DURING an internet discussion over the identity of a photographed bird, working from a single pic, the two proposed identities were female Klaas’s Cuckoo and Stierling’s Wren-warbler. It was somewhat of a deadlock with both sides adamant that they were correct. The photographer was asked if he had any other shots of the bird and he duly produced a couple, including a close-up of the feet. ‘Aha!’ said one of the group, ‘It can’t be a cuckoo as it does not have zygodactyl feet!’.
This was news to me and I hastened over to Wikipedia to remind myself (and now you, should you need a refresher) about the various configurations of the avian foot.
The most common foot arrangement, anisodactyly, is the “big toe backwards, three toes forwards” which we see in more than three-quarters of bird species, including sparrows, weavers, pigeons, eagles and (as webbed feet), ducks and penguins. This type is well suited to perching and grasping.
Another solution to the grasping problem is zygodactyly – the second and third digits point forwards while the first and fourth toes point backwards. This foot type is found in cuckoos, parrots and woodpeckers. Some birds (for example owls and ospreys) have the ability to rotate the outer (fourth) toe forwards or backwards; this is known as semi-zygodactyly.
Kingfishers, hornbills and bee-eaters exhibit syndactyly, in which two or more toes are fused to varying degrees. Swifts and some mousebirds have yet another configuration (known as pamprodactyly) and all four digits face forwards, allowing these species to hang their weight on their toes and even feed upside down.
Then of course there are those families which have lost two along the way – think of the two-toed ostrich (didactyl) or the three-toed bustards and quails (tridactyl).