I learned the other day that synchronized swimmers don’t shave their legs, at least not to the point of smoothness. Apparently the hair on their legs gives them a sense of where their legs are in the water. Try lightly running your hand along your hairy arm or leg – there clearly is a sense of touch connected with our hirsuteness, as meager as it may be. We also know that cats, mice, dogs, and many other mammals use their whiskers, bristles, vibrissae, or whatever one calls them, to sense their environment by touch. Now, lots of birds have bristles around their jaws and face such as this barbet. These are called rictal bristles because they are located at the rictus, the gape of a bird’s mouth.
For many years it was thought, and logically so I think, that the bristles that surround a bird’s beak served as an insect net or sweep to scoop up insects as the bird foraged through the air. It made some sense. When I started graduate school (back in the Mesozoic) I was looking for a research project and happened to read about rictal bristles; they intrigued me. Exactly how did these bristles work? It seemed to me flycatchers might provide some answers, so I captured some flycatchers and put them in a large glass-fronted cage. I inserted house flies into the cage one at a time and the birds learned to fly down to capture the insect as it was introduced into the cage near the glass. As the birds swooped down to capture the fly, I filmed them at 400 frames per second, not a big deal today but at that time it was high tech. Remember, movies were shot at 24 or 32 frames per second. So I’d go through a roll of film in about 11 seconds and ended up with a fairly large stack of developed film. (No pixels those days.)
After spending months filming the birds I edited the movies. Most of the film fell on the cutting room floor, as they say, because the actual insect capture took only a second. But I got plenty of shots that showed me that rictal bristles had nothing to do with prey capture, at least nothing that I could see. The flies were invariably caught in the tips of the flycatchers’ bills, which are hooked at the tip. (If you are interested in reading about what I did, you can access my published research paper: The Role of Avian Rictal Bristles.)
It seems obvious that bristles would serve as insect nets – look at this nightjar, an aerial insect eater. Makes sense, right? But after discovering that Cedar Waxwings, Bald Eagles, Kiwis, and various other birds that typically don’t catch insects mid-air, I wondered what these modified hairs did do. Nobody knows for sure, but the consensus seems to be that rictal bristles have a sensory function, letting the bird know its speed and orientation in the air. The Kiwi doesn’t fly but is a nocturnal forager with very long rictal bristles which the bird probably uses to find food in the soil.