> Does anyone know what factors determine the
> distribution of chickadees, robins, catbirds,
> mockingbirds, and cardinals? I lived near Washington
> DC and now live on the Oregon Coast near Nehalem, OR.
>
> I wondered if type of availablility of particular
> foods, holly berries, particular types of shrubs etc
> made much difference.
>
> Hope Stanton
> Nehalem OR
>

Geographic isolation is always a factor in species divergence. In North
America, the Rocky Mountains and dry interior help to isolate eastern forest
birds from western. Simple genetic drift causes separated species to
gradually differentiate, even if they live in similar biomes.

More important than geographic isolation between the Pacific Northwest and
the eastern US is the difference in climate patterns. In the eastern US,
rainfall is fairly evenly distributed through the year (compared to the
Northwest) with peaks in spring and fall.

In the Northwest, there are distinct wet and dry seasons. From about
mid-July to mid or late September, rain is scarce. Rain increases through
the fall and peaks in November, December, and January.

Birds are probably less influenced by particular plant species than the
plant species phenology (the timing of growth, fruiting, etc.). Here, many
plants become senescent by July, which means many insects that live on the
plants also disappear (dormant or dead). The lack of water also means that
less decay of fallen leaves occurs during the dry season. Many birds depend
on invertebrates that live in leaf litter.

Birds and other animals in the Northwest must be able to get through the
dryness of late summer and early fall. The dry conditions are ameliorated
somewhat in cities, where people provide food for some types of birds
(seed-eaters, mostly) and water lawns. On the other hand, people have
drained marshes, diverted streams through concrete ditches, and built along
waterways, making it more difficult for some species to cluster around water
in the dry season.

Kelly Cassidy
Pullman, WA
mailto:lostriver at completebbs.com