The Red Crossbill
By David Cronenwett
The winter of 2009-2010, if one can remember, was troublesome in Montana due to mild temperatures and scant mountain snowpack. But then, in mid May, it began to rain, and did so on and off for weeks on end. The extended period of moisture caught us up on precipitation and, at least in North Central Montana where I live, helped produce one of the most impressive conifer cone crops in recent memory. The limbs of Douglas fir, Limber Pine, Engelmann Spruce and others were heavily laden with cones that year, and as birders might guess, this situation can be a boon for several species.
Chickadees, nuthatches, Clark’s Nutcrackers and Pine Siskins can be seen feeding on the bounty hereabouts. But one beast is especially abundant and visible now; the Red Crossbill (Loxia curvirostra). These gregarious birds are included in the family Fringillidae, (the finches) and have remarkable bills obviously adapted to the specific task of dismembering conifer cones. During the summer of 2010 on the Rocky Mountain Front, one could observe large flocks of Crossbills (both White- Winged and Red, often together) absolutely mauling cones across a huge area; in some places, there are cone piles several inches deep, with their telltale open scales, evidence of Crossbill activity. These birds will famously travel great distances seeking abundant crops and exhibit some unusual behaviors like the ability of sub-adults, in juvenile plumage, to mate and raise young in nearly all seasons. This may be an adaptation stemming from the highly nomadic nature of the bird; it must reproduce quickly, before moving on to the next feeding ground.
The most astonishing thing about this creature however, are the several “Types” (1 through 9) that occur across its vast range. Most ornithologists only recognize one of the Types as a distinct species; there simply are not significant enough differences within the other variants to do so. Yet there are differences. Nine variations have been described; the morphological distinctions are minute, mostly reflected in bill size. But each Type nearly always identifies and mates with the same. Also, the Types have strong feeding preferences for particular conifers.
However this behavior can vary and shouldn’t be used as a dependable means of identification. The most reliable way to discern between Types is by flight calls. I have significant difficulty determining the call subtleties between the two Types (4 & 5) that are present in my area. But that summer, I was able spend some field time with my friend David Sibley who was able to point out what to listen for. After significant help, I was able to recognize each; one call slightly ascending, one descending. I stood in amazement at my guide’s uncommon observational powers and the subtlety with which species diversify and express themselves across the landscape. With Red Crossbills, we seem to be witnessing evolutionary incarnation before our eyes; the Types are “species in the making”, a gray area that we find hard to classify, but where Nature seems to frequently and comfortably operate.