Presented by Colorado Bird Atlas Partnership:

The Second Colorado Breeding Bird Atlas


Atlas Methods ~ Breeding Codes & Their Interpretation

Observers used specific criteria (Breeding Codes) to determine breeding status. These criteria categorized species as Confirmed, Probable, or Possible breeders, or Observed (non-breeders). They applied the following Atlas Breeding Codes to all species seen or heard during that species’ breeding season and in appropriate habitat.

Observed (Nonbreeders; not incorporated into distribution maps or block counts)

O – Migrants and nonbreeding species

Possible Breeder

# – Species seen or calls heard
X – Singing male present

Probable Breeder

M – Multiple males (≥7 singing males heard in one block on one day)
P – Male-female pair
T – Territorial behavior, including two or more males counter-singing or chasing, or presence of a singing male in the same location on occasions at least a week apart
C – Courtship behavior between a male and female, including display, food exchange, and copulation
V – Visiting probable nest site, but no further evidence obtained
A – Agitated behavior or anxiety call from an adult that indicate nearby nest site and/or young
N – Nest building by Bald Eagles, Curve-billed Thrashers, or Marsh Wrens; excavation of cavities by woodpeckers; or filling cavities with twigs, etc., by Bewick’s or House Wrens

Confirmed Breeder

NB – Nest building or adult carrying nest material
PE – Physiological evidence; used only by banders or those handling birds and indicates brood patches and cloacal protuberances
DD – Distraction display or injury feigning by adult
UN – Used nest or egg shell found; only for unique and unmistakable shells or nests
FL – Recently fledged young (or downy young of precocial species such as geese, ducks, grouse, and sandpipers) with limited mobility
ON – Occupied nest (contents undetermined)
CF – Adult carrying food for nestling or fledgling
FF – Adult feeding fledgling
FS – Adult carrying fecal sac
NE – Nest with eggs
NY – Nest with young

singing male
Photo by Charles Hundertmark

fledgling birds
Photo by Marilyn Rhodes