Saturday, April 12, 2014

Eagles on Ice

A young Bald Eagle (right) harasses an adult that was trying hard to eat a fish.
The surface ice that's covered our lake all winter finally blew off this week, here at the end of the road. High winds and rising temperatures were more than the rotting ice could withstand, and it all broke apart over two afternoons (time-lapse video below).

One one side of the lake, our white-headed neighbors have been refurbishing their nest for a couple of cold and snowy weeks now. When the first cracks appeared in the ice, one the eagles flew out, plucked a fish from the narrow lead, and landed on the lake ice nearby. No doubt hungry, he had just set about tearing the fish apart when a young, sub-adult eagle landed a few meters away.

Now we had a stand-off.

Whenever the adult turned his attention to lunch, the younger eagle would feign a mild bluff charge. Sometimes this just meant jumping a few steps towards the adult. Other times the youngster would fly at and over the adult. After 15-20 minutes of intermingled stare downs, feeding bouts and food mantling (covering the fish with spread wings), the adult gave up and flew off to his nest. The youngster quickly hopped over to eat what was left of the fish.

What intrigued me about the young bird's behavior was how it resembled the Bald Eagles' duck hunting technique in spring. When there are large rafts of tightly-grouped ducks - especially Coots - the Bald Eagles make frequent flights over the nervous ducks. (I know, Coots are rails not ducks, but stick with me here.) Eagles fly towards these rafts at a moderate speed and altitude, just low enough to make the ducks skitter in a semi-mild panic.

The eagles are essentially shuffling the deck, looking for the odd duck that does not move like the rest. Most of the time, the ducks just skitter off a ways and the eagle returns to a tree branch, still hungry. The eagle spent very little energy in the effort. But every now and again the shuffle allows an eagle to spot a weak or injured duck, and the chase begins for real. Now the eagle focuses intense efforts on the odd duck, and he stooping in fast and low. If he misses the first time, he'll keep circling and stooping until succeeding or giving up.

The eagle's high-energy efforts were saved for the duck that stood a better chance of becoming lunch. Until then, he just made low-energy bluff charges. Efficient behaviors are well-honed in wild animals, and inefficient animals are soon removed from the gene pool - guess it's a good thing that I'm somewhat domesticated instead.