Sibling species of treecreepers (Certhia)
Sibling species are closely-related
species that are nearly 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 hind toe. Behavioural patterns and ecology are
quite distinct. (from Futuyma 1997)