Crossbills are very unusual songbirds, in that their upper and lower bills are crossed at the tips, instead of coming together in a point. This peculiar configuration is a clever adaptation that allows crossbills to open the cones of conifer trees and extract the seeds.
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Some birds have bills that cross to the right, and others have bills crossed to the left. The crossed bill tips are inserted between two scales of the cone, and the lower bill is moved sideways, prying up the upper scale, so the tongue can extract the seed. Straight-billed seedeaters, such as redpolls or pine siskins, can't do this - they can only exploit cones on which the scales have already opened. Crossbills are so specialized to opening cones and eating conifer seeds that they aren't very efficient at eating other kinds of seeds that are nevertheless sometimes included in the diet.
Because they are so specialized, crossbills really depend on access to large crops of conifer cones. Large, mature conifer trees generally produce larger cone crops than young trees, and they do so more reliably. But even large trees typically do not produce large cone crops every year in any geographic area. This means that crossbills have to move around from region to region, in order to find places that offer good cone crops. As a result, the population size of crossbills in any area is likely to vary a lot from year to year.
In our area, the summer of 2005 was one of tremendous crops of spruce and hemlock cones, and we had hordes of crossbills, but the summer of 2006 offered poor cone crops, and crossbills were few.
In the years when cone crop failure is widespread, crossbills roam widely in search of better foraging, but mortality reportedly can be very high. The birds can rebuild their populations rapidly, however, if they succeed in finding a region of good cone crops. Once they find a good area, each pair of crossbills may be able to rear several broods of chicks in one year. Even birds less than a year old can mate and raise chicks when foraging is good.
The birds are able to judge food availability by their rate of food intake, and they quit trying to breed if intake rates drop too much.
There are two species of crossbill in North America. The more brightly colored white-winged crossbill focuses chiefly on the cones of white and black spruce and tamarack, and it is not usually very common in Southeast.
The more common red crossbill is, arguably, more interesting in an ecological sense. It turns out that there are at least eight types of red crossbill in North America, distinguished by their flight calls, body size, and bill size. Birds of each type apparently breed only with their own kind, and the so-called types may, in fact, be separate species.
Each of these types is specialized to a particular kind of conifer: Each type of bill is adapted to the kind of cone that the birds depend on most heavily during the seasons of food scarcity, which is usually winter and early spring. The size of the bill determines the efficiency of seed harvesting on different kinds of cones. For example, types of crossbills with large bills are most efficient at foraging on pines, a type with a medium bills focuses on Douglas fir, and the small-billed type is most efficient on hemlocks.
Here in Southeast, the common type is a small crossbill with a small bill, i.e. a 'hemlock crossbill." More rarely we have a "Sitka spruce crossbill" and occasionally a "Douglas fir crossbill" from the Pacific Northwest.
Our hemlock crossbill, whose bill is the smallest of all North American red crossbills, is most efficient at opening the tiny cones and extracting the small seeds of hemlocks, although it can forage (less efficiently) on other species too. Their population size in Southeast depends principally on the existence of sizable stands containing large hemlock trees that periodically produce large cone crops.
The reported rarity of the sitka spruce crossbill in Southeast is interesting. It is known that industrial, clearcut logging in the 1950s, '60s, and '70s preferentially harvested stands of large spruces along rivers and in certain other habitats. This clearly must have reduced the availability of large spruce-cone crops that would be the preferred food of the sitka spruce crossbills.
One has to wonder if the relative rarity of these spruce crossbills might have been caused by the massive reduction in their food supply. There is evidence from other crossbill populations that declines in the proportion of the landscape with older forests (and good cone crops) are associated with disproportionately large declines in crossbill populations there. This should send us a cautionary signal about maintaining our regional crossbill populations.
Mary F. Willson is a retired professor of ecology and a Trail Mix board member.