion of all banders and observers in the process is intended to help resolve
such problems and to mitigate any one person's tendency to over- or under-estimate numbers. Such subjective judgements about overlap and
numbers will likely result in some day-to-day variability in the accuracy of ETs, but this should not greatly affect year-to-year consistency if
the same procedure for deriving ETs is followed consistently from year to year (see McCracken et al. 1993: 16-18).
Because the components of the ET involve overlap, each of the components is at least partially redundant. For example, some of the birds
censused may also be included in the banding total. The procedure for estimating the daily total (ET) involves somewhat subjective judgements
about such overlap and redundancy (see above). Nevertheless the ET procedure also takes advantage of the partial redundancy of methods, in the
sense that an individual detected by any of the component methods can be included in the ET. When weather conditions preclude netting, many
birds that would have been captured are included in the observation totals. Moreover, additional observation effort can compensate for the
smaller sample of captured birds when netting effort is reduced. Redundancy of component methods and compensatory adjustments of effort may
contribute to accuracy of ETs as a measure of the actual numbers occurring at a site.
ETs are likely to be most consistent when the
component procedures are highly standardized and strict rules are followed for making judgements about overlap. Special attention should also
be given to adopting a protocol that avoids excessive variation in the time and effort devoted to incidental observations.