of Western North America
Birds in western North America tend to follow bird highways on
their autumnal migrations for winter food supplies.
Ornithologists have described three different migration patterns
which can be seen in the habits of birds which frequent North Cascades
1) Complete Migration
When an entire species such as the rufous
hummingbird migrates afar to wintering grounds each year they
are undertaking a complete migration. Most of North Cascades neotropical
migrants follow a complete migration when they head to the tropics
for the winter.
2) Partial Migration
Some species such as the bald
eagle may follow partial migrations, and not all eagles migrate.
There are bald eagles that live in the Skagit River drainage all
year long, while others migrate to Alaska when food is scarce.
3) Irruptive Migration
Some migration patterns are less predictable. Pine
siskins will flock to locations with the best cone crop. Irruptive
species will migrate where the food is best. In 1988, when the cone
crop was poor in wild areas, pine siskins where noted in backyard
feeders throughout the United States.
Scientists have defined other migration patters as well. Differential
migrations are when males and females migrate to different locations.
Some seabirds follow the ocean's winds and complete full loop
migrations around an entire ocean basin.