Raptor Migration 2015

Author: 
Robert Walton

The swifts, swallows, warblers and tanagers have completed their marathon migration to warmer climes and are now settled for the winter with strange exotic tropical species. The sparrows and towhees have traveled to the southern states. The second week of October saw thousands of Broad-winged Hawks following the Maumee down across Ft Wayne on their way to Mexico and Central America. Sharp-shinned Hawks made a brief visit just after Halloween on their way south. In mid-November a few Golden Eagles passed by. They were hunting ducks and other small game on their passage to the southern Appalachians. However, our hawks and owls seem to be staying put and our wintering raptors have not appeared. 

Migration for raptors is complex. Migration or movement of animals (including humans) is almost always related to the search for food and protection from the weather. The trigger for raptor migration is almost always food related. If there is a good, readily accessible source of food, the birds tend to stay in place. As food grows scarce and is less available, the birds will move on.

In early fall, birds that depend on fish like Osprey and Bald Eagles disperse (post breeding dispersal) from the more northern roosts and travel towards large open bodies of water such as rivers and the ocean. They feed at ponds and lakes along the way until the fishing turns bad or the water freezes. The Osprey will continue to the ocean while the Bald Eagles may gather at the river dams where dead fish and carrion may collect at the spillways.

But still there is no sign of our regular winter visitors this year. Why? First, the weather has been mild with no significant snowfalls to hide prey from view.  So hunting is still good. Second, there may be a bumper crop of prey available and the birds are gorging themselves. Third, there may have been a major breeding failure this year due to a lack of prey, destruction of habitat or widespread disease such as avian flu or West Nile Virus.

Raptors have very little control over food resources. There are boom and bust years of prey availability with some cyclical regularity (usually a 3 or 4 year cycle). Rodent prey numbers may increase exponentially if their food is readily available. However, the prey may outstrip the food source and their numbers crash. Meanwhile, predator numbers increase with the increase in prey and then must deal with any collapse of the prey base.  So the raptors move on to an area of good hunting.

Eventually winter comes and depending on the severity and the raptor population, the birds move south. With the winter snowfall, prey usually borrows out of site under the snow, logs, etc. Hawks which rely on keen eyesight cannot find their food and move on to better hunting areas. Owls with their remarkable hearing can still find rodents moving under the snow.

In our area, for example, lakes and rivers to the north are now freezing as evidenced by over 1000 Sandhill Cranes heading south over us this past Saturday.  The birds of prey will not be far behind.   Already we have had reports of Snowy Owls in Allen County. So look out for Short-eared Owls, Harriers and Rough-legs over the fields and Long-eared owls in copses of conifers. Oh, and don’t forget the cute little Saw-whet Owls snuggled up in vine covered trees!

This Saw-whet spent last winter at the Tiger exhibit in the Ft Wayne Zoo.