This week in the woods, we heard the sharp, harsh jip calls from a gregarious flock of red crossbills and watched them alight on this red spruce and begin to forage. While a medieval European legend held that the bird twisted its bill trying to extract the nails holding Christ to the cross (and turned red from his blood), the heavy mandibles with crossed tips evolved naturally to extract conifer seeds – the bird’s near-exclusive source of nutrition. This dependence determines a great deal for the bird, from physiology to migration to reproduction.
Species with less specialized bills can eat conifer seeds only after the cones have opened, but our two crossbill species (red and white-winged) can reach them even in closed cones and thus take advantage of them earlier in the year. When the crossbill bites a cone, it spreads apart woody scales with its separating mandible tips, exposes a seed at the base of a scale, and uses its tongue to fish out the seed. The crossbill husks the seed using the back, aligned parts of its bill and swallows the kernel whole.
A single crossbill can remove all seeds from a cone in minutes, starting at the tip (the part with the most seeds) and spiraling around toward the base. Depending on their bill tips’ orientation, crossbills can be left- or right-billed, which determines the direction of their spiral movement; as with the left-billed male pictured, a crossbill’s lower mandible must cross toward the cone in order to tweeze out the seed.
The conifers that produce crossbills’ limited diet – white pine, spruces, eastern hemlock, and larch for the bird populations we see – tend to produce cones in boom-and-bust cycles; synchronously within a species, these trees produce fewer seeds some years and more seeds in mast years than wildlife can consume. When cone crops dwindle in crossbills’ usual boreal forest residence in Alaska and Canada or breeding success pushes populations beyond what those northern landscapes can sustain, the birds can wander up to 2,000 miles per year searching for cone-bearing trees. In such irregular cases (called irruptions), we in New England – and locales as far south as Mexico – host these nomadic visitors. (For more on predicting irruptions of red crossbills and other winter finches, see Jack Beaudoin’s recent Outside Story article.)
As Bryan Pfeiffer writes in the Winter 2015 issue of Northern Woodlands, day length triggers hormone production for breeding in virtually all other songbirds. The longer, warmer spring days coincide with a burst of food availability, especially of insects, which most small bird species’ young require. However, food availability itself prompts breeding for crossbills, and their young thrive without eating invertebrates. Access to abundant cones means crossbills can get the calories essential for egg production and regurgitate enough partially digested seeds for their young. Because of seeds’ unpredictable availability, crossbills must build nests and breed at any time – even in the depths of winter. Observers have found their nests in every month of the year.
What have you noticed in the woods this week? Submit a recent photo for possible inclusion in our monthly online Reader Photo Gallery.