I see the level 3 paper had a question on indigobird evolution. This is quite a neat example of rapid sympatric evolution in an animal (& one that I use in my own teaching here at Waikato), so I thought I might flesh it out a bit for you here.
Indigobirds are host-specific brood parasites i.e. each indigobird species parasitises a particular species. They lay their eggs in the hosts' nests, where their chicks are raised and fed alongside the host's own babies. The parents are stimulated to provide food by the cheeping and the wide gaping mouths of the chicks – and baby indigobirds' mouth markings mimic those of their host species. This mimicry goes further: adult male indigobrids mimic their host's song, and females use this to find and choose a mate of their own species, and also to find their host's nests.
Sorenson & his co-authors (2003) suggest that these indigobird behaviour patterns, as well as maintaining host specificity, would also provide a reproductive isolating mechanism: if an indigobird made an error in selecting a host nest, the next generation of chicks would imprint on and mimic that new host's song. And because both males and females imprint in this way, those chicks would be more likely to breed with each other than with chicks who'd imprinted on the 'right' host species. Ultimately this could lead to quite rapid sympatric speciation in indigobirds.
The researchers tested this prediction by comparing both mtDNA and nuclear DNA data from several indigobird species, and from their firefinch hosts. The mtDNA data were used to generate a phylogenetic tree for both hosts and parasites – you can see the full version here. This supports the idea of rapid indigobird speciation. Analysis of nuclear DNA found that indigobird species in a given region showed significant genetic differences, which indicated that indigobirds raised by a given host are most likely to breed with other individuals raised by the same host species i.e. that imprinting on host song can indeed act as a behavioural isolating mechanism. (This would then be followed by selection for mouth patterns in nestlings.)
This story has it all, really: sympatric speciation in a vertebrate species; a key role for sexual selection (females choosing mates on the basis of their song); behavioural isolating mechanisms… I like it 🙂
You'll find more images of indigobirds here – & a heap of other stuff as well. There are pictures of the fire-finches that indigobirds parasitise, and also sound files, so that you can hear how well the indigobirds mimic their hosts' songs.
M.D. Sorenson, K.M.Sefc & R.B. Payne (2003) Speciation by host switch in brood parasitic indigobirds. Nature 424: 928-931