In my last blog, I discussed the various categories of migratory birds that can be seen at Lake Arrowhead, here in Southern California’s San Bernardino Mountains. While I jokingly offered the explanation “Because it’s too far to walk” when I posed the question of why birds fly south for the winter, allow me to delve into the topic more seriously.
Seasonal migration typically occurs when birds are seeking resources, usually nesting sites or food. For example, in the first installment of this blog I mentioned that red-necked phalaropes winter on the open Pacific Ocean off the coast of South America. Obviously, there is no way to build a nest on the open ocean. They fly thousands of miles north to the Canadian or Alaskan tundra where there is abundant food and places for nesting.
Why not just fly a few hundred miles east to the mainland on South America? There are two good reasons: the northern tundra offers more food and more daylight hours to hunt for food. In the arctic, aquatic insects that phalaropes eat breed like crazy during the summer, meaning there is ample food for their brood of three to six chicks. The longer number of daylight hours also means there is more time to forage for food. This may be important because the chicks are walking, swimming, and feeding on their own within a day of hatching. The females often fly south a few days after the last egg hatches and provide no care for the young. Males may hang around for a couple of weeks, but the young are pretty much on their own to find food.
The extended hours of daylight and more abundant seasonal food is also a driving force in most migratory songbirds. While rain forest habitat can be quite hospitable for birds, the conditions for raising a family are better in higher latitudes during the summer when food is more abundant. Additionally, song birds feed most actively at dawn and dusk. If you’ve ever spend time in the tropics near the equator, dawn and dusk are relatively brief as the sun breaks over the horizon and rises straight up. But if you’ve spent any time in northern states or in Canada during the summer, the sun begins to illuminate the sky long before it rises at a low angle and slowly makes its way over the horizon to create a leisurely, drawn out dawn. This gives parents more time for catching insects to stuff into the hungry, gaping mouths that await them at the nest.
Depending upon the species, southward migration is generally thought to be triggered primarily by changing day length, especially for those birds that undertake long journeys. If they wait for food to become scarce, they may not be able to consume enough calories to sustain them for an arduous voyage. Research suggests that northward migration from the tropics for long distance migrants is fostered by some type of biological “hardwired” clock that compels them to leave the comfort of their southern haunts.
It is worth mentioning that we also see “altitudinal” migration here in the mountains. These are much shorter journeys where certain species move to lower elevations when conditions on the mountain become inhospitable. We have a few types of warblers and hummingbirds that do this.
Now that the fall migration is in full swing, get out there and witness the spectacle for yourself. With hundreds of thousands of birds passing through the area each night, you never know what species you might see!
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