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I have been trying for the past several weeks to get a good look at a yellow-billed cuckoo, so far without success. I have heard its call many times, but always from thickets so dense I couldn't locate the bird.

If I had had unlimited time to search I'm sure I could have spotted several, but I was engaged in a bird census that required me to cover a large area in a short time. So I couldn't just hang around somewhere and search for cuckoos.

The fact that they are hard to see is of course one of the reasons I wanted to see one, but it wasn't the only reason. They are very striking birds: long tailed and slender, with sturdy curving beaks and a posture that makes them look like they're hunching their shoulders.

They belong to a family filled with strange species. Roadrunners are cuckoos. There are species in the tropics that take the basic slender, long-tailed body plan and extend it out to nearly two feet. The rufous-vented ground cuckoo, a terrestrial bird like the roadrunner, has a stout curved beak, bronze wings, and a tail almost as long as its body.

As a group the cuckoos are reproductively innovative. The three species of the genus Crotophagus, commonly known as anis, are group nesters. They hang together in flocks of as many as 20 birds, and when the urge strikes they build a communal nest. All the females in the flock lay eggs in the nest, and everybody joins in to incubate the eggs and feed the young after they hatch.

Other cuckoos, including some of the tropical species, are brood parasites. The common cuckoo, a species whose range includes all of Europe, is a brood parasite whose habit of laying its eggs in other birds' nests is the source of the word "cuckold." It is also the source of the joke in the rhyme about flying over the cuckoo's nest. There ain't no such thing as a cuckoo's nest.

Common cuckoos have carried the finer points of parasitism further than any other parasitic species. Even their eggs vary to match their favored hosts'. Cuckoos in Finland lay blue eggs that match the eggs of redstarts and whinchats. Hungarian cuckoos of the same species lay green eggs blotched with brown and black, which match the eggs of the great reed warbler.

Cuckoo embryos develop very fast, so the blind, naked cuckoo usually hatches before its nest mates. Within ten hours of hatching the young cuckoo begins to clear the nest of any possible competitors for parental attention. The young bird develops a sensitive area on its back. If any solid object--an egg or another newly hatched bird--touches the area the cuckoo begins to push against it. And the cuckoo keeps pushing until the object is ejected from the nest. From then on the cuckoo gets all the food its foster parents bring.

There are lots of songs about cuckoos. Some are bawdy ditties that play off the bird's reproductive habits. Others are about the bird's two-note song. Cuc-koo gets monotonous after a while, but the birds begin to sing early in spring, and at that time their song is welcomed as a harbinger of the softer seasons.

Some of the English songs survive in the southern Appalachians, but the words have changed from "Oh, the cuckoo, she's a pretty bird / She sings as she flies" to "She warbles as she flies" and ultimately to "She wobbles as she flies / She don't never holler cuckoo 'til the fourth day of July." A person singing those words obviously knows nothing about common cuckoos. The two cuckoos of eastern North America, the yellow-billed and the more northerly black-billed, are late migrants, yet even they are likely to be singing by Memorial Day. Yet I've never heard anybody sing "She don't never holler cuckoo 'til the last weekend in May."

These changes in the song are part of what is called the "folk process," which a friend of mine once defined as "getting things wrong." It would be more accurate to say it is "getting things wrong creatively," since the new versions of songs and stories created by the folk process may be better than the older versions.

Getting things wrong creatively is a major source of innovation in both human affairs and biological evolution, though we humans have a strong tendency to try to stifle the process. I suspect that if humans had been put in charge of creating life on earth there would still be nothing around except bacteria and maybe a few blue-green algae. We would have done everything we possibly could to make DNA a 100-percent-accurate transmitter of genetic information. Unauthorized innovation would be strictly prohibited. No mutations unless the CEO personally approves each one for cost-effectiveness and appropriateness to our marketing strategy.

Both the yellow-billed and black-billed cuckoos of eastern North America build nests and incubate and feed their own young. However, there are instances reported in the literature of yellow-billed cuckoos laying eggs in other birds' nests. There are also reports of other birds laying their eggs in the nests of yellow-billed cuckoos.

In Life Histories of North American Birds, A.C. Bent quotes from a report published in 1887 by someone named J.L. Davison on a robin's nest that had been taken over by a yellow-billed cuckoo. The robin had laid one egg in the nest when the cuckoo moved in; the cuckoo laid two more and started incubating. Subsequently a mourning dove laid two eggs in the nest, and it too started incubating. When Davison found the nest the mourning dove and the yellow-billed cuckoo were sitting side by side on the same nest, jointly incubating the two dove's eggs, the two cuckoo's eggs, and the robin's egg.

This is more a case of dumping than of nest parasitism. Dumping is common behavior. Eggs, like human babies, come when they are ready to come. If everything is as it should be, an appropriate nest will be ready to receive the eggs. But if a storm, a raccoon, or something else has destroyed the nest the bird has to find another place to deposit its eggs. The nest of another bird--of the same or different species--becomes the avian equivalent of the church steps, and there the foundlings are deposited.

One of the areas I was surveying for nesting birds this year is the Swallow Cliff Forest Preserve on U.S. 45 just south of Illinois 83. Someone who owned this land before the Forest Preserve District planted a grove of white pines. This summer at least one pair of black-throated green warblers nested in these pines. This is a North Woods species. Swallow Cliff is maybe 150 miles south of the normal breeding range, but the pines were apparently all this pair needed to keep it here.

But the story gets weirder. I was out with Dan Niven, a graduate student from Champaign who was also working on the survey, and twice we saw a male black-throated green warbler that was singing the song of the hooded warbler. The hooded warbler is a fairly common bird in the preserves around Swallow Cliff. The likeliest explanation for our unorthodox singer was that it was dumped in a hooded warbler nest as a baby and learned to sing by listening to its foster father.

The more time we spend in natural surroundings the more weird stories we discover. Black-throated green warblers singing like hooded warblers, cuckoos and mourning doves sitting side by side incubating eggs on the same nest. We are out there trying to apply reason to this confusing welter of events, but the birds are just trying to get through the next few hours. "Oh my God, I've got an egg coming and the nest just blew down? What'll I do?" "That must be dad's song now. I'll sing just like him."

They live in a world of constant emergencies, a world crowded with other living things. Your nest may be invaded at any moment. You may even find yourself sharing a nest with someone you wouldn't ordinarily speak to. This crowded, jostling, exigent world is the world of biological diversity. This is life as it has existed for at least 600 million years. It is the kind of environment that produced all the living species on earth, including us. It is the kind of environment whose very existence is now threatened by human actions. We don't know what will happen when it is gone.

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