Monday, June 29, 2009

The call of the wild -- Part 2

Writing about cuckoos and global warming yesterday, reminds me of some cuckoo research I once read about. But first, a bit more background.

In Britain and Ireland, four species of birds are victimized by cuckoos -- reed warblers, meadow pipits, dunnocks, and pied wagtails -- all of them much smaller birds than the cuckoo. There seem to be four genetically distinct strains of female cuckoos, each specializing in one host species. Except for the cuckoo that lays her egg in the dunnock's nest, each female's egg closely resembles the eggs of the selected host.

Nicholas Davies and Michael Brooke are (were?) two Cambridge University ornithologists who specialize in cuckoos. They described their work some years ago in Scientific American. You'd have to be a little cuckoo to do what these guys do, but their research elegantly demonstrates the power of evolutionary theory to explain natural curiosities.

Our intrepid researchers armed themselves with phony cuckoo eggs, made of resin, the exact size and weight of real cuckoo eggs, and painted to resemble the different eggs laid by the four strains of female cuckoos.

Then they played cuckoo.

They snitched real eggs from reed warbler nests and replaced them with phony cuckoo eggs. The warblers accepted eggs that resembled their own and rejected most of the others, pushing them out of the nests.

Clearly, reed warblers aren't without some powers of discrimination, and natural selection would favor a cuckoo egg that closely resembles the host's. The evolved similarity of eggs is a classic example of mimicry.

But this was just the beginning. Davies and Brooke systematically replaced eggs in the nests of all four species of host birds, with phony eggs of every type, at different times of the day, removing different numbers of host eggs, and every other combination of thieving and confounding they could think of. They even went to Iceland to try their surreptitious switches on meadow pipits and wagtails that have long lived in isolation from cuckoos.

Every response of the cuckoos and their hosts to phony eggs was consistent with natural selection. For example, Icelandic birds were more easily fooled by phony eggs than their British cousins; they have not needed to evolve defenses against cuckoo trickery. And the cuckoo that lays its eggs in the dunnock's nest has no need of egg mimicry; the dunnock accepts almost any egg as its own, regardless of color or pattern, perhaps because it has only recently been parasitized by cuckoos.

What we have here appears to be a case of coevolution: Cuckoos have responded to the host's defenses by evolving eggs that closely resemble host eggs. Hosts, in turn, have adapted to cuckoo parasitism by becoming ever more discriminating and less likely to be fooled. And all of this inscribed in the "four-letter" code of the DNA.

The most delightful thing about this story is the thought of the two cuckoo-ologists, their pockets full of phony cuckoo eggs, skulking around in marsh and moor trying to unravel the ways of evolution. Such behavior on the part of humans is a natural curiosity as worthy of investigation as the egg-laying habits of birds.