Friday, September 30, 2016

Why do birds return north after winter—why don’t they just stay where it’s warm?

Mark Pokras, V84, associate professor of environmental and population health at the Cummings School of Veterinary Medicine, fills us in

It’s pretty easy to explain why a lot of birds (and some other animals like certain bat species and monarch butterflies) go south for the winter. In our northern climes, winter is a tough season. It takes a lot of extra energy just to keep warm, and food can be very hard to come by, especially if fruits, insects, worms or other invertebrates are an important part of your diet.

But once arriving in those warmer southern climates, why should animals come back north in the summer? It takes a great deal of energy to migrate back and forth, and there are many risks along the way. A lot of animals don’t make it.

Unfortunately, despite what the Jimmy Buffett song indicates, life in the tropics is not as ideal as it might seem. For one thing, the tropics are not chock-full of unused food resources. The migrants from the north have to compete with a huge variety of tropical species that live there year-round. A more subtle issue is that warmer climates also tend to be home to a great many more infectious diseases and parasites.

It also turns out that there are some real advantages to making the trip north. Spring migrants time their return to coincide with a virtual explosion of food resources. As New England emerges from the grip of winter, virtually every local plant and animal begins to reproduce, and it’s not long before there is a huge abundance of seeds, fruits and invertebrates. Migrant species take advantage of these resources to have their own young.

What’s more, day length is more favorable during northern summers. In the tropics, there is little seasonal variation in the number of hours of daylight. As you travel farther north, summer days get longer and longer—in fact, above the Arctic Circle, there are weeks when the sun never sets. These longer summer days mean that there are more hours of daylight in which migrant birds can gather food and feed the hungry mouths of their rapidly growing young.

So is migration worth it? It’s really a question of how a species’ ecological necessities balance out. Where and when is food most available? How much competition is there with other species? How many predators and diseases are there? It’s a delicate calculus that varies tremendously over time.

This balance can shift relatively frequently. There is some evidence that climate change is already affecting the migration of some species. Robins are arriving several weeks earlier than they did 20 years ago. Similarly, some wrens that used to migrate to Latin America now go no farther south than the Gulf Coast.

As climates change and species evolve, who knows what alternations we’ll see in migration behavior over the next 100 or 1,000 years? As François de la Rochefoucauld said, “The only thing constant in life is change.”