Documenting Breeding Evidence
The goal of the breeding survey was not just to produce a species list, but to determine which species actually breed within each block. Finding an active nest with eggs or young is generally the strongest evidence of breeding, but many other things, especially the behaviors of birds, can provide evidence for breeding. The Connecticut Bird Atlas used breeding criteria agreed upon by the North American Ornithological Atlas Committee (NORAC; Beck et al. 2018), with minor modifications to facilitate data entry via the eBird app.
Breeding categories and codes: The list of breeding codes is divided into four categories of breeding evidence: Observed, Possible, Probable, and Confirmed. Within each category there are a number of different codes, associated with different breeding behaviors. Codes are listed below in increasing order of certainty of breeding. Volunteers were encouraged to keep a separate checklist of species and record of the breeding codes observed on each visit to a block. By doing this, we had several goals. First, we wanted to ensure that the strongest evidence for breeding was recorded for each block. For example, if a pair (code P) had been recorded on one visit, and an adult carrying nest material (code CN) on the second, then we wanted to ensure that the CN code was recognized as indicating confirmed breeding. And, if a nest with young was found on a third visit, we wanted to recognize that the evidence for confirmed breeding was especially strong.
Even when breeding codes do not provide stronger evidence of breeding in a block, they contribute important information. Recording breeding behaviors throughout the breeding season helps us build an understanding of the timing (or phenology) of breeding. Knowing the range of dates within which a given species is nesting can be useful for conservation because it helps us know when certain types of activities are most likely to affect a species adversely, and can help refine the timing of species-specific research. We are also increasingly recognizing that the timing of many key events in the lives of birds, including migration and nesting, are in flux. Many studies have now demonstrated phenological change as a common response to climate change, for instance. Timing of breeding might also vary among habitats, with possible consequences for species as a result of land conversion. Reporting breeding codes for every checklist also provides benefits when checklists are located to specific locations within a block – for instance, knowing that there are five specific places within a block where a given species nests is more valuable than simply knowing that a species nests somewhere in the block. Not all observers collected this level of detail – doing so is a much bigger commitment, than the traditional atlasing approach of simply reporting the breeding code that provides the strongest evidence of breeding for a block – and users of the atlas data should bear this in mind. But, many observers did go above and beyond the traditional approach, enriching the data available for analysis.
OBSERVED CODE
X: Species (male or female) observed in the block during its breeding season, but not in suitable nesting habitat. No evidence of breeding. This code could apply to species such as vultures soaring overhead, summering ducks on an urban pond without nesting habitat, or a colonial nesting species not at a colony. This code should be used when an observation does not meet the standards of one of the higher breeding codes. X is not listed as a breeding code in eBird since it does not indicate breeding. (Note, F for “flyover” is listed as a code in eBird, but is not a standard atlasing breeding code and also suggests birds that may not be breeding nearby. For the Connecticut Bird Atlas, the F code was treated as equivalent to X, on the grounds that the H code would have been used in situations where a bird may have been a breeder.)
POSSIBLE BREEDING CODES
H: Individual(s) of species (male or female) observed in suitable breeding habitat during the breeding season. Observers were warned to be cautious during migration periods when birds may just be passing through or lingering at wintering sites.
S: Singing bird present or breeding calls heard in breeding season in suitable nesting habitat.
PROBABLE BREEDING CODES
M: Multiple singing males observed in suitable breeding habitat. This code is designed to identify species that are sufficiently common that they are likely to be breeding. By North American atlasing convention, this code should be used in cases when an observer has detected at least 7 singing males in suitable breeding habitat within a block.
P: Pair observed together in suitable nesting habitat during the breeding season. Observers were told that this code should not be used simply because two individuals of a species are seen in the same place, but should be used when there is a male and female that are interacting with one another. Warnings were also given that some migratory species pair before reaching the breeding grounds (e.g., many waterfowl); such records were excluded from the final atlas block lists based on safe dates.
T: Permanent breeding territory presumed due to defensive territorial behaviors (e.g., chasing individuals of the same species, or counter-singing) in breeding habitat and season, or territorial song at the same location on at least two occasions seven days or more apart. A male American Robin chasing another falls under this code, as would two males counter-singing against each. Some species, especially raptors and hummingbirds, exhibit territorial behavior in defense of feeding areas, favorite perches, etc. even while wintering or migrating, so observers were warned to be careful to limit use of this code to cases where defense is likely linked to nesting.
C: Courtship or display behaviors, indicating interaction between a male and a female, including courtship feeding or copulation.
N: Visiting probable nest site, but no further evidence obtained. This code is especially useful for cavity-nesters, for which it may be difficult to discern whether visits to a cavity confirm occupancy.
A: Agitated behavior or anxiety calls of an adult, indicating a nest site or recently-fledged young in the vicinity. A pair of birds circling just above an observers head or a Northern Goshawk distress call falls into this category. Observers were told not to use this code if agitated behavior was induced by “pishing” or using taped calls.
B: Nest building by wrens or excavation of holes by woodpeckers and wrens. Woodpeckers and other cavity excavators will make holes for roosting as well as nesting. Similarly, some wrens, including unmated males, make nests that are intended to attract mates and that are not used for breeding. Consequently, nest building by these species does not confirm breeding and the B code should be used instead of NB. If observers used the NB code for these species, we changed it to B for use in our analyses.
CONFIRMED BREEDING CODES
PE: Physiological evidence in the form of a brood patch on an adult female or a cloacal protuberance on an adult male. These features are usually detectable only when the bird is in the hand, and are used primarily by banders.
CN: Adults seen carrying nesting material (e.g. sticks, grass, mud, cobwebs). This code can be used with all species except wrens, which build dummy nests that are not used for nesting.
NB: Nest building at the nest site. This code cannot be used for wrens or woodpeckers (see B, above).
DD: Distraction display, defense of unknown nest or young, or injury feigning. Used when adult birds are seen trying to lead people away from a nest or young, most commonly in ground-nesters (e.g. Killdeer, Ovenbird). The code could also be used for active defense, such as a Cooper’s Hawk diving at an observer in a way that indicates close proximity to a nest, but not simply for agitated behavior (see A, above).
UN: Used nests or eggshells. Observers were asked to use this code only for unmistakable egg shells and nests that were used during the atlas period, and to not use it if they were unsure about the identity of an unoccupied nest, or whether it was used recently. We also warned against using it for species that build multiple nests in a breeding season, and that do not use all nests for breeding, such as Marsh Wren.
FL: Recently-fledged young of nidicolous species (those that stay in the nest for a while after hatch) or downy young of nidifugous species (those that leave almost immediately after hatching, e.g., ducks and shorebirds) including those incapable of sustained flight. Observers were warned to take care with use of this code for older fledglings or those seen close to a block boundary, and that the code should be used only if they were reasonably confident that the fledgling was hatched within the block. For instance, a duck brood on an isolated pond would merit this code, but older ducklings on a river may not. We also warned observers to be especially cautious about using this code for swallows, starlings, and blackbirds, which can fly considerable distances while barely-fledged. The observation of a recently-fledged cowbirds was taken to confirm breeding for both the cowbird and, if seen, the host.
ON: Adults entering, occupying (e.g. sitting on nest), or leaving a nest site in circumstances indicating an occupied nest.
CF: Adult carrying food for their young. Observers were warned to be cautious when using this code because it was important to avoid using the code for birds carrying food for their own consumption. In general, birds flying more than a few meters or hopping around with beaks full of food are usually taking it to a nest, and repeated observations of birds flying back and forth, taking repeated beakfuls to the same spot, would be especially compelling evidence. On the other hand, observers were told that the code should not be used for raptors (hawks, eagles, osprey, falcons), corvids (jays, ravens, crows), or kingfishers, which often carry food some distance before eating it themselves. We also warned against using it for species like terns, which may carry food from distances far from the nest and might be seen with food in a block in which they are not nesting. If observers submitted records for these species using this code, we changed the code (usually to H, unless there were notes or other evidence indicating that a different change was warranted).
FY: Adult feeding young outside the nest. Young cowbirds begging for food were taken to confirm breeding for both the cowbird and, if seen, the host.
FS: Adult carrying fecal sac. Many passerines keep their nests clean by carrying fecal sacs produced by their nestlings away from the nest and observations of this behavior were taken as evidence of a nest nearby.
NE: Nest containing egg(s). Observers were warned to take great care with nest identifications, unless an attending adult was present. A cowbird egg in a nest was taken to confirm both the cowbird and the host.
NY: Nest with young seen or heard. Observers were warned to take great care with nest identifications, unless an attending adult was present. A cowbird chick in a nest was taken to confirm both the cowbird and the host.