As Michael T says, 'species' is essentially a human construct. With evolution being a constant process (more or less) at any one point in time there are bound to be taxa where, no matter what definition of species we use, it is very hard to define whether a taxon is a distinct 'species' or a well-marked 'sub-species'. The Red and Black-throated Thrush complex is one of these and whether it has been treated as one or two species has followed fashion to some extent: two species in the 'splitting' C19, one species in the 'lumping' C20 and perhaps two again in the 'splitting' C21.
In determining what is a species we are looking for isolating factors: geographical distribution, habitat, behaviour (song and display) and morphological features which serve to reduce gene flow to a point where genetic lineages remain distinct from each other. In the case of these thrushes it appears (from the literature) that there are some differences in geographical range, some differences in habitat preferences, some differences in vocalisation, and (obviously) some very marked morphological differences. However, the literature (e.g. HBW) suggests that in some areas a high proportion of birds are hybrids, so these isolating factors are, apparently, incomplete. Is the frequency of hybridisation high enough to suggest that mate selection is random (in which case we are probably dealing with subspecies) or is there evidence that it is selective, in which case treatment as two species may be appropriate. From my reading of the literature there is little consensus on this point and hence I am slightly surprised that the usually taxonomically conservative BOU has recently taken the decision to recognise two species.
Mike Leven
