Saturday, October 17, 2015

Where do falcons and eagles go in winter?

Lately I've been getting asked where our falcons go to winter. The answer is complex. Birds do not fly south simply to escape the cold. Snow and ice seal the food supply and/or habitat of many birds away until spring: hummingbirds don't have flowers, waterfowl don't have water, and most insectivores don't have insects. However, peregrine falcons and bald eagles are able to weather winter since their food supply remains locally abundant, or at least present, year-round. Both species are partial migrators: that is, some birds migrate and others don't.

Bald eagles eat a wide variety of food, including other birds, mammals, reptiles, and invertebrates. They take their food live, freshly killed, frozen and left in the nest, or as carrion, and they can gorge food quickly and fast for days if need be. Peregrine falcons eat almost solely other birds they catch in flight, but many birds winter in Minnesota and Iowa, especially in cities and along open stretches of water. While there has historically been some natural open water along the Mississippi river during the winter (Eagle Valley, for example), the number of open water spots has increased with industrialization. Power plants discharge warm water that keeps stretches of rivers and lakes ice-free, providing spots for birds like swans and geese to winter (especially when people feed them, as they do here). Cities and industrial sites provide food and shelter for non-migratory birds like rock doves, which builds populations and makes wintering in place much easier. Even in the harshest winter, falcons and eagles have ample access to food in places like Minneapolis, St. Paul, Bayport, Decorah, Red Wing, and even northerly Duluth. 

Even with ample access to food, our winter weather is pretty gnarly. Birds have wings, so why don't they leave? We don't know for sure, but we do know that migration is costly and risky, and a good territory is hard to find. Falcons and eagles that stay on site have an advantage against territorial interlopers and are less likely to find themselves replaced by another mate come springtime. Years of occupancy on site yield a deep map of the the territory - places to find food, niches to shelter, and a knowledge of the local fauna - that helps animals survive even under adverse conditions (see this blog for more information about how eagles handle winter). 

So I've talked about peregrines and eagles that stay, but what about peregrines and eagles that leave? Transmitter studies indicate that mates don't migrate or spend time together off their breeding grounds. Among falcons and eagles, young don't migrate with parents, and we don't think they spend much time together after they disperse. 

Bald eagles tolerate - maybe even like? - cold. Many eagles that migrate don't fly into latitudes free of ice and snow. Eagles in central Canada might shift south as far as the Mississippi river in Minnesota, Iowa, and Wisconsin. Alaskan eagles go west to the sea. Eagles in central north America might fly to an open body of water not too distant from their breeding grounds, joining winter eagle congregations in places like Eagle Valley, Wisconsin; Clinton, Iowa (Eagle Point Park); Bayport, Minnesota (south of Xcel Energy's Allen S. King plant); and many other places.

Peregrine Falcon Migration Map
Peregrine falcons have been documented traveling a bit further south. We've had band returns from south Florida, an oil rig in the Gulf of Mexico, and Costa Rica (click these links to read about Island Girl and Immaculata). Thousands of Tundra peregrines have been trapped and banded at the Padre Islands in south Texas as they fly from an area up above the arctic circle to Central and South America - a distance of up to 15,500 miles! Florida reported a world record number of migrating peregrines just last week - an amazing 1506 in a single day count! 

In short, migratory eagles from Minnesota, Iowa, and Wisconsin might easily be wintering fairly close to home, spending time on open stretches of water along large rivers with eagles from central Canada who flew several hundred or a few thousand miles to get there. D1, D14, and Four all wintered in NE Iowa and (in D1's case) SE Minnesota (explore the cluster map for details). However, peregrine falcons from the same area might fly thousands of miles south to warm their feathers in Central and South America before returning to our area in late February and early March. 

The whole question of photoperiod length becomes even more interesting when migration is factored in. In late February, our latitude gets around 11 hours of sunlight daily. We peak on June 21, at 15 hours, 36 minutes. Non-migratory birds and Canadian migrants to our area also experience the shortest day, at 8 hours and 46 minutes on December 21. Falcons that migrate to Costa Rica experience a different photo-year than their non-migratory counterparts, however. A falcon that left on October 1 has a shortest day-length of around 11 hours. Its days will get slightly longer as it wings south. The shortest day in October in Costa Rica is 11 hours and 46 minutes long. The shortest day of the year is 11 hours and 32 minutes long. These birds have more average daylight over the course of the year, and a much flatter photo period map during the time they spend near the equator. The difference is even greater for birds like the Tundrius falcon, which summers at and above the arctic circle.

Some things that helped me write this post: