Bird Wise: The natural perils of bird migration
This is the time of year to expect to see throngs of migratory birds returning from the south, as they have for millennia, for another attempt at nesting and producing young.
The earliest to arrive are typically those species that spend the winter mainly south of Minnesota but within the U.S. Common examples are American robins, eastern phoebes, dark-eyed juncos and sandhill cranes.
During May, the neotropical migrants — those species that winter south of the U.S. through Central and South America — arrive en masse. These include some of "our" most colorful species like warblers, tanagers and grosbeaks, but also more subtly plumaged species like flycatchers and vireos. Many have Minnesota as their destination, while others continue much farther north into Canada.
What triggers this annual phenomenon? For the majority of species, we know it is the gradually increasing period of daylight, or photoperiod, which stimulates the pituitary gland to secrete hormones that cause birds to feed more, accumulate fat, become hyperactive and eventually migrate. These behaviors are purely instinctive, not able to be controlled. In effect, the birds are little more than robots under the complete control of their unique genetic make-ups. The evolutionary process of natural selection ultimately determines how each species responds to environmental cues, such as increasing photoperiod. Local weather conditions, like storms or adverse winds, also play a role in fine-tuning the precise migration schedules of species each year.
Natural selection favors individuals that are able to produce the most surviving and ultimately reproducing young. Among the many factors that affect reproductive success is the timing of arrival at their nesting grounds. Individuals that arrive earliest can nest earlier. An early start means a greater chance of being able to raise two or more broods of young. When this happens, the genes that promote early migration are passed on to the young, which will then be prone to migrating early themselves the following year, and this trait will become more common in the population over time.
But those early arrivals may also face a greater chance than late arrivals of encountering severe weather and food shortage. We see this often: Early swallows or bluebirds dying in their nest boxes from cold and lack of food. Or, as I saw in late April last year, thousands of hermit thrushes trying to eke out a living along roadsides because their normal feeding areas had been coated in ice. I suspect many of these birds perished. So the drive to arrive early and produce more young is sometimes offset by the risks of arriving early and not surviving to reproduce.
How these forces play out over time determine whether early or late arrivers "win" in the long run. To be sure, migration is a perilous undertaking. But because it is an essential part of the life cycle of migratory birds, it will continue to happen year after year, much to our delight and fascination in being able to witness the spectacle.