Bills


Some years ago now, excellent birder and bird photographer Richard Ditch and WC co-authored a couple of articles on bird identification for bird photographers. Richard did most of the work, and made an excellent point: a bird’s bill is a good starting point for identifying the bird’s species and figuring out how the bird makes a living. With that credit to Rich, WC will undertake to give some examples of the astonishing evolutionary adaptations of bird’s bills to bird’s business.

Raptors have bills designed to tear and rend their prey. You’d expect their bills to be large, strong and sharp, with a hook for grasping. And that’s how they are built.

Bald Eagle, Cordova, Alaska

Bald Eagle, Cordova, Alaska

Here’s a classic raptor bill. A Bald Eagle also feeds on carrion, so its bill is even heavier. The hook in the upper bill allows an eagle to pierce hide and tear. A Bald Eagle takes on big prey, and has a bill that evolved to manage that.

A Rough-legged Hawk, by contrast, hunts mostly voles, mice and ground squirrels, and evolution has provided it with a smaller bill.

Rough-legged Hawk, Delta Barley Project, Alaska

Rough-legged Hawk, Delta Barley Project, Alaska

Note that the bill isn’t just smaller; it’s smaller in proportion to the bird. Weight is critical for a creature that flies; the bill is just heavy enough to dismember a vole, like the one this bird has, and no heavier.

The Southern Crested Caracara is a South American bird, about the same size as a Rough-legged Hawk, but has a comparatively huge bill.

Southern Crested Caracara, Pantanal. Brazil

Southern Crested Caracara, Pantanal. Brazil

That’s because the Caracara is primarily a carrion feeder,and that heavy bill – proportionately much larger than a Bald Eagle’s – has evolved to allow it to tear and dismember carcasses.

There’s a number of raptors who specialize in fishing and, as you’d expect, their bills are adapted to the task of tearing up their slippery prey.

Black-collared Hawk, Pantanal, Brazil

Black-collared Hawk, Pantanal, Brazil

Note how much thinner this bird’s bill is than the Caracara’s or Bald Eagle’s, and the long narrow hook. That hook – technically, it’s called a “nail” – is perfect for tearing fish flesh.

Not all raptors are big, of course. The American Kestrel is North America’s smallest falcon.

American Kestrel, Malheur National Wildlife Refuge, Oregon

American Kestrel, Malheur National Wildlife Refuge, Oregon

A Kestrel hunts even smaller prey than a Rough-legged Hawk, and, as you’d expect, its bill is proportionately even smaller. It has the same general raptor shape, with that nail on the upper mandible, but it’s shrunk considerably.

One last raptor bill, the Snail Kite.

Snail Kite, Pantanal, Brazil

Snail Kite, Pantanal, Brazil

As the bird’s name suggests, this bird feeds almost exclusively on snails, in particular Apple Snails. That bill, with its very long nail, has evolved to hook the body of a snail out of its shell. The curve of the nail is a perfect match to the curve of an Apple Snail’s shell.

Snail Kite Feeding, Patnanal. Brazil

Snail Kite Feeding, Patnanal. Brazil

WC finds bird endlessly fascinating,1 and the adaptations of bird bills are just one of the reasons. But we’ll visit other kinds of bills in future posts. But if you are lucky enough to be close to a bird, take a moment to look at its bill.

 

 


  1. This might be a classic example of stating the obvious. 
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