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Handbook for Atlasing
North American Breeding Birds

Standardized Breeding Criteria Codes:
Recommendations for North American
Breeding Bird Atlas Projects

Sarah B. Laughlin, Janet R. Carroll, and Sally M. Sutcliffe
Vermont, New York, and New Hampshire Atlas Projects

The Working Group on Atlas Codes reached consensus on recommended codes for North American Atlas projects. For the most part this was relatively simple to agree upon; however, the MULTIPLE SINGING MALES issue was difficult and incurred some somewhat heated discussion. It was AGREED that "multiple singing males" is NOT an acceptable code for CONFIRMATION because it is not a hard, factual, biologically valid criteria like the other codes. Now that atlas results are becoming legal criteria for land-use issues and endangered and threatened species lists, it seems especially vital to keep our criteria above reproach. The same arguments hold true for "multiple singing males" as PROBABLE. One side argues that if you have seven singing territorial males on one block-busting trip you should intuitively know that you have a probable breeding species. The other side argues that if there are indeed those numbers of males behaving in a territorial way that you should be able to achieve probable statue with one of the existing codes, and that it's not worth jeopardizing our data by using "sloppy" codes. The Working Group finally voted NOT to recommend it as an acceptable code for PROBABLE either (voting NOT ACCEPTABLE were New Hampshire, New York, Maryland, Quebec and Virginia; voting YES, ACCEPTABLE were West Virginia and Florida).

Code1— Evidence


O—Species (male or female) observed in a block during its breeding season, but no evidence of breeding. Not in suitable nesting habitat. Includes a wide range of species such as vultures or raptors, or a colonial nesting species not at the nesting colony.


Species (male or female) observed in suitable nesting habitat during its breeding season.

X—Singing male present in suitable nesting habitat during its breeding season.


PPair observed in suitable habitat during its breeding season.

S—Permanent territory presumed through song at same location on at least two occasions 7 days or more apart.

T—Permanent territory presumed through defense of territory (chasing individuals of the same species).

CCourtship behavior or copulation.

N—Visiting probable nest-site.

AAgitated behavior or anxiety calls from adult.

B—Nest building by wrens or excavation of holes by woodpeckers.


CNCarrying nesting material, such as sticks or other material. Please submit full details including location within the block of the observation.

NBNest building at the actual nest-site.

PEPhysiological evidence of breeding (e.g. highly vascularized, edematous incubation [brood] patch or egg in oviduct based on bird in hand. To be used by experienced bird banders on local birds during the nesting season).

DDDistraction display or injury feigning.

UNUsed nests or eggshells found. Caution: these must be carefully identified, if they are to be accepted.

PYPrecocial young. Flightless young of precocial species restricted to the natal area by dependence on adults or limited ability.

FL—Recently fledged young (either precocial or altricial) incapable of sustained flight, restricted to natal area by dependence on adults or limited mobility.

ONOccupied nest: adults entering or leaving a nest site in circumstances indicating occupied nest. To be used for nests which are too high (e.g. the tops of trees) or enclosed (e.g. chimneys) for the contents to be seen.

CFCarrying food: adult carrying food for the young.

FY—Adult feeding recently fledged young.

FS—Adult carrying fecal sac.

NENest with egg(s).2

NYNest with young seen or heard.2

1. The letter code is entered by the field workers in the appropriate space on the field report form. Possible and Probable categories are represented by single letters or a symbol; Confirmed by double letters. Letters have been selected as a mnemonic aid, keyed to bolded words in criteria definitions.
2. Presence of cowbird eggs or young is confirmation of both cowbird and host species.

General Recommendations

1. The date the code was observed should be recorded on the recording sheet as on the New Hampshire recording sheets. This is valuable data for timing of breeding activities.

2. A data information booklet on what codes are appropriate for what species within what dates should be prepared by each state and province, as "safe dates" for breeding vary with geographic location. The Maritimes have set an excellent standard for this. Computer proofing programs should be set up to check the entered date against these criteria.

3. Good training of field workers in the use of the codes cannot be over-emphasized.

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