Bill Shape and Feather Shape



This post was prompted by a comment/question in a previous post about a green heron preening in our Wetlands. The question concerns the existence of a relationship between bill shape and feather shape, “I only thought about beak shape in terms of feeding, but I wonder if there’s a beak shape/feather shape relationship too?”

The short answer is, no. But read on.

Any question that stimulates the thought process is a good question. This question started me thinking long and hard. I too, have always thought of birds’ bill shape as having evolved essentially to serve the feeding process, the other uses of the bill, whatever they are, being secondary.

Here are some thoughts on the subject.

The needle-like, nectar sucking bills of hummingbirds, the conical shaped, seed-cracking beaks of finches, the upside down filter feeding bills of flamingoes, and the fish snatching, clog-like schnoz of the shoebill of Africa all serve their owners as preening tools. As diverse as all of those beaks are, they all apparently function as preening apparatus with no special adaptations that I’m aware of. By the way, birds also use their feet to preen. This comes in handy in places that are out of reach of the bill. The head comes to mind.

However, the mere fact that birds preen with their bills, as well as eat, cut meat, crack seeds and nuts, build nests, and defend themselves, is in itself a special adaptation. We humans need our hands to do all of those things. Birds squeeze many diverse functions into one apparatus, their beaks.

The real question was, though, whether or not there is a bill shape/feather shape relationship in birds. I’m not aware of any adaptations whereby a bird’s bill evolved to fit the shape of a particular feather shape or to function specifically to serve the form of a particular feather. But, keep reading.

Most species’ feather tracts are similar, there are body or contour feathers and flight feathers. Some species have special feather types like plumes (herons, egrets, birds of paradise) and gorgets (the shiny feathers of a hummingbird’s throat), and how about those outrageous tail feathers of peacocks.

Different species have different feather quality. Woodpeckers have very stiff tail feathers which they use to prop themselves up on tree trunks. Penguins have small, stiff, dense feathers over their entire bodies. Owl feathers have a sound-deadening quality. Their flight feathers are “softer” than, say, a hawk’s feathers. The leading edge of the wing feathers are serrated. The same feathers are fringed on the trailing edge. Those two modifications of the wing feathers breaks up the airflow over the wing and allows the owl to fly through the night air unheard by its prey. But that still doesn’t answer the question of whether or not bills have evolved in ways to serve the shape of feathers, or the other way around.

One study suggests that bird species, or different populations within the same species, with a hook-like structure at the tip of the maxillary (upper bill) have fewer feather lice than species lacking such a structure. An experiment was done where a group of rock pigeons (your basic pigeon), which do have such a hook, had their hooks removed, while another group in the study were left untouched (their bills still had the hook). After a period of time, the birds with the hook removed had more parasites than the group that retained the hook. It sounds as though, at least from that particular study, that bill shape has evolved to assist in preening in some species, as far as parasite removal, although there doesn’t seem to be a correlation between bill shape and feather shape.

Here’s the website which contains the study above.

Another study I found concentrated on bill length in relation to preening. Shorebirds were used for this study. Shorebirds have a wide range of bill lengths throughout the different species, from the short, pecking bills of plovers to the very long, thin, decurved probing bills of long-billed curlews.

Observers recorded the length of time various species spent preening, when that species was actually engaged in preening. According to the study, birds with longer bills spent more time preening than birds with shorter bills. This suggest that the birds with longer bills were less efficient at preening than shorter billed birds. But, there was no significant relationship between bill shape and time spent preening. There were no differences recorded in time spent preening between curved billed and straight billed species with comparable bill lengths. But that still doesn’t address a correlation between bill morphology and feather morphology.

Click here for that study

I’m sure that somewhere out there, someone is conducting, or contemplating conducting, a study of whether or not birds’ bills have evolved in some way relative to the shape of at least some of their feathers. There may be a bird in deepest, darkest Borneo, Amazonia, or tropical Africa that performs some bizarre, never before witnessed, preening behavior whereby its bill and perhaps some specialized feathers (plumes?) have evolved together to perform such an act. But until that study has been done, if it hasn’t been done already, or that bird is discovered, I’m going to say that there probably is not a bill shape/feather shape relationship.

But back to bill length and preening behavior for a moment. There’s a hummingbird in South America which has a bill which is longer than its body length, the sword-billed hummingbird. The long bill is excellent for feeding on the nectar tucked away in the back reaches of long tubular flowers. But, how does a bird with a bill longer than its body reach feathers on, say, its belly in order to preen? Imagine trying to scratch your own belly if your arms didn’t bend (there’s no joint in a bird’s bill). It’s difficult, for sure.

This hummingbird uses it’s feet to preen. But hummingbirds have tiny feet and very short legs. With their enhanced aerial ability, hummingbird feet became perching-only feet. Hummingbirds don’t do any walking, running, or hopping. Nearly everything they need to do, they can do while flying. Their feet don’t need to be as big or their legs don’t need to be as long as a sparrow, sandpiper, or even a chicken (in relation to their body size) making them suitable for walking, running, or even hopping.

The exception is the sword-billed hummingbird. It has apparently evolved in a way that as the bill grew longer in order to reach the nectar deep inside long tubular flowers, so too the feet grew to compensate for the loss in bill-preening ability. Why, what big feet you have. The better to preen with!

You learn something every day. Thanks Jennifer!

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