Bird feet come in all sorts of shapes and sizes, presumably relating to both evolutionary history and current function. The two-toed feet of ostriches are typical of runners, being relatively small and light; webbed duck feet,while functional on the land, have clearly evolved under pressure to perform well in water; powerful talons characterise the raptors; and jacanas have weirdly long toes, consistent with their lily-trotting habits. But some traits have less clear-cut functional relationships. Many birds – for instance the skylark – have strikingly long claws, and yet are definitely not raptorial.
One idea – the `snowshoe hypothesis' – is that, somewhat like jacanas, the larger foot spread afforded by long toes and claws might allow birds including `longspurs', `longclaws' and some larks to handle life on relatively long, `rank' (as in thick/luscious, not smelly) grasses. In order to test this, Rhys Green, Keith Barnes and Michael Brooke from Cambridge and Cape Town compared the feet of `grass-dwelling' and `bare-ground' bird species. Their hypothesis was that grass-dwellers – typified by the skylark – should have relatively long toes and claws compared with larks from bare ground habitats. And that is exactly what they found. What is more,this was not an oddity of larks: matched pairs of closely related birds from a range of families, one from a stony/bare habitat (e.g. grey wagtail) and one from a grassy habitat (e.g. yellow wagtail), showed the same trend. So, it appears that elongated toes and claws (or, perhaps, relatively small bodies and wings) may be an adaptation to standing in and moving across long grass. While the mechanism behind this benefit is not as clear as in the case of lily-trotters, probably relating to both the unevenness and instability of grass tops, the relationship does tie in with behavioural observations:skylarks prosper in `untopped' (long) stubble, whereas granivorous birds,including those with toes and claws of ordinary length, do well on cut and shortened crops.
So, what is the down side to having extended toes and claws? One is presumably an increased risk of damage. Indeed, the authors' unpublished observations of Roso larks, a species confined to a barren rocky island, show that around half of older birds have toe or claw damage. A stronger tendency towards lengthening was observed for claws than toes, suggesting that extending the toes may be somewhat more costly. It was suggested that this may be because toes require a blood supply, resulting in heat loss. So, temperate and tropical lark feet were compared, with the idea that heat loss would be less severe in tropical birds. While the direction is as predicted (temperate birds extending claws relatively more than tropical), the effect was not statistically significant. This highlights the problem with such comparative studies: evolution has only occurred a limited number of times, so if the sample size or signal is too small, there is not much that can be done about it, even if the underlying mechanism is perfectly valid.
Overall, however, this study successfully demonstrates a relationship which was not, before, absolutely obvious. And the finding should inspire further study of the physical mechanisms of bird gripping and locomotion, not least because of the growing interest within biomechanics and robotics into movement over complex, unstable substrates.
