Biology Comics

A cheerful Biology Comic?!


Dooryard Views

Watching from my dooryard I think the local raptor population is increasing. I see more and more small birdeating hawks: sharpshins (a small short winged forest hawk), cooper’s hawks (a larger edition of the same), merlins (a tiny forest falcon, the size of a robin).

Three or four autumns ago I watched an immature Cooper’s hawk perched in the top of a dead aspen in my meadow for an hour. If it hadn’t perched there so long (preening and ruffling its feathers) I wouldn’t have been able to identify it: separating sharpshins and Cooper’s hawks is difficult.

Several times a summer a sharpshin arcs over the yard. If one stands and looks up on a sunny September day with a north wind, one sees them soaring south, flicking their wings like butterflies, among the stately redtails.

The call of our common summer hawk, the broadwing, still drops from the sky on sunny days.

A friend found a Cooper’s hawk nest a few weeks ago in a red pine beside a busy mountain trail. Going to look for the nest, I happened upon the hawk perched in a dead elm next to an abandoned beaver pond. A bronzed grackle was perched in the same tree. Upright and gray, larger than the grackle, the hawk stared straight ahead. Neither bird was moving. Perhaps the hawk wasn’t hungry. The safest thing for the grackle (who glanced at the hawk from time to time) was not to fly.

And so what?

Well such hawks, like the fisheating American mergansers that have become common on the river, occupy the top of the food chain. They eat migratory birds and migrate themselves. Thus they concentrate pollutants (absorbed by the birds with their insect prey) from all over. That, plus shooting during migrations (when they are most vulnerable), is thought to be reason for their enormous decline in the middle part of the last century.

So their increase is a good thing: shooting has stopped; the chemicals that worst affected them are declining in the environment (though many bad boys, including mercury, are rising). Of course their prey species (songbirds) have also been declining.

A good thing in more than one way: raptors (day flying hawks) sculpt their surroundings. Cowbirds are a nest parasite from the prairies. Cowbirds spread east during the last century (clearing the eastern forests for farmland created an eastern prairie habitat for them) and increased in number tremendously. The female cowbird lays its egg in the nests of small songbirds (such as warblers and thrushes). The egg isn’t recognized as foreign by the parents. The young cowbird hatches out quickly, grows quickly and pushes the other hatchlings out of the nest. A warbler raises a cowbird chick several times its size.

A female cowbird will parasitize several nests – nice work if you can get it. So some warblers, wood thrushes and other neotropical migrants have been declining. (There are additional causes for this but abundant cowbirds are a factor.)

Since the 1970s, at least one male cowbird has perched in my apple tree gurgling its liquid notes all spring. Often two females accompany him. This year I saw one once, briefly. Their habit of sitting in a treetop displaying makes them vulnerable to our bright eyed hawks.

Perhaps as a result I have heard wood thrushes singing near the house the last two summers for the first time in 30 years. (But mercury accumulating in the insects of the forest floor also affects wood thrushes, as well as habitat change in the tropical forests where they winter.)

So fewer cowbirds (or scared cowbirds) are a good thing. I see also fewer blue jays. The beautiful blue jay (illuminations in blue and violet) is a nest predator. I once watched a blue jay go from nest to nest in the carved stonework above the doorway of a church, plucking out baby sparrows, one from each nest, while the parents chirped and squawked nearby. What could they do?

Sharpshins enjoy tasty blue jays, though the jays are slightly larger than they. One winter afternoon I watched a sharpie out my window eat the breast meat off a jay too large for it to carry away. Jays eat at feeders and the hawks (not stupid) cruise from feeder to feeder. As the hawk fed, the movements of the jay’s spread wings became feebler and feebler. Finally the little hawk flew off with the still remains through the woods.

So this is good. Though the merlins and the sharpshins will undoubtedly also strike at the indigo bunting that now sings in the dead aspen, the scarlet tanager singing in the pines, the crested flycatcher that whoops from the woods.

That’s how it is! In a better world than ours all these birds would be much more numerous, eat the insects that eat the trees and produce many many young.