Bills, Bills, Bills

By Bill Pulliam

There is a principle in biology that if you want to know how an animal lives, look at its mouth and its limbs.  These will tell you most of what you need to know.

Birds are excellent examples of this idea in action.  If you look at a bird’s bill, wings and feet, you will learn much of what there is to know about its lifestyle.  In the next few articles, I’ll talk about these body parts of birds and what they reveal.

This month, it’s time to talk about mouths.  All living birds have bills on their mouths.  In fact, this is one of the few things that distinguish living birds from their close-related dinosaur cousins.  Dinosaurs mostly had reptile-like toothy jaws; modern birds are toothless.

Some birds do seem to have “teeth” on their bills, and hatchlings often have an “egg tooth” for cracking the eggshell from the inside.  These are really just serrations or bumps on the bills, and are not related to real teeth.  The same applies to the “tooth” or “nail” that some birds have on the tips of their bills.  Real teeth are made of bone; bills, “egg teeth,” and these other structures are all made of keratin, the same stuff that makes up skin, feathers, hair, claws and fingernails.

Birds’ bills are all built from the same basic parts.  There are two mandibles, the upper and lower, just as with mammals’ jaws.  The upper mandible usually has two nostril holes (“nares”) in it.  As with mammals, the upper mandible is strongly tied to the skull, while the lower mandible moves freely up and down at the jaw hinge.  A bird’s upper mandible is more flexible than the upper jaw of most mammals, however, and can be flexed upwards a bit in most species independently of the skull.  Humans and other mammals tend to have the upper jaw bones fused rigidly to the skull.

Beyond these similarities, birds’ bills come in a wide range of specific shapes.  Bills are mostly used for feeding.  Since birds feed on an enormous variety of different things, their bills come in an equally enormous variety of sizes and shapes.

Some of these bills seem quite extreme to us.  The pelicans have developed an enormous flexible pouch on their lower mandibles to be used as a dip net and hold giant mouthfuls of fresh seafood.  Some shorebirds have very long slender bills that are almost as long as the rest of the bird’s body.  These are for probing into wet mud, sand, and soil for small prey animals.  And hummingbirds have thin straight tubular bills for extracting nectar from flowers.

Flesh-eating birds like hawks, falcons and vultures have strong, sharply-hooked beaks for tearing and crushing.  Interestingly, parrots have similar bills that they use for devouring the flesh of juicy fruit.  And, it turns out when you examine their DNA, you find that the flesh-eating falcons and the fruit-eating parrots are actually very closely related to each other and quite distant from the other birds of prey.

Among the smaller birds, one of the biggest distinctions is between seed-eaters and insect-eaters.  This is not an absolute black/white line, of course.  Many birds that mostly eat insects at some seasons will also eat seeds and berries at other times of year, and vice versa.  But the overall pattern is pretty clear.

Seedeaters generally have short, thick, conical bills.  These are built for cracking seeds and extracting the kernel from the husk.  Some seed-eaters have dainty little conical bills, others have big hefty things.  Both are adapted for cracking and crushing.  Sparrows, buntings, grosbeaks, cardinals and cowbirds all show typical seed-eater bills.

The crossbills are a distinctive group of finches that occur mostly in the north and west, feeding on the seed of spruces, firs and northern pines.  The tips of their mandibles do indeed cross each other.  This is an adaptation for prying seeds out of these small, tight cones.  The bill is wedged deeply into the cone, and the crossed tips become pliers prying the space open.  Then the bird pulls the seed out with its tongue.

Insect eaters tend to have longer, thinner bills.  These are for probing and snatching, like two stiff fingers or a pair of tweezers attached to the bird’s mouth.  Insectivore’s bills also vary greatly, from dainty needle-like warbler bills to heavier beaks of vireos and flycatchers.  Some, like creepers and thrashers, have long curved bills for probing through brush, under bark and into other blind spots.

Birds that are more generalized in their eating habits often have fairly robust, moderately long, all-purpose bills.  You will see these on crows, jays, grackles, starlings and blackbirds, among others.

Since birds’ bills are so varied, and tied to how they live, they are also one of the first things you should look at when you are interested in identifying an unfamiliar bird.  Whatever the bill looks like, it will greatly narrow down the options for what the bird might be.  If it is small and conical, don’t even bother with the warblers, vireos, flycatchers, etc.  Go straight to the sparrows and finches.  Try to learn to look at the fine details of the size and shape of the bills on the birds you see.  Every species has one that is a little shorter, sharper, slightly more curved, etc.  Also, don’t forget that bills come in colors.  This can also be a very important identification point.

Next month we’ll move down to the birds’ feet, and see how they are also finely adjusted to how each species lives in the world.

This article was originally published in the June 2014 issue of Validity.

Bill Pulliam got started in birdwatching by his junior high science teacher in 1974, and has been an avid birder ever since in 48 U.S. states and 7 foreign countries.  He is currently the Tennessee editor for eBird, a online project that compiles millions of observations from tens of thousands of birders around the world.

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