Oregon Breeding Bird Atlas
Methods

Evidence of Breeding
For each observation (visual or auditory) participants made, they applied one of the following 20 codes. Atlasers were instructed to use whichever code closest to the bottom of the list was applicable, because codes are listed in order of increasing certainty of breeding. These standardized codes are similar to those recommended by the North American Ornithological Atlas Committee (NORAC 1991) and used by atlas projects in other states.

Atlasers were not asked to report the calendar date of every observation because of the extreme amount of tedium that would have required. Instead, participants were asked to write in the calendar date when they (1) confirmed breeding of a species, (2) observed a species not "expected" to occur in a particular atlas unit, or (3) observed a species that is reputedly an especially late or early migrant or vagrant - such species were denoted on the field cards. For species observed but not confirmed on multiple dates in the same hexagon or square, participants were asked to report only the date closest to June 20. This approximates the middle of the breeding season for a majority of species in Oregon.

Two issues emerge when trying to judge breeding from the above codes.
First, the codes categorized as Possible or Probable should constitute evidence of breeding only if those behaviors are observed during the season during which a species normally nests. Otherwise, the observations might be only of late (or early) migrants, vagrants, or individuals that ultimately fail to breed. However, many volunteers are unfamiliar with when a particular species "should" be breeding, and in years with unusual weather, even the "normal" dates of breeding may shift. To help alleviate this interpretive situation and provide greater meaning to the Probable and Possible codes:

Nonetheless, it is important to recognize that some of the records still containing codes in the Possible or Probable categories may represent nonbreeding individuals.

A second confounding issue is that behaviors in the Possible and Probable categories sometimes are observed in habitat that reputedly is unsuitable for breeding. An extreme example is an American Bittern - normally a bird that nests in tall dense herbaceous vegetation - observed just once along the steep, barren, rocky shore of a wastewater pond. Such an observation surely represents an individual that was not nesting in the habitat where it was seen. However, many volunteers are unfamiliar with which habitats a species "should" nest in, and many species nest occasionally in habitats where reputedly they are not "supposed to" nest. Although it is possible from the species range maps to identify instances of species being reported from hexagons where habitat is allegedly unsuitable, there is no certain way to identify observations from supposedly anomalous habitats in this or any other database. Opinions differ greatly as to what constitutes minimally suitable habitat for most species, and the coarse scale at which vegetation is photointerpreted, classified, and mapped often aggravates the uncertainty of after-the-fact interpretations. Participants were not asked to classify and report the habitats associated with every observation because of the skills required and the extreme amount of tedium this would introduce.

We did not include a code for individuals observed in an atlas unit but not believed to be breeding there, due to inappropriate habitat, unexpected date of observation, or other evidence. Some atlas projects denote such records as "O" (Observed) or "X" (Casual or Vagrant) and generally do not map them.

Atlasers were also asked to recall and report, in a separate column on the field card, observations of species they made during the breeding season in atlas units prior to 1995 (the beginning of the atlas project) but later than 1980 (an arbitrary cutoff), provided the species had not already been found in that unit during the current effort. These are termed "historic" records. They were not mapped but are contained in the list of observations found under the Species and Hex sections and included in the database of all observations found on this CD (see Observations in the Data files in the Intro section). No effort was made to comprehensively extract historic information from Oregon Birds or other published literature or databases for use in this database.