07 December 2009

Why do Birds Migrate?

SURVIVAL WILL BE ENHANCED. A seasonal shortage of food is the main factor. Weather, per se, is probably not so important. Partially migrating species can be held for the winter in the North. Small birds can withstand great drops in temperature, like up to 140 to 60 degrees F.

The distance migrated varies even within species, as I have said, some birds are partially migratory. In these cases, the young of migrants tend to be migrants also (so if something happens to the nonmigrants, the population can be replaced. Birds probably migrate just far enough to assure their survival. Shorter distances or longer distances must be disadvantagous. Likewise, migrating at all must be a tradeoff to the dangers of trying to survive the winter.

The dangers of migration include getting lost, storms, energy loss (exhaustion). Blackpolls migrate from Canada to Ecuador. They gain half their body weight in fat.

In partially migratory species, its the females and young that migrate: dominant, territory-holding, males tend to stay put--the cost of migration must be less for migrants than if they did stick it out.
Eruptive species: food supply varies markedly from year to year. What happens with many winter finches is that they feed on pine seeds. The trees go through roughly a four year cycle in seed production. When you have lots of seeds, you have lots of finches. The next year, with few seeds, you have a lot of hungry finches pouring into the United States from Canada. Examples of winter finches include Red Crossbills and Purple Finches.

Migrants usually build up fat reserves--young Manx Shearwaters leave British nesting grounds 50% heavier than adult normal weight and go to Brazil and Argentine waters. One banded bird made it in 14 days (570 km/day). Sanderlings build up enough fat reserves for a 2000 km trip!
You might try Googling bird migration and see what you come up with.

Pine Siskin

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