For the Birds Radio Program: Crossbills
Why does a crossbill’s bill cross? And who cares? Laura Erickson has the answers. 4:50
Among the most intriguing birds in the world are the crossbills. These medium-sized finches are designed to eat seeds from the cones of conifers–food available throughout the snowiest of winters in the northern forest. We seldom see crossbills at feeders, though once in a while a small group does figure out that sunflower seeds are edible.
We’re most likely to notice crossbills when we get bonked on the head by a cone dropping from a spruce, or when a flock gathers on a roadside to pick up salt and grit. Because of their limited diet, these finches avoid mineral deficiencies by eating any grit they can find in winter–I see them much more often in my children’s sandbox than anywhere else in my yard. They sometimes even cling to the sides of brick buildings and chimneys and scrape off and eat small bits of mortar.
Extracting seeds from a pine or spruce cone isn’t as easy as it might seem–the easiest way is to grab the cone from the side, opening the sheaths one at a time with one mandible while bracing the cone in place with the other mandible, all the time using the muscular tongue to pull out the seeds. The trouble most birds would have with this method is that their upper mandible would block the lower one unless it could bend out of the way. The crossbill solves this problem in a simple, elegant way–its upper and lower mandibles twist in opposite directions, giving it its uniquely crooked appearance and even its name.
There are two species of crossbills in North America–the Red Crossbill, which is usually associated with pine trees, and the White-winged Crossbill, associated primarily with spruce and larch. In Red Crossbills, the ratio of birds whose lower mandible crosses to the right to birds whose bill crosses left is just about exactly 50:50. But in White-winged Crossbills, the ratio of right- to left-billed birds is 3:1. Ornithologists have noticed this since the early 1900’s, but no one spent much time puzzling about it until just recently, when a couple of ornithologists set to work to figure out just why there was such a noticeable discrepancy. The two of them have completely different theories about the reasons that bill crossing is different in the two species, and working out the final answer will take years of thought, observation, and experimentation.
So far it appears that the different seeds the two species eat are a critical factor in accounting for the difference. The pine cones preferred by Red Crossbills are thick and tough enough that the birds usually leave the cones on the tree, each sitting next to a cone to extract whichever seeds are closest. Since the birds seldom remain in one spot long enough to remove all the seeds from a given cone, the flock may return to the same tree time after time. If a right-handed, or more appropriately right-billed, bird has fed on a given cone, the next bird will find more seeds if it happens to be left-billed. Having equal numbers of right-and left-billed birds allows a flock of Red Crossbills to forage most efficiently. On the other hand, White-winged Crossbills cut spruce and larch cones right off the branch as they extract the seeds, so it makes no difference how many birds in a given flock are right- or left-billed, since each one is feeding on entirely different cones.
One of the current ornithologists believes that genetically, bill crossing is much like our brown and blue eyes. He figures the right-crossing lower bill is dominant, because that’s the characteristic found in 75% of White-winged Crossbills. Assuming that the right- and left genes are equally distributed, that’s the ratio Mendel long ago predicted that a genetic property with simple dominant and recessive characteristics should tum out.
The problem with this theory is that when another ornithologist bred left-billed birds in captivity, he ended up with some right-billed young. If the right-billed characteristic really were dominant, this would be comparable to two blue-eyed people producing brown-eyed babies, which just doesn’t happen. Also, so far all the right-billed pairs have produced only right-billed young. So he interprets the data the opposite way–that the left crossing bill is dominant, and that the two genes are not equally distributed. He makes a complicated mathematical case for the gene frequency to be 15% left- and 85% right-billed. The whole argument gives me a headache, reminding me why college genetics wasn’t my favorite class.
The issue may seem trivial, but it turns out that there are actually several different kinds of Red Crossbills, with measurable differences in bill size and shape, depending on which species of pine tree they’re adapted to. For example, ponderosa pine cones require a bigger bill and stronger tongue musculature than other cones. And there appear to be differences in the right and left-billed gene frequency in the various groups of Red Crossbills, along with other differences in their bills and tongues.
The elegant adaptations these birds have made to America’s rich forest diversity may end up hurting them in the face of modem forest management. In the western United States, industrial forest lands are managed on a 40- to 60- year rotation, which limits cone production of many conifers, and now some species of pine trees, and the Red Crossbills associated with them, are declining. Crossbills are so uniquely pleasant to watch, and understanding them is so challenging to the human mind, that I hope every kind of them manages to hold on in our world for a long time to come.