This story from a couple of weeks back:
BBC Nature: Related birds evolve different songs and colours
Canadian Researchers have discovered a pattern in birds' songs and plumage that help explain some of the colourful and tuneful variety in nature.
The team found that closely related birds that share the same habitat tend to look and sound different.
This evolutionary rule of thumb seems to help birds to identify members of their own species.
The gist of the story—as portrayed by the BBC, at least—is that birdsong and plumage help closely related species living in close proximity to tell each other apart. While I'm sure this must be the case, the story overlooks the interesting possibility that preference for certain styles of song or plumage are what caused these closely related species to diverge in the first place.
For many years now, I have had a pet hunch that Darwinian Sexual Selection (which is a special case of Natural Selection) might be a far more important driver in the creation of new species than it is generally given credit for.
The real power of Sexual Selection is that it can sometimes be based entirely on whim. Imagine a population of blue-coloured birds of a particular species where a small number of females happen to have a random genetic preference for males with redder plumage. Any males which happen (through random mutation) to develop slightly redder plumage are likely to be selected as mates by these females. Any male offspring from such unions will tend to inherit their fathers' redder plumage, and any female offspring will tend to inherit their mothers' preference for males with redder plumage. In subsequent generations, the females' preference for redder plumage will tend to act as a driver for males to develop redder and redder plumage. In this way, the blue and red populations can begin to diverge—and ultimately speciate—without their having to be physically isolated from each other to prevent inter-breeding. The sexual whims of the females in my admittedly simplistic example could be as much a barrier to inter-breeding as locating the birds on different islands. And, where you get such sexual segregation, you also tend to get speciation.
As I say, it's just a hunch. But it's a hunch I stand by for the time-being.