General Technical Report
Nest searches provide the most direct measurement of nest success in specific habitats. They also allow identification of important habitat features associated with successful nests and insight into habitat requirements and species coexistence. Knowledge of the appropriate cues and techniques for finding nests allows large numbers to be found, thereby providing vital information about many species. Nest searches have an advantage over constant-effort mist netting, in that the measures of success are direct and habitat-specific. However, they are more limited as to the area surveyed and do not measure individual survivorship. Mist nets sample birds from a larger area, and the data derived may therefore have wider applicability, but are not habitat specific.
In this section we describe aids and standardized techniques for locating and monitoring success of nests, adapted from Martin and Geupel (in press).
Nest finding is labor intensive (DeSante and Geupel 1987, Ricklefs and Bloom 1977), but most observers can improve their ability to locate nests in a matter of days with training and practice.
The behavioral observations and clues described below work effectively for a variety of species. However, our experience includes a small subset of species and habitats and, in particular, is largely restricted to forest and shrub habitats. Other methods may be more effective in other habitats. For example, cable-dragging (Higgins and others 1969) and rope-dragging (Labisky 1957) may be more effective for many grassland species. In particular, all species, and even some individuals, differ in nest placement and behaviors near the nest. The patience and alertness of observers, and their familiarity with the habitat and behavior of individual species, are the most important influences on effectively locating nests.
Nest finding can be a frustrating task; patience is an important asset. It is a good idea to set a goal of finding at least one nest daily. More than one nest will be found on many days, but if at least one nest can be consistently found every day, the numbers of nests over the season will rapidly accumulate.
The particulars of plot sizes and numbers will vary according to the purpose of the study or activity, the habitat involved, and the density of birds. As a general guideline, we recommend that two study plots be established for each person that searches for nests. The searchers should work alternating days on these two plots for the entire nesting season. This provides consistent monitoring and allows the person to become familiar with the plot. In general, eight plots, each 40-50 ha, would usually be necessary to be established in forest habitat to find sufficient numbers of nests (ca. 20 nests per species) for the range of species typically found in any given forest, but smaller plots (ca. 10 ha) can be established in areas with higher densities.
In general, one should try to develop as quickly as possible a search image for the nests of various species. T. Sherry (pers. comm.) notes that he routinely finds 25-50 percent of his nests by constantly scanning appropriate potential nest locations in the vicinity of an active female.
During Nest Construction
Ideally, nests should be located during construction to provide the best estimates of nest success. This is also usually the easiest time to find nests because of the high level of activity and, in some areas, forests are not leafed out, making the task of following the female much simpler (T. Sherry, pers. comm.). We advise biologists to spend the maximum amount of time early in the season when the finding rate is maximum. Nest building begins by May in most areas of North America, although permanent residents and some ground-nesting species will begin earlier. Only the female constructs the nest and incubates the eggs for most small terrestrial birds (Kendeigh 1952, Silver and others 1985). Exceptions include woodpeckers, vireos, and wrens. Thus, the most effective way of finding most nests is by locating and following females, although males may provide some cues. Some nests in the shrub layer can be found by random search. Ground nests in forests are usually the most difficult to find. It is best to watch the female as she is gathering nesting material without using binoculars, because when she flies, she can be followed more easily with the naked eye.
Females tend to be extremely furtive during nest building. A mated female can be recognized by copulations or by her movements around the territory unharassed by the male. Females should always be checked with binoculars, especially during and after long, direct flights, to determine whether nesting material is being carried. Many birds will carry very fine material, not obvious upon casual inspection, such as spider webbing and hair for lining nests.
Sitting near sources of nesting material (e.g., failed nests, thistles) or open areas with a good view of the territory can help detection of nest-building females. Observers should use different paths across plots to increase the probability of randomly encountering females near undiscovered nests.
Follow a bird with nesting material from a distance to avoid disturbance. Do not interrupt a long flight. If the bird disappears in a patch of vegetation, begin to scan for potential nest sites. Be patient and wait for another visit by the bird. If the area where the female disappears is near the nest, the female will spend time in the area. At the same time, be aware that the female may move out of the back side of the patch to another patch that contains the nest.
Some individuals tolerate nearby observers and behave normally, but most species are very wary of observers. If the observer is too close to the nest, the bird often will sit on a perch somewhere near the nest site until the observer leaves. Eventually the bird will drop the nesting material if the observer does not move away. Thus, such behavior is an indication that the observer is too near the nest and should move quickly away. Obtain a new position at some distance (ca. 15 m) hidden by vegetation. Observe the female arrive with nest material and leave without it from the same location several times. Be aware that a female can skulk into one patch of vegetation and leave unobserved to move to a different patch, then return the same way, to give the appearance of nesting in the first patch. Some species such as MacGillivrays Warbler, Hooded Warblers, and Sage Sparrows will walk on the ground for several meters to approach the nest secretly. Birds can often be detected by watching for movement of the vegetation where they are otherwise hidden. Where the vegetation stops moving is usually the nest site.
Mapping the males position as he sings around the territory can often reveal a center of activity from which the male can often see the nest (T. Sherry, pers. comm.). The observer then can scan appropriate nest sites nearby, or at least increase the chance of catching a glimpse of a wary female.
Once the suspected nest site has been identified, back away quickly. Verify the status and location a few hours later, being careful that the female is absent. Do not approach the nest while the female is watching; disturbance at this early stage can cause abandonment. After quick verification, the area should be left and not visited for four days.
This is the most difficult stage for finding nests because the female may visit the nest only when she lays an egg, and most species lay one egg per day. The female will sometimes sit on the nest during egg-laying when weather is particularly harsh. Nest visitation becomes more frequent with more eggs in the nest (Kendeigh 1952).
Behavioral cues are useful at this stage. When either parent gets near the nest, they will look at it. If an egg-laying female detects a predator in the area, such as an observer following her, she will sometimes check the nest. Another good cue is a female staying in an area without actively feeding. She will often look at the nest site repeatedly, aiding location of the nest.
Finally, copulatory behavior can be used during both nest-building and egg-laying. Copulation often occurs in the same tree above a nest, on the same branch, or in the next tree. Examine carefully the area immediately adjacent to copulatory activity.
The beginning of incubation can be estimated as when females suddenly vanish, and males increase singing. Some behavioral cues can help locate nests. Females start foraging faste during the incubation and nestling stages, probably because their time is more limited. Females that are making rapid hops, quick short flights, and rapid wing flicks will probably return to the nest soon. On average, most passerine females are off the nest for 6-10 minutes and on for 20-30 minutes (e.g., Zerba and Morton 1983).
Observers can find females by alertly moving through the study plot, but sitting down in a spot for 20-30 minutes is also useful. A female leaving a nearby nest can thereby be detected. Females can also be detected by call notes, although species differ in the types of sounds. Females of many taxa (e.g., gnatcatchers, warblers, Emberizine finches) chip or call just before leaving, or just after leaving, the nest. This behavior seems to be a communication note to the mate. Females of other species use other vocal signals, e.g., thrushes give a chuck or mew sound; tanagers often give a characteristic sound near the nest or during copulation; and some taxa (e.g., Emberizine finches and icterines) have a nest departure call (McDonald and Greenberg 1991), often answered by the male. If you detect, follow, but then lose a vocalizing female, immediately return to the original location where she was detected, and you may often find her again before she returns to the nest.
Males can also be of some help. When the female is off the nest, some males quietly guard the nest or follow her (for example, the Gray Catbird) (Slack 1976). A quiet male may indicate presence of a foraging female or a nest nearby. In many species, especially cavity-nesters, males will feed incubating females (e.g., Lyon and Montgomerie 1987; Martin and Geupel, unpubl. data; Silver and others 1985). Males of some species (e.g., Chestnut-sided Warbler) use singing perches that are in direct view of the nest. Males sitting on a perch, looking towards the same spot, may indicate a nest.
Males can sing anywhere in the territory while a female is incubating, but he can become silent when the female is about to leave, or has left, the nest (T. Sherry, pers. comm.). When this occurs, he will often make a long flight over to where the female is starting to forage (and sometimes will incite her to leave the nest). Sherry suggests being alert to these flights because they provide valuable clues to where the nest vicinity is, and can also help the observer detect females, which are often difficult to find considering how long they stay motionless during incubation.
A female foraging off the nest is fairly tolerant of people, but observers should be inconspicuous. As she returns to the nest, she is more cautious. This can be used to an observers advantage. First, a relatively long flight after foraging is probably a return to the nest, and is often along the same route. Quickly running in her direction for about 25 m may often result in a resighting, because the disturbance will keep her from returning to the nest, giving more time to relocate her. If she is near the nest, but cautious about approaching, she will bounce between a few branches, and may also forage rapidly. Eventually, she will start to move down toward the nest several times and then suddenly fly back up, apparently indecisive. If the observer is too close to the nest, the bird will continue to bounce, and will sometimes fly off, only to return within a few minutes. The observer should then back off and watch. If it is cold, do not keep her off the nest for too long. If the female has been followed for more than 30 minutes without results, then she probably is not on a nest, unless both sexes incubate.
If a female disappears into a tree or shrub, the nest is probably in or next to it. Memorize the area where the female disappeared and choose potential nesting sites before approaching. Moving quietly, begin tapping potential nest shrubs with a stick. Listen for the flush of the female off the nest. If unsuccessful, the site can be revisited for careful searches.
In many species, nest site preference seems to be an evolutionarily conservative trait (Martin 1992). Some birds greatly prefer their nest to be in or under certain plant species, or in particular patch types (Martin and Roper 1988, Martin unpubl. data). Describe and visit nest sites from previous years to aid new observers in finding nests.
During the Nestling Stage
Finding nests during the nestling period is the easiest, because both males and females commonly bring food and remove fecal sacs. Males are normally the easiest to follow, as they tend to be less cautious. Nests can usually be found from a distance using binoculars because of the constant activity of the parents.
In some species a singing male can indicate the nest location. He may sing, for example, less and less as he starts to gather food to carry to the nest, become silent when he is about to approach the nest, and then resume loud song immediately after leaving the nest (T. Sherry, pers. comm.). Additionally, Sherry notes that birds will often become reticent to go to a nest with a human nearby, so that if a bird becomes relatively inactive (hopping around, not taking long flights) in a particular area, or dropping prey, then the nest is probably nearby. In this case, the observer should either search intensively in the vicinity, if likely nest spots are nearby, or back away to give the bird a chance to become calm and go to the nest.
Knowledge of the nesting cycle allows an observer to anticipate when to start looking for a new nest. Most species will renest after a nesting failure, although this varies among and within species (Geupel and DeSante 1990a, Martin and Li 1992). Reconstruction usually begins within 10 days, and the earlier in the nesting cycle that failure occurred, the farther apart the nests are likely to be (citations in Martin 1992). Multi-brooded species may renest in as little as 8 days after fledging. Sometimes the female will begin nesting while the male is still tending the fledglings of the previous brood (Burley 1980).
Each nest found needs to be checked every 3 to 4 days to determine its status. Careful attention to checking nests is critical for data quality, because the number of days that nests have eggs or young is used to calculate daily mortality rates, the most effective measure of nest success (Mayfield 1961, 1975). Nests should be checked from a distance the day before expected fledging, and every other day thereafter. A chart showing nests as they are found and the expected date of fledging is extremely helpful. If nestlings appear ready to fledge before the next scheduled visit, then the next visit should be sooner. Calculations of nest success should terminate with the last day that young were observed in the nest. Nests should also be checked more frequently about the time of hatching, if the length of the incubation period is desired.
With canopy nests, mirrors attached to telescoping aluminum poles can check contents of nests. These are available from stores stocking swimming pool supplies, and are commonly up to 4-5 m. A window-washing pole to 12 m is also available (Tucker Manufacturing Company, 613 Second Ave. S.E., Cedar Rapids, Iowa 52406; 319 363-3591). T. Sherry (pers. comm.) suggests a convex mirror to allow views from a variety of angles from the ground. Mounting a small flashlight next to the mirror can illuminate the nest contents in cloudy or rainy weather. Often binoculars must be used to view the nest in the mirror.
Careful and detailed observations should be recorded if a nest predation event is observed. If the nest appears inactive from a distance, it should be approached to verify. If the eggs or young appear to be gone, then check the nest structure and immediate area, perhaps up to 6-10 m (T. Sherry, pers. comm.) for evidence. Any evidence (e.g., shell fragments, hole in nest, nest torn up) should be fastidiously noted. When the young fledge, they commonly perch on the edge, flattening it, and leave fecal droppings in (or on the edge of) the nest. These would indicate possible successful fledging. Observers should try to verify success by seeing fledglings or by hearing adult alarm calls or begging calls of the young. Fledglings normally do not move very far in the first couple of days, although some, such as Rufous-sided Towhee, may move 100 m in a few hours. Some species or individuals may carry food up to 24 hours or longer after predation of their nest, including to unrelated fledglings from neighboring territories.
Nestlings may be banded when the primaries first break sheath. Banding may provide valuable information on juvenile survival and dispersal. Always have an assistant with you to record data, and be careful the nestlings do not jump out as you try to remove them (use two hands). Avoid banding in the morning or during cold or wet periods.
Filling Out the Forms
Two types of data sheets are used to record data about the nest site and nest activity. One set (Nest Check Form fig. 11) is used in the field to record information when nests are checked. To prevent loss, and serve as a backup and summary record for each nest, the Nest Record Form (fig. 12) should be maintained at some permanent location. The Record Form should be updated daily, to prevent data loss.
Figure 11An example of a Nest Check Form for recording in the field the status of nests and information on where the nest is located.
Figure 12An example of a Nest Record Form that is kept at a permanent location for recording data from the Nest Check Form, as well as the nest site and characteristics data.
All observations should be recorded on the Check Form and transferred to the Record Form, including visits with no activity. This is particularly critical for canopy or cavity-nests where nest contents cannot be viewed.
Nest Check Form
Data are collected in the field and are recorded on the Check Form. One to several nests can be recorded on a single form. When a new nest is found, its location is carefully noted at the bottom of the form, and the form may be needed in the field over the next few visits to relocate the nest. The data taken should include:
- State or provinceThe 2-column code for each.
- RegionAn 8-column code, designated by the investigator. Often, the name of the USGS quad, a prominent landmark, or a nearby town will provide the best code name.
- StationA 4-letter code for the station that contains the nest search plot.
- Observers initials.
- Nest numberA unique, identifying 2-column number for the nest site. We would expect that at each station, for each species, no more than 100 nests would be found.
- Species nameThe 4-letter code, based on CWS and USFWS (1991).
- DateMonth, Day, Year.
- TimeUse the 24-hour clock.
- The activity of an adult if either building (build.) or incubating (on), by putting an X in the blank.
- The observer should record the contents of the nest whenever it is approached close enough for careful observation. If the contents are actually observed, this should be noted by an X in the observed box (obs.). If the contents are counted accurately, the number of eggs, young, or both, are noted. Age of the nestlings should be estimated when possible because it can help determine the nest fate by providing information on length of time that nests were active. Age estimates should be recorded in Notes.
The form also includes space for a description of one or more nest sites that the observer finds on this day. The description should be sufficiently detailed to allow anyone to locate the nest. Take compass readings from a fixed point (e.g., a stake or grid point) to establish a reference location.
Nest Record Form
This form is filled out each day upon return from the field, and should contain the following data:
- Header data
- State or Province
- Species code
- Nest number
- The number of attempts at nesting that this record represents for that pair for that season.
- Nest Checks. These are the data transcribed from the Check Form, and are the same as for that form.
- Dates and Period
- The following dates should be tabulated, as they become available: date of finding of nest (and contents when found), date of first egg laid, date of clutch completion (and number of eggs laid in final clutch), date of hatching of last egg (and number of nestlings produced), date of fledging (and the number of fledglings), or nest failure, and date when last active.
- Outcome, a written description of the fate of the nest.
- Cause of failure (codes: UN = unknown because not revisited; FY = fledged, with at least one young seen leaving or in vicinity of nest; FP = fledged young, as determined by parents behaving as if dependent fledgling(s) nearby, FU = Suspected fledging of at least one young, but uncertain (e.g., no adult behavior observed); FC = fledged at least one host young with cowbird parasitism; PO = predation observed; PE = probable predation, nest empty and intact; PD = predation, damage to nest structure; AB = nest abandoned prior to eggs; DE = deserted with egg(s) or young; CO = failure due to cowbirds; WE = failure due to weather; HA = failure due to human activities; and OT = other).
- Period = the number of days nest was observed for the following: days during the egg laying, incubation, and nestling period.
- Success = for each period, based on the following codes: S = Successful, D = Depredated, N = status unknown/nest not occupied, U = status Unknown/nest occupied fate unknown, M = Mortality other than predation, A = Abandoned, F = Female died, Z = abandoned, no (zero) eggs laid.
Predation Risk from Monitoring
Locating and monitoring nests have potential to increase predation (Major 1989, Picozzi 1975, Westmoreland and Best 1985). With proper precautions, such biases can be eliminated or minimized (Gottfried and Thompson 1978, Willis 1973). Finding the nest normally creates the most distress to adults and disturbance to the nest site because subsequent visits are brief. Some evidence suggests that predation rates are higher on the first or early visits than subsequent visits (Bart 1977, but see Bart and Robson 1982).
Therefore, we suggest the following when locating nests:
We suggest two methods of vegetation measurement: (1) the nest and the plant containing it; and (2) the nest site and random points in the plot. The entire plot should be measured with a series of points, as outlined in the section Methods of habitat assessment below.
The Nest and Nest Plant
Measurement of the vegetation of the nest site is an important research tool and has some application to monitoring. If you wish to determine this aspect of habitat, we suggest that you measure the vegetation as soon as a nesting attempt terminates. Be careful at the beginning of the season, as an empty nest may not yet have eggs. Some species or individuals will delay laying as long as eight days after completing nests. Do not delay measuring the vegetation, because foliage density around the nest changes rapidly.
We suggest the following measurements (fig. 12), of the plant containing the nest. All measurements should be in centimeters.
- Plant species common name.
- Plant species genus.
- Plant height.
- Nest height.
- Plant dbh (diameter at breast height), stem diameter of the nest substrate, usually measured at 0.25 m above the ground, because many nests are in substrates less than breast height.
- Nest distance from edgeDistance from edge of plant, inward to the nest.
- Canopy coverThe canopy cover at chest height should be measured using a densiometer. This is a measure of the tree canopy, and should be measured as close to the nest as possible, but not under the canopy of the nest plant if it is a shrub.
- Nest distance from center/stemDistance of the nest laterally from the main stem.
- Number of support branchesThe number of branches actually supporting the nest.
- Diameter support branchesAverage diameter of stems supporting the nest.
- Nest concealmentMeasured by estimating percent of the nest concealed by foliage cover in a 25-cm circle centered on the nest from a distance of 1 m from above (overhead cover), from below, and from the side (side cover) in each of the four cardinal directions.
- Compass directionDirection from the nest to the main stem of the substrate.
- Total percent cover nest substrateThe percent cover of the plant containing the nest, using the outer margin of the plant as the boundary. This is most useful in shrubs.
The Nest Site and Random Points
Vegetation in the patch surrounding the nest can provide information on differences in microhabitat choice among species.
We recommend using vegetation sampling methods based on a series of points, as outlined in the section Methods of habitat assessment, below, or those described in Martin and Roper (1988) with some modifications (obtainable from Martin). The point method involves measuring habitat features in the nest patch in circular releves of 11.2-m radius centered on the nest, smaller than the 25- to 50-m releves for general habitat assessment, detailed below. In addition, non-use sites should be sampled with the same protocol at 35 m from the nest in a direction parallel to the contour of the plot (to stay within the same microhabitat type when possible). The sampling plot should be centered on the plant stem nearest to the 35-m point that is of the same species and size as that used for the nest. Random plots can also be established in a grid to obtain a stratified random sample of the vegetation. Comparisons of random versus nest plots can indicate choice of microhabitat types. Comparisons of nest versus non-use plots then provide information on choice of habitat patches within a microhabitat type. These sampling protocols keep the methods relatively compatible with other sampling schemes (e.g., James and Shugart 1970), but also allow tests of hypotheses about the interactions between choice of nest site and predation risk or habitats chosen for nesting.