Why Move?

dark-eyed-junco_blkforest-co_lah_4174The seasons have changed. The grosbeaks, hummingbirds, and other birds of summer have left for more tropical climates, but they’ve been replaced. Ducks, loons and grebes that spent the summer in the far north are showing up on local ponds. Rough-legged Hawks sit where Swainson’s hung out a month or two ago. Sandhill Cranes are headed for their winter feeding grounds in New Mexico. Instead of Chipping Sparrows at my millet feeders, I have flocks of Juncos.

Since the actual number of birds doesn’t really seem to change that much, I often wonder why birds bother to migrate at all. If Juncos can survive the winter here, why not Chipping Sparrows?

Clearly some places become pretty inhospitable in the winter. Leaving the dark and frozen tundra seems very sensible. Likewise, species that migrate vertically, such as juncos, are avoiding the severe winters found at high elevations. Food is hidden under many feet of snow. It takes a lot of energy to melt snow and ice for drinking water. Some species have found it more profitable to move rather than try to survive under such harsh conditions.

Other birds migrate south because their diet consists mostly of insects and other small invertebrates. During our winters, that food source is largely nonexistent. So, the birds fly south to a buggier place. This accounts for the long-distance flights of neotropical migrants such as warblers.

Similarly, birds that feed on nectar and other sources of sugar need to go where the flowers are still in bloom and fruit ripens year round. Hummingbirds and orioles are two examples (although they also eat insects and other sources of protein).

house-finch-blackforestco-2008oct08-lah-002rBut I still haven’t answered my original question. Why do some seed-eating birds migrate while others stay put? Chipping and other summer sparrows go south, but American Tree Sparrows and Dark-eyed Juncos come for the winter. I have Pine Siskins, House Finches, and Spotted Towhees in my yard all year long, but not Evening and Black-headed Grosbeaks, or American and Lesser Goldfinches.

Well, after extensive research, it turns out that… we don’t know! (Anyone need a thesis idea?)

It seems to be a balance between the perils of winter—snow, cold, ice, lack of cover from deciduous plants, etc.—and the perils of migration. While we can hop a plane or take the interstate to a tropical vacation, birds have to fly there the hard way. That is a huge investment of time and energy. And it isn’t very safe either. In addition to the traditional hazards such as hawks and other predators, storms, and just plain exhaustion, there is a whole slew of man-made dangers—plate glass windows, wind turbines, outdoor cats, electric wires…. Even the lights of a city can confuse night-flying birds and throw them off course.

It seems that some bird species are willing to risk all for a more comfortable winter, while others prefer to stay put and take their chances here. I’m glad they do. It would be a long, dull winter if there weren’t any birds at my feeders!

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