Why do Florida Eagles migrate the "wrong way" after breeding?

[NewMexiKen]

Those of us who have been watching our local pair of Bald Eagles and their progeny for the past three breeding seasons are well aware that our Florida eagles are "contrarians" when it comes to migration. While nearly all other migratory birds head south after nesting and rearing their young, our local eagles do just the opposite-- they turn north. Well, this is not exactly the case, as Florida's adult Bald Eagles, especially those from the southernmost end of the peninsula, don't exhibit much wanderlust. Generally, the adults tend to wander locally, or at most, regionally in the lower third of the Sunshine State, while younger (especially first-year) birds often become long-distance travelers.

Thanks to satellite radio tracking, we know that younger birds counter the prevailing trend by turning north in late June and July. Those that follow the Atlantic coast commonly end up at or near Chesapeake Bay, joined later by others of their species from more northern climes. Some young eagles may fly as far north as New England and Canada.

Mary Lou and I, as "migrants" between Florida and Illinois, assumed we understood the rationale for this deviant behavior. Our Pembroke Pines eaglets hatched out in late December and January, just about the nicest time for them (and us) to enjoy life in South Florida. By then, hurricanes are but a memory, rainstorms are few, and bugs are less troublesome. Fish and long-legged wading birds, both important prey species for eagles, are concentrated in smaller pools of receding water. Parent eagles have no trouble finding food for their hungry and fast-growing broods. Who can blame the youngsters for getting out of the heat and incessant blackbird and mockingbird attacks, and for staying away until October or November, when the nights turn cool and the storms abate?

As it turns out, our understanding of eagle behavior was rather simplistic. Like so many biologic phenomena, the explanation for the young eagles' "reverse migration" is no simple matter. First, why do the adults stick around while the youngsters take off for distant parts? Can it be that there are other hidden "migrations" that contribute to this deviant behavior?

A more insightful rationale has been offered by Hawk Mountain scientists Keith Bildstein and Sarkis Acopian in  "Wrong" Way Migrants, an article in this spring's Hawk Mountain News (cited in Illinois Audubon magazine, Summer 2010):

"...Bald Eagles that breed at the southeastern terminus of their range in peninsular Florida travel north during the non-breeding season rather than south, and their southbound journeys in autumn are flights that take them back to their breeding grounds, not away from them. Farther west, eagles that breed in southern Arizona and southern California do the same thing."

The authors suggest that the seasonal vertical migration of fish is a very important causal factor in the migration of eagles. Up north, the fish retreat to deeper and warmer water as winter progresses. Northern eagles must move southward as fish become harder to find, and as lakes freeze up. Quite the opposite occurs in subtropical climes, as fish move down into deeper and cooler water in the heat of summer. Young eagles, less experienced hunters, are forced to seek colder bodies of water with more accessible fish. The eagles return south in autumn, when fish once again move up as the surface water cools:

"In Florida, vertical fish migrations create a situation in which prey are decidedly more available to surface-feeding eagles in winter-- and in comparison decidedly less so in summer-- exactly the opposite of what occurs farther north on the continent. Not surprisingly, in response to this phenomenon Florida eagles shift the timing of their nesting when surface fishing is easier..."

Adult eagles appear to be motivated to remain near their nesting and feeding territories all year long, to defend them against rivals. Interestingly, young eagles follow two general routes northward, either the Atlantic coastal plain, or the Appalachian mountain chain. Those who choose the latter tend to travel farther north, probably because they benefit from mountain-associated updrafts.