Sibling species of treecreepers (Certhia)

Sibling species are closely-related species that are nearly morphologically indistinguishable. The short-toed treecreeper (Certhia brachydactyla) (left) differs subtly from the common treecreeper (C. familiaris) (right) in a number of minor characters, including wing pattern and size of the claw on the hind toe. Behavioural patterns and ecology are quite distinct. (from Futuyma 1997).