This timeline illustrates the migration/dispersal period and the breeding period for each species. Use it to learn when species are nesting in Maryland and DC, and when their migration or dispersal overlaps with their nesting. During their migration/dispersal period, weak codes should not be used (e.g., code H, code S, or code M), while other Probable codes should be used with acute caution and always supplemented with comments.
Each bird population has a general range of dates in which nesting occurs determined by their physiology (or how their body operates), food availability, and migration route. Many species both breed in and migrate through the same region. For example, Eastern Wood-Pewees tend to arrive on the mid-Atlantic coast in early May, but many continue moving further north. This means an atlaser who detects a pewee in May doesn’t know if the bird is a migrant or a breeder without further evidence. It is important the Atlas only receive records of breeding individuals, so potential migrants should be excluded. This means atlasers should be conservative when using breeding codes during a species’ migratory period. To help with this, we have provided safe dates that show, based on historical data, when a species is ‘safe’ to count as a potential breeder and not as a migrant.
Of course, birds do occasionally nest outside of safe dates. Outside of safe dates, do not use Possible codes (code H or S), and make extra effort to find stronger evidence. Record the evidence you observe with photos, recordings, or detailed field notes and submit the evidence with your observation.
Marcia Watson has put together the following graphical representation for each species:
Graphical representation for each species in Beginning Safe Date order:
Graphical representation for each species in End Safe Date order: