The paper is:
Selection-based biodiversity at a small spatial scale in a low-dispersing insular bird
Jacques Blondel et al
Science 285:1399-1402 (Aug 27, 1999)

Picture a landscape with clumps of different forest types--pines or oak, for example. The different clumps will have different microhabitats, and in same ways they may be markedly different. For example, in response to the difference in foliage, caterpillar populations may peak at different times, and this may affect caterpillar-eating birds. You might expect, then, that those birds living in the oak habitat (where caterpillars reach an earlier peak) might nest earlier than their counterparts in the pine habitat.

By the way, anyone who reached this page via a search for "cool tits" ... you are probably going to be disappointed.

As the more cunning of you may have guessed, this isn't a purely hypothetical picture. The observation, though, isn't what I (in my naivety) would have expected; in general, birds in either microhabitat nest at pretty much the same time. The example in the paper is the blue tit Parus caeruleus, a caterpillar-eating bird in the Mediterranean, and the result is that birds in the oak habitat (which is less common than the pine habitat) are maladapted for their local microenvironment; they nest too late to take advantage of the abundance of caterpillars, though they would be well-adapted for life in the pine microenvironment. This is apparently because the birds in the different microhabitats interbreed, to the point that a variety more suited for the oak habitat can't arise. (That is, there is selection pressure such that blue tits genetically prone to nest earlier, are fitter in the oak environment; but the same genes are maladaptive in the more abundant pine habitat. In a sense, the gene pool dilutes out any genes that select for early nesting.) As the authors put it, "In such mobile organisms as birds, local differentiation of fitness-related traits as a response to variation in known environmental factors is supposed to be rare at a microgeographical scale becase of the homgenizing effects of gene flow that override local adaptations."

As a side note, less-mobile organisms--notably plants, but also insects, as well as some fishes and mammals (the exception to every rule, the naked mole rat, seems to be the only mammalian example) do show asuch adaptation to microenvironments.

Blondel et al here find another exception, which is the blue tits in the island of Corsica, as opposed to the mainland. As on the mainland, there are pine and oak microhabitats in Corsica; as on the mainland, caterpillars peak about a month earlier in the oak than in the pine habitats; but unlike the mainland, blue tits in the Corsican oak habitats nest about a month earlier than do those in the pine habitats.

They go on to show that the populations of tits in the habitats are distinct in a number of other ways (size, bill length) and by banding several thousand nestlings, show that on the island, there is virtually no migration between the habitats. Previous studies on the mainland blue tits showed that there was some migration between the population--not a huge amount, but it doesn't take very much gene flow to keep populations homogenous. They also show that nesting time is under genetic control. (What actually happened is that when they put birds from the different populations in the same captive envirnment, the pine birds nested at the same time as their wild kin, while the oak birds nested much later than either the pine birds or their wild kin. I guess this suggests that the one population has relatively hard-wired timing, while the oak population is more dependent on local cues.)

So the Corsican (island) populations are diverging and forming distinct varieties, while the mainland populations are remaining homogenous because there's gene flow. How come?

(And here's the cool part.) Here's what some guy called Darwin said:

... there is a very curious point in the astounding proportion of Coleoptera that are apterous; & I think I have grasped the reason, viz that powers of flight wd be injurious to insects inhabiting a confined locality & expose them to be blown out to the sea; to test this, I find that the insects inhabiting the Dezerta Grande, a quite small islet, would be still more exposed to this danger, & here the proportion of apterous insects is even greater than on Madeira proper. (Letter to J.D. Hooker, 7 March 1855)

Darwin, as usual, got it right and saw the general principle underlying the observation. Island species, not only of insects but of birds, tend to be flightless, or if they still fly tend to be much less apt to disperse wisely; because if they did cover large distances, they wouldn't be island species, they'd be dead at sea. There's a general selection pressure for island species to keep to a smaller habitat. (There are obviously exceptions, but it's a pretty good general rule.)

So what's happened is that the blue tits on the island of Corsica evolved a tendency to stay put -- because if they fly about merrily hither and yon, they get blown out to sea and die. Because they're more sedentary (Norwegian Blue jokes will be sternly ignored) they're more likely to stay in either an oak, or a pine, habitat. Because they're more likely to stay in a particular habitat, they're less likely to interbreed with birds in other habitats. Because they're not interbreeding, they're speciating.

In other words, the general tendency for island species to remain localized has led to a specific tendency for blue tits to remain in the same habitat; and this tendency for the island birds to remain in the same habitat, has enabled them to develop locally beneficial adaptations that mainland birds can not.

Islands are well-known hotspots for speciation: In proportion to their size, islands are incredibly rich in species. This paper suggests yet another reason for this: physical islands may lead to adaptations that in turn lead to genetic islands. Recursive islands, or as the authors observed, as case of "nested insularity".

Well, I thought it was cool.

Footnote: After I wrote this there was a letter, or an article, that argued that Darwin's explanation of insular apterism (i.e. that flying beetles get blown out to sea if they live on islands, so there's selection pressure for flightlessness) is not correct, or at least is only part of the story; the authors, if I recall correctly, concluded that flightlessness correlated with body size ... unfortuanately I can't turn up that article any more. If anyone knows what I'm talking about, drop me a note.