Pacific Southwest Research Station

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

Preface

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

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 monitoring.

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 monitoring.

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 Hussell (1995).

The Mirgration Count

Depending on 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 year.

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

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 be adopted.

Advantages of visible migration counts are:

1. Birds involved in unidirectional flights are clearly in migration, are new arrivals, and are unlikely to be counted more than once.
2. Counts are unlikely to be affected by year-to-year changes in food abundance or other local site changes.
3. Procedures can be highly standardized.
4. Counts are relatively unselective with respect to species, in that potentially all birds seen can be counted (but see Disadvantage #1).
5. No special skills for handling birds are needed, and birders like to participate in these counts.
6. 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:

1. 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.
2. If large numbers of birds are involved, it may be difficult to count or estimate their numbers accurately and consistently.
3. Because birds are not handled, age and sex composition of the migrants will be unavailable for most species.
4. 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 has the following advantages:

1. It is relatively unselective with regard to species (except that secretive species are likely to be missed).
2. 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.
3. Procedures can be highly standardized.
4. It can be completed in a relatively short time, by one person.
5. No special skills for handling birds are needed, and birders like to participate in censuses.

Disadvantages of a census are:

1. Some individual birds may be missed, especially in heavy cover, and others may be counted twice.
2. Observers must be skilled in identification and detection of birds. Training necessary to bring observers up to acceptable standards can be time-consuming.
3. The ability of observers to detect birds accurately varies.
4. 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.
5. Small birds moving rapidly through an area may be difficult to detect and identify.
6. Secretive species are likely to be missed.
7. 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).

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:

1. It is objective (all birds caught are identified and counted).
2. It is relatively unaffected by differences among the operators' abilities.
3. 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.
4. Previously-banded stopover migrants or residents can be identified as individuals and separated.
5. It provides detailed demographic information on age and sex classes which can be used to formulate hypotheses about causes of changes of population levels.
6. 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:

1. 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.
2. Its efficiency is likely to be adversely affected by severe weather conditions, and weather will dictate complete closure of the operation on some days.
3. 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.
4. Large day-to-day variation in numbers of birds at some sites may make it difficult to operate nets in consistent numbers or locations.
5. Netting and banding requires special skills, training, and permits.

Incidental observations have the following advantages:

1. They are relatively unselective as to species and can be made in any weather.
2. They provide a measure of species which are rare at the site or at a particular time of year.
3. They can detect presence and/or numbers of birds at a time of day or in a part of the site not covered by standardized procedures.
4. 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.
5. No special skills for handling birds are needed.

Disadvantages of incidental observations:

1. Many individual birds may be missed and others may be counted twice or more.
2. Observers must be skilled in identification and their abilities to detect and identify birds will vary.
3. 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.
4. Because they are not well standardized, they cannot be used as a sole migration count.

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:

1. 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 each procedure.
2. 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 and individuals.
3. 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.
4. 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:

1. 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.
2. Variation in effort devoted to any of the component procedures may introduce a bias into the estimates.
3. 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.
4. 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.

Count Area

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 field protocol.

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.

Count Period

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

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 stage.

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 stopovers.

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 locations.

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

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.

1. 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.
2. 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.
3. 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 volunteer commitment.
4. 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.
5. Preferably, the site should not be frequented regularly by the public and it should be well protected from any sources of development and disturbance.
6. 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).
7. 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 counting)
8. 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.

Habitat Management

Personnel

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:

1. 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.
2. 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.
3. 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

1. 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).
2. 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.
3. Definition of the daily time period(s) during which the count(s) are to be conducted (see Count Period).
4. 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.
5. 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.
6. A description of record-keeping procedures.
7. Statements specifying the knowledge, skills and experience required of participants and descriptions of training programs to bring inexperienced personnel up to the required levels.
8. A discussion of potential changes in the habitat at the site and, if necessary, a management plan designed to maintain a stable situation.

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

• 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 protocol.
• 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 criteria.
• 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 be documented.

Acknowledgments

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Appendix 1