February 2017

Winter is a time to keep warm and contemplate. So I have been mostly contemplating feet. Nothing for chiropodists to worry about, I wouldn’t want to tread on any toes, so instead I’ve been thinking about birds’ feet.  Birds’ feet are particularly well adapted for what they do, which is to support the bird that they are attached to, in the way that the bird needs them to – which is to perch, hop, walk, run or cling.

Feet of a young robin - standard perching feet

The first thing to notice about a bird’s foot is that they are quite small – not because they don’t need to be bigger, but because they are lighter that way.  The whole of a bird’s anatomy is tuned to being as light as possible from beaks to hollow bones, not to mention the arrangement of the lungs and the amazingly strong and light feathers so that flight involves the least effort.  If you look at any flightless bird such as a kiwi, you will notice how stocky the legs are – the same basic pattern, but much heavier.

A pukeko, another flightless bird from New Zealand with
very sturdy legs.

A poor photo, but you can still see the 'tree-trunkiness' of the
 legs relative to the size of the kiwi.

  At the other end of the scale are swifts’ legs; these are short and almost useless because swifts spend the whole of their life apart from nesting, mating and fledging in the air.  If you see a swift-like bird perched on a wire then it will be a swallow or a martin, never a swift.

When it comes to perching, there are a number of ways this is done; most birds have three forward pointing toes and claws, and one pointing backwards.   This is an ideal arrangement for gripping a twig or a branch, but it’s not all about perching or roosting; woodpeckers and their ilk use the same arrangement to cling to the trunks of trees.  The three forward, one back arrangement works here quite well, but the relatively heavy woodpecker has also developed short and strong tail feathers which are used to brace it against the trunk. 

A young woodpecker, not a good photo but it shows long claws
 and the two very stiff tail feathers

Nuthatches and treecreepers don’t need to use their tail for balance; they rely instead on strong toes and longer claws than other birds.  The nuthatch is renowned for being able to climb down the trunk as well as up and it relies on an exceptionally strong rear claw to achieve this.  This gives them a different range of insects perhaps invisible to an upward facing bird like the treecreeper that doesn’t have that ability.  (I was once told that nuthatches have two backward facing toes but this appears not to be the case.) Whilst on the subject of perching – when did you last see a gull or a duck perched in a tree?  It is a rare sight, and you will only see them do it on relatively wide, flat branches and it is simply because they don’t have the rearward facing toe like other birds.  Their feet are webbed for swimming and the rear toe has been reduced to a small spur on the back of their legs. 

If you do happen to see a large gull sat in a tree, you may want to know if it is a greater or a lesser black-backed gull.  The most reliable indicator is not the greyness or blackness of their wings but rather that one has yellow legs, the other has pink legs.  All you need to do then is remember which is which!  Identification by birdwatchers is one thing that evolution hasn’t worked out yet, but feet can be quite distinctive.  Some birdwatchers reckon to be able to tell the difference between a willow warbler and a chiffchaff by the colour of their legs.  One has darker legs, but that really needs them sat side by side.  Apparently the only reliable indicator is that one has a notch in the third primary wing feather, the other doesn’t.  So you just need to catch it and then remember which is which! 

The Common Kestrel - in India

Same kestrel slightly enlarged showing the shiny black claws.

Recently I took a photo of a kestrel in a place where the lesser kestrel was also a possibility.  Fortunately, the difference is clear – the lesser kestrel has yellow legs and pale claws and the common kestrel has yellow legs and black claws – and beautifully shiny black claws they were too.