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The first two paragraphs below borrow freely from Mattocks (1988).

What is a Breeding Bird Atlas (BBA)?

The basic objectives of the breeding bird atlas are to develop a map of the nesting occurrence of each species known to breed in the four-county area, and to make these maps available in a socially useful and scientifically valid manner. BBA is a presence/absence study, not a quantitative study. It seeks to determine whether or not a species is present in a given area (atlas block) in the breeding season and to weigh the evidence for its actually nesting there. But it is not designed to measure population density.

Breeding Evidence

The BBA is based on the fieldwork of volunteer participants who record evidence of breeding for each species occurring in an atlas block, following a standardized protocol employed by atlas projects in dozens of other counties, states, and countries. Each observation during the nesting season can be classified in one of four categories: OBserved, POssible breeder, PRobable breeder, or COnfirmed breeder. The specific type of breeding evidence is recorded on field cards under one or another of these categories, as appropriate, using standard codes (see Table 1). On our maps, each species is depicted at the highest evidence level at which it was recorded by BBA observers on each individual atlas block. Only POssible, PRobable, and COnfirmed records are reflected on the maps; the OBserved category is not used.

O – Species observed, but no evidence of breeding. (In practice, used in cases where the observer believes that the individual observed is a migrant, vagrant, or post-breeding dispersant; or that no suitable nesting habitat is present on the atlas block.)
√ – Species in suitable habitat during nesting season
X – Singing male present in suitable habitat
M – Multiple singing males (7) found during one visit
P – Pair observed in suitable habitat
T – Territory established; also, singing male present at same location on two dates a week or more apart
C – Courtship behavior, copulation, or enlarged cloacal protuberance
V – Visiting probable nest site
A – Agitated behavior from adults
N – Nest-building or excavation of nest cavity
PE – Physiological evidence: brood patch or egg in oviduct
DD – Distraction display
UN – Used nest or eggshell (of positive identity)
FL – Recently fledged young incapable of sustained flight
ON – Occupied nest: adults entering, leaving, or incubating, but nest contents unseen
FN – Adult bringing food to nest
FS – Adult removing fecal sac from nest
NE – Nest with eggs
NY – Nest with young seen or heard

Table 1 — Breeding Evidence Definitions and Codes

Data from Washington Department of Fish and Wildlife (WDFW)

Through the generosity of WDFW, breeding records from their Heritage and Spotted Owl databases have been included in the BBA. Those for the years 1987–1994 are reflected on the maps in Smith et al. (1997). To these we have added the records from 1995 to the end of the four-county atlas period (2002 for Island County, 2000 for the other three counties). These databases contain detailed information about breeding sites of many sensitive species. By agreement with WDFW, our maps and data tables for Greater Sage-Grouse, Northern Goshawk, Ferruginous Hawk, Golden Eagle, Peregrine Falcon, Prairie Falcon, and Spotted Owl have been generalized to full township/ranges, to help safeguard these species from disturbance. For information concerning records from these databases, please contact:

Washington Department of Fish and Wildlife
600 Capital Way N
Olympia, WA 98501-1091

Integrity of data

In 1998, a block-by-block comparison was made of the hardcopy field cards for King and Kittitas Counties with the final database of BBA records used for the maps of the bird volume of the Washington Gap report (Smith et al. 1997). This exercise revealed an unacceptable number of discrepancies. Many of these can be attributed to lost or otherwise absent field cards, while others appear to result from data-entry errors ranging from random keystroke mistakes to entire field cards having been entered under the wrong atlas-block coordinates. As a result of this comparison it was decided to create a fresh database for the four-county atlas, starting with the reentry of all of the data from the original field cards of the first (statewide) atlas phase. It was also decided that no data from new fieldwork would be entered except on the basis of a hardcopy record in standard field-card format, thus permitting the later verification of any record.

After cessation of fieldwork the completed database was cross-checked against the field cards for a random sample of 60 blocks, disclosing three errors in 2,113 datapoints. This error rate of <0.15 percent was considered acceptable.

Anomalous and outlier records were reviewed upon data entry and again by the authors of the species accounts. Almost without exception such records were at the POssible level, reflecting isolated observations of single birds in atypical locations. In many instances reviewers decided to downgrade these to OBserved, reasoning that they represented migrants or birds otherwise not breeding on a particular block. The smaller number of cases where such records have been allowed to remain on the maps are individually discussed in the species accounts. It should be noted, too, that reviewers have upgraded many records from OBserved to POssible in cases where this action was clearly justified by date of observation and by habitat. In addition, observers were occasionally asked to confirm unusual sightings to be certain they did not result from recording errors in the field.

Availability of Data

Maintained in the form of Excel spreadsheets, the four-county database of bird records was limited to the fields necessary to generate the maps—atlas block, species name, and highest breeding-evidence level (PO, PR, CO)—and to track observer hours and survey years. These data can be viewed and downloaded from the present website. The Seattle Audubon Society maintains the archive of field cards for the entire BBA (1987ff.) and will make these cards available to qualified researchers who are seeking more detailed information about individual records. Correspondence should be sent to:

Chair, Science Committee
Seattle Audubon Society
8050 35th Avenue NE
Seattle, WA 98115