Each block consists of 1/6 of a USGS 7.5 minute topographic quad. 538 blocks were further divided into quarter-blocks. These are the north-west (NW) block of each quad, most blocks in Baltimore Co., Garrett Co., Howard Co., Montgomery Co., Prince George Co., Summerset Co., and the District of Columbia, and a few blocks in Carroll Co. and Charles Co.
(see handbook for full description & use suggestions)
OB Category - Observed: not breeding evidence
Species (male or female) observed within safe dates but not in breeding circumstances. This code is primarily for birds that are not believed to breed in the block.
PO Category - possible breeding evidence
Species (male or female) observed in suitable nesting habitat during its safe dates.
PR Category - probable breeding evidence
Agitated behavior or anxiety calls from adult.
Pair observed in suitable nesting habitat within safe dates.
Territorial behavior or singing male present at same location on at least 2 different days, 5 days or more apart. Territoriality can be presumed from defensive encounters between individuals of the same species, or by observing a male singing from a variety of perches within a small area.
Courtship behavior or copulation.
Visiting probable nest site.
Nest building by wrens or excavation of holes by woodpeckers.
CO Category - confirmed evidence of breeding
Nest building by any species except wrens and woodpeckers or adults carrying nesting material.
Distraction display or injury feigning.
Used nest found. Must be carefully identified if it is to be accepted.
Recently fledged young or downy young. This includes dependent young only.
Adult carrying fecal sac.
Adult carrying food for young.
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.
Physiological evidence of breeding, e.g. highly vascularized, edematous brood patch or egg in oviduct based on bird in hand. (For banders.)