Making ends meet with a crossed bill
Steven D. Faccio
Crossbills are one of our most specialized groups of birds, feeding almost exclusively on conifer seeds. These hardy, nomadic finches have evolved oddly-shaped bills that allow them to exploit a food source before it becomes available to most other birds. However, being so specialized and relying on a single primary source of food means that when that food is unavailable, they have to search far and wide to make ends meet.
North America has two species of crossbills - white-winged crossbills (Loxia leucoptera) and red crossbills (Loxia curvirostra). Both are widespread across boreal regions dominated by conifer trees, and populations extend south into mountainous areas, with red crossbills reaching as far south as Mexico. In New England, the more slender-billed white-winged crossbill, which is more commonly observed, spends most of its time foraging on the relatively small cones of spruce, balsam fir, hemlock, and tamarack, while red crossbills are typically associated with large-coned white and red pines.
Based on their distinctive flight calls, ornithologists have identified 9 or 10 types, or "morphs," of red crossbills. Although some researchers believe that many of these types deserve species status, taxonomists have yet to agree. Moreover, they can’t seem to agree on just what to call them - are they morphs, super-species, sibling species, or sub-species? What they do know is that six red crossbill types have evolved bills that are each adapted to feed on cones from a single species of conifer tree. Such specialization requires that crossbills depend on finding a particular species of conifer seed, because, compared to other finches, crossbills are rather inefficient at foraging on non-conifer seeds. For crossbills, the most important characteristic of a conifer tree is that its cones stay closed, or partially closed, through late winter and into spring. If cones open too early, less specialized species, such as pine siskins or nuthatches, will eat the seeds.
The specialized mandibles for which crossbills are named enable them to exert and withstand the strong forces that are needed to pry open the woody scales of a conifer cone. Both their upper and lower mandibles are curved at the tips, where they also cross each other, not unlike pruning shears. However, when a crossbill is extracting a conifer seed from a cone, it doesn’t use its bill for shearing.
Instead, as the tips of the mandibles bite the cone scale, the lower mandible, due to its asymmetrical curve, spreads the cone scale to the side, slightly exposing the seed at the base of the scale. Then it uses its long tongue, which extends around the back of their skull, to lift the seed out. Finally, like any other finch, crossbills husk the seed using the back of their mandibles and swallow the kernel.
Of course, it takes much longer to describe how a crossbill extracts a seed then it does for the bird to actually do it. A crossbill can remove all the seeds in a cone in a matter of minutes, as it systematically moves around the cone, often focusing on the tip where most of the seeds are located. As a crossbill feeds, it spirals around the cone in one direction or the other, depending on which side its bill crosses. Crossbills can be left-billed or right-billed, and when they forage, they need to have their lower mandible crossing toward the cone in order to feed.
Like many species of trees, most conifers are highly variable when it comes to seed production. For example, in the Northeastern U.S., balsam fir and eastern hemlock usually produce cone crops every other year, while white pine produces a large cone crop every three to five years. So, as any birder will tell you, there’s no need to watch for crossbills unless there’s a significant cone crop. In addition, if crossbills are around one year, it’s unlikely they’ll be around the next.
Cones begin to develop during the summer, which is the time crossbills begin their nomadic wanderings in search of large developing cone crops. In our area, crossbills typically show up in autumn during years with abundant cones, but they can arrive as early as August and remain as late as April if the cone crop is really good. During these "irruptions," crossbills will nest and breed at any time of year, even in mid-winter - the availability of food apparently being the driving force behind when and where they nest.
Steven D. Faccio is a conservation biologist at the Vermont Center for Ecostudies; he lives in Strafford, Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org.
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