I live near a highway. All winter, I almost never went west on that road, only east, but a couple of times I did go west. And, in a tree off to the right in that direction, I noticed a mass of stuff that resembled a Bald Eagle’s nest as much as any mass of stuff ever could, but of course, it was not a bald eagle nest.

First, it was really close to the road, close enough that the birds in the nest, if it was a nest at all, would feel the wind from the larger, faster passing semis. Eagles like their nests a bit farther from roads than that. Then, the nest was too low. It was 2/3rds of the way up a tree, but it was a small tree. A kid could give another kid a boost and with very little climbing reach the nest, if it was a nest. Eagles don’t like their nests so low to the ground unless they are up on a cliff or something. This mass of stuff is even low for, say, a robin’s nest. The tree overlooks a large marsh, with a stream running through it, and that’s a good place for an eagle, but on the other side is a busy housing development. Eagles don’t like being near developed landscapes. Eagles will, of course nest in parks in cities along major rivers and stuff, but this nest is only a very short distance, as the eagle flies, from some very large parks with plenty of big trees and extensive bodies of water.

But despite all these things, the mass of stuff looked exactly like an eagle’s nest, even though it simply could not be. Funny, that.

Today, we drove west on the highway and there was the eagle sitting on the damn nest, sitting there being majestically improbable. Its feathers were practically fluttering from the wind of the passing semis, and the bird was clearly staring suspiciously at the nearby housing development were surely a band of 11 year old boys was plotting their climb into this tree.

And this got me thinking about nests, central places, and the distribution of super carnivores. Obviously.

Ask yourself this question: Why don’t the super carnivores in a given ecosystem just eat all the prey? By “super carnivore” I mean top-of-the-food-chain kinds of critters, like lions or wolf packs, etc., and in the world of birds, a pair of eagles qualifies. So, this pair (I assume there are two, though I only saw one today) of eagles has a territory including small dogs and stray children, baby coyotes, crows, but mainly squirrels and marmots right around its nest, and a major river backed up by a large dam about a mile or two away (the Mississippi) and several lakes. But, this pair of eagles will not consume enough of the available prey to wipe out any of these food sources (well, maybe the small dogs in the housing development, whose demise by the way will be blamed on the coyotes).

It is almost as though the eagles, and other territorial carnivores, have spaced themselves out on the landscape just right so that they don’t overuse their food resources, so that they, the eagles, can continue to do their thing without threat of eating up their own food supply. This make sense in a certain way; a species of carnivores that over-concentrated in space would go extinct. A little bit of prudential restraint is good for the species as a whole, and thus it evolved.

But of course, this can’t be true, for a simple reason: As majestic as the great eagles is standing there on the nest, the bird is more than capable of cheating. Any of them, really, not just the one I saw today. Consider this thought experiment. Imagine that there was a gene that caused the eagles to spread out from each other when they nest, so they would be far enough apart that in most years there would be enough food for a pair of eagles and one offspring per nest. Imagine this is happening every year over a long period of time, and everything is just fine, the spaced-out gene acting for the survival of the species.

Then, one day, a mutation arises in this gene and the bird that has that mutation builds a nest in closer to some other nests, but right on top of a really good food source. That particular bird then gets more food than it otherwise would if it mainly followed the “spacing out” rule rather than the “nest near the food” rule that the new mutation causes. And, that bird might well do better than it otherwise might. There is a certain chance that this bird’s offspring will have the same, novel gene, and that this bird will be able to produce more offspring, with its mate, than the other eagles. The cheating version of the gene might then spread in this population over a few decades, and pretty soon the “space out” rule among the eagles would be gone.

The same argument can be made regarding prudential restraint in the number of eggs and chicks a pair of birds produce and nurtures in a nest. In some colonial nesting birds it has been observed that the number of eggs per nest varies from year to year, seemingly in correspondence to variation in food supply from year to year. If this was true species-supportive prudential restraint, than a cheating version of the genetic code underlying this behavior would spread like wildfire once it emerged, and it probably would emerge eventually.

(This way of thinking about bird behavior oversimplifies the link between genes and behaviors, but it is still a useful way of thinking about it.)

So, there must be another reason that eagles nest far enough apart that they don’t eat all the prey. I suspect the answer is direct competition, and that this competition is only partly over food-rich territory and mainly over mates. Eagles do mate for life (serially) and they are usually very closely bonded. So, on one hand, “messing around” (called “cuckoldry” or “philandery” in birds) is unlikely to occur because of bonding, but on the other hand it is very very costly (genetically). It is in the interest of each member of the pair to have a good enough distance between all of the use areas that there is little overlap in space during the breeding season; thus the spread out nesting sites, which are more or less central to the territory. This rule of thumb probably applies to all non-colonial nesting birds, and to other carnivores as well. At the same time, maintaining a large enough territory for food related purposes not by agreeing to space out nesting sites (due to a “spaced out” gene) but rather due to getting your birdy ass kicked if you try to build a nest to close to an established pair’s nest, may be a factor. The fact that eagles often feed in close proximity to each other suggests, however, that breeding territoriality is the main reason for spaced-out nests.

Eagles are supposed to leave the wintering grounds, where lakes and rivers freeze over and there is no fishing and there are few other things to eat, migrating to larger water bodies or to the south. But of course, some eagles don’t do that in a given year, and here in central Minnesota, it seems that eagles have been wintering more and more commonly. Although this is not anything like an official count, I don’t see fewer eagles where I live, not far from the Mississippi, during the winter than the summer. I assume eagles from the hinterland move to near the river when that body of water remains open. And this winter, there were even small ponds near where I live that stayed unfrozen all year (other than a skim easily broken by the ducks and geese that never left as well). I imagine that our local eagles are doing very well these days. This pair of eagles seems to have build a nest in a suboptimal place because there were no other places to build a nest. In fact, there are thousands of suitable trees in the area, but many of them are near existing eagle nests in the aforementioned park and along the aforementioned river. In the case of these eagles, territoriality trumps nest quality.

This might not end well, especially if the kind of brush fire forms that occurred here a few years ago. That particular tree is in a stand that was inundated with smoke for hours, and that caught on fire, in a major marsh fire a few years ago, big enough for us to see from 60 miles away. The eggs or chicks would not survive. As it happens, in May of 2009, I got a shot of that fire while driving by, showing the tree the eagles have subsequently nested in:

Natural selection can be hard on a bird.

Written by Greg
Greg Laden has been watching birds since they were still dinosaurs, but has remained the consummate amateur. This is probably because he needs better binoculars. Based in the Twin Cities, Minnesota, Greg is a biological anthropologist and Africanist, who writes and teaches about Evolution, especially of humans. He also blogs at Scienceblogs.com. Greg's beat is Bird Evolutionary Biology. One could say that knowing the science of birds can make the birds more interesting. But really, knowing about the birds that go with the science is more likely to make the science more interesting. And thus, birding and Neo Darwinian Theory go hand in hand. Darwin was, after all, a pretty serious birder. Greg has seen a bird eat a monkey in the wild.