Surveys - Breeding codes

Documenting Breeding Evidence

The goal of the breeding survey is not just to produce a species list, but to determine which species are actually breeding within each block. Finding a nest, however, is not the only evidence that can confirm breeding. In fact, nests need not be seen at all, and there is no need for volunteers to spend a lot of time searching for nests. Many other things, especially behavior, can be used as evidence for breeding. The Connecticut Bird Atlas will use breeding criteria agreed upon by the North American Ornithological Atlas Committee (NORAC), with minor modifications to facilitate data entry via the eBird app for those who wish to use it. The following codes and definitions are listed on the Field Card for reference while surveying. These standardized codes were chosen to ensure that everyone involved in the project collects information in the same way. Consequently, it is critically important to use these codes correctly and put them in the right place on the field cards. Study the breeding criteria carefully and clear up any questions with your regional coordinator before you start to collect data. Also, please feel free to ask questions if they come after you begin field work.

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. These codes are listed below in increasing order of certainty of breeding. As you spend more time in your block, you will be continually upgrading these codes for your final block list as you find stronger evidence of breeding. Please use only those breeding codes that are listed and be careful to use them as described. Even if you believe that breeding is more likely than the codes suggest, it is important that you wait until you have observed behaviors associated with a higher code before using one. For example, if you see a bird visiting a possible nest site you should record it as N to indicate probable breeding, even if you feel confident that the birds are definitely breeding in the block. If, on a subsequent visit, you observe birds repeatedly visiting the nest site then you can upgrade to ON (occupied nest), which confirms breeding. By only recording what you see, even if you think there is stronger evidence for nesting, you will improve the quality of the data and the variety of ways in which we can use it to accurately describe the status of the state’s birds.

Make a complete checklist on each visit: Each time you visit a block you should keep a separate checklist of species and record the breeding codes that you obtain on that visit. Even if you have confirmed a species on a previous visit, it is important to also record that species, along with the strongest evidence of nesting obtained on the new visit. In other words, if you find a Northern Cardinal nest with eggs on your third visit to a block, then see a pair of cardinals on the fourth visit, record the code “NE” for cardinal on the checklist for the third visit and the code “P” for cardinal on the checklist for the fourth visit. (Note that this requirement to maintain separate checklists on each visit is different from some other atlas projects that you might have been involved with, but will allow us to do much more with the data.)

A word of warning: Many of the coded breeding behaviors that appear under the possible and probable categories may also be seen in birds during migration (e.g., singing male warblers or a pair of ducks). If you find a species exhibiting apparent breeding behavior in a place where it is unlikely to nest, either because the habitat is unsuitable or because you are outside its normal breeding range, and you are still within the migration period of that species, then recording the bird as simply Observed (code X, see below) is probably appropriate.

Safe dates: To help you determine when breeding is likely to be plausible, we have provided a table that gives “safe dates” for each species . These are dates between which breeding is known to occur. Reliance on these dates is not perfect. Some late migrants, especially of species that also nest far north of Connecticut, may be seen after local nesting has begun. Also, individual birds do occasionally nest earlier or later than the majority of their species. Our safe date list is based on one being used by the Rhode Island Bird Atlas, and has been found to be accurate for most species over the past two summers. If, however, you detect strong breeding evidence outside of a species’ safe dates, the record is likely to be particularly important and we want to document the case carefully. Even if it is a common species, please provide a careful description to confirm the identity of the species and the behaviors you observed so that we can enter that information into the long-term scientific record.

Breeding Codes:


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. Note that if you use eBird to enter data, X is not listed as a breeding code and you should enter it manually in the checklist or enter the number of individuals seen, just as you would in a normal eBird checklist.


H-Individual(s) of species (male or female) observed in suitable breeding habitat
during the breeding season. Be cautious during migration periods when birds may just be passing through or may linger at wintering sites.

S-Singing bird present or breeding calls heard in breeding season in suitable nesting


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 you have detected at least 7 singing males in suitable breeding habitat within your block. If you find singing birds in the same location at least a week apart, or detect birds counter-singing in response to each other, then code T for territorial behavior would be more appropriate.

P-Pair observed together in suitable nesting habitat during the breeding season. Note that this code should not be used simply because two individuals of a species are seen in the same place. Use it when there is a male and female that are interacting with one another. Also be aware that some migratory species will pair before reaching the breeding grounds (e.g., many waterfowl).

T-Permanent breeding territory presumed due to territorial song, the occurrence of an adult bird, or defensive territorial behaviors (e.g., chasing individuals of the same species), at the same location in breeding habitat and season, 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 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 your head or a Northern Goshawk distress call falls into this category. Do not include this code if agitated behavior was induced by "pishing" or using taped calls (note that use of tapes is generally discouraged to reduce the risk of disturbance).

B-Nest building by wrens or excavation of holes by woodpeckers and wrens. Woodpeckers and other cavity excavators usually make only one nest hole, but will make other holes for roosting. Wrens, including unmated males, will make nests that are intended to attract mates and which may not be used for breeding. Consequently, nest building by these species does not confirm breeding (see also NB, below).


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 will be used primarily by banders. Care should be taken not to use this code for birds that may be nesting in a different block (e.g., if a banding station is close to a block boundary, or if the bird is a species that could travel a long distance from its nest).

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. Do not use for wrens or woodpeckers (see B, above).

DD-Distraction display, defense of unknown nest or young, or injury feigning. Used if adult bird is seen trying to lead people away from a nest or young. Commonly seen in most ground nesters (e.g. Killdeer, Ovenbird). Also use this code for active defense such as a Cooper's Hawk diving at you. Do not use this code for agitated behavior (see A, above).

UN-Used nests or eggshells found within atlas years. Unless carefully identified, use this code only for unmistakable egg shells and nests that were used during the atlas period. If you are unsure about the identity of an unoccupied nest, or about whether it was used recently, don't use this code. Do not use this code for species that build multiple nests in a breeding season, such as Marsh Wren. Do not collect nests, because some species roost in them year-round and because it is illegal to collect nests or eggs without a permit.

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. Take care with use of this code for older fledglings or those seen close to a block boundary; the code should not be used unless you have reasonable confidence that the fledgling was hatched within the block. A duck brood on an isolated pond would merit this code, but older ducklings on a river may not. Note too that barely fledged blackbirds and swallows can fly considerable distances. The presence of young cowbirds confirms both the cowbird and 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 the young. Be cautious when using this code. Birds flying more than a few meters or hopping around with beaks full of food are usually taking it to a nest, but avoid using the code for birds with food for their own consumption. Do not use the code for raptors (hawks, eagles, osprey, falcons), corvids (jays, ravens, crows), or kingfishers, which often carry food some distance before eating it themselves. Generally, avoid using it for species like terns, which may carry food from distances far from the nest and might be seen in a block in which they are not nesting.

FY-Adult feeding young (out of the nest). Young cowbirds begging for food confirm both the cowbird and 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. Fecal sacs generally appear as a gleaming white blob in the bird’s bill.

NE-Nest containing egg(s). Unless you see an attending adult, the same warnings under the UN code apply here. Cowbird eggs confirm both the cowbird and the host. If you find an active nest, be careful not to linger nearby or to make repeated visits, or you may risk disturbing the birds.

NY-Nest with young seen or heard. Use only when you actually see or hear the young. A cowbird chick in a nest confirms both the cowbird and the host. If you find an active nest, be careful not to linger nearby or to make repeated visits, or you may risk disturbing the birds.