Is this a winter for a big winter finch irruption?
Waiting for pine grosbeaks and crossbills
By BILL CHAISSON
Of a Feather
I have never seen a pine grosbeak or either of the crossbills. When you consult range maps in bird guides, it shows the species’s winter ranges as reaching down into the northern U.S., but in fact they only ‘irrupt’ in years during which there is either a shortage of conifer seeds — their primary food — or if there has been a run of good years for conifer seeds that has caused the populations to increase beyond the carrying capacity of the Canadian forests.
Crossbills don’t really have individual breeding territories, nor do they breed during a particular time of the year. They are among the birds, many of them high latitude dwellers, who breed opportunistically. Whenever there is enough food to feed their young, they make nests, lay eggs and raise the next generation.
Pine grosbeaks are more orderly, setting up individual breeding territories and not wandering around in large flocks all year like the crossbills. During the winter they do assemble in groups of five to 15 individuals and forage together.
Different references portray the distribution of the pine grosbeak differently. David Sibley shows the winter range extending down to the Massachusetts/New Hampshire border and across to central New York. Cornell’s allaboutbirds.com site shows the winter range only extending as far south as very northern New Hampshire and the far northern Adirondacks. The difference is perhaps in how they treat the irruptions. Cornell is showing you where you will regularly see the birds in winter, while Sibley is telling folks further south to look sharp, because you may get lucky. The Cornell site has a separate “sightings map” that shows occurrences down into Connecticut and even New Jersey.
Irrupt is not a word we use often. It means to break or burst in forcibly, which is certainly distinct from “erupt,” and it has a more specialized meaning applied to animal populations that suddenly increase in numbers and consequently surge into surrounding regions.
Several northern bird species exhibit this pattern. In addition to the crossbills and pine grosbeak, other finches like pine siskins, evening grosbeaks, and the redpolls do it as well. Perhaps more dramatically, snowy owls irrupt southward in some years, occasionally even showing up in the Carolinas or Georgia.
My first experience with the phenomenon was the sudden appearance of evening grosbeaks at my feeders in the mid Hudson Valley in the early 1970s. They had been classified as a western species before that time but are now firmly established in the eastern U.S., even having observable differences between the eastern and western populations. The eastern birds have smaller beaks and a wider yellow band that wraps around the front of the head and over the eyes.
Like the crossbills, the evening grosbeaks tend to gather in flocks and wander erratically. It is perhaps because of this erratic group behavior that they segregate into separate breeding populations and their appearance and behavior begin to differ. The nine distinct populations of red crossbill have been identified based on their flight calls. Pine grosbeaks are also subdivided by flight call and groups with different calls have been observed to remain separated at bird feeders.
Students of evolutionary biology will recognize this as an instance of allopatry. When populations are physically separated from one another — in the cases of the evening grosbeak and red crossbill, by ceaseless wandering in flocks — any sort of mutation, like a new flight call, that arises in that smaller group will tend to be preserved and passed on to the next generation. When the groups encounter one another again, the new behavior or appearance may lead to a tendency for more similar birds to mate and for the difference to be preserved. This is one road to creation of a new species. Of course, the difference may not significant enough to prevent random mating between two populations and they may blend together again, like a river that splits into two channels at the upstream end of an island and then becomes one current again at the downstream end.
In spite of sharing the vernacular name “grosbeak,” the evening and pine grosbeaks are not closely related. They are in the same subfamily within the finches, Carduelinae, but are in separate genera. Although “gross” in modern German means “large,” the modern German word for a grosbeak is “kernbeißer,” which means “seed biter.” The German name for the bird describes their behavior well in that they are primarily vegetarians, although they will forage for insects in order to feed them to their young. The English vernacular name is derived from the French “gros” or thick and “bec” or beak. (The German word for beak is “schnabel” like the surname of the painter and filmmaker, Julian Schnabel.)
The closely related (they interbreed) rose-breasted and black-headed grosbeaks, which is not associated with coniferous forests, are in a separate family, the Cardinalidae, with the blue grosbeak, with the latter in a different genus from the other two. This is a splendid example of how misleading vernacular names can be. Many bird watchers do not delve into avian systematics and thereby get to know the Linnaean names (binomial nomenclature), but it is a fascinating topic, especially since the advent of molecular techniques that trace relatedness through DNA.
The grosbeaks, as their name insists, all have thick beaks. But their lack of close genetic relatedness means that this is an example of convergent evolution, the tendency of species to develop similar attributes to do the same job. The job in this case is to break open seeds. The short torque arm of a high conical beak allows the birds to bring to bear more force on their meals than other finches. Conical beaks are a general characteristic of finches, but sparrows for example, are smaller birds than grosbeaks with narrower bills.
Crossbills, as their name suggests, have mandibles that actually overlap. This is an adaptation to extracting seeds from cones on spruce, fir and pine trees. The various types of red crossbills mentioned above, in addition to having different flight calls, are also associated with foraging on particular tree species. Type 1 is the subspecies Loxia curvirostra neogaea, which forages on white pine and white spruce. The same subspecies in the western U.S. has a different flight call and is associated with Douglas fir.
Now that we have experienced our first snowstorm of the year, I am thinking more about these northern species, hoping that this will be a “good year” for them. I am, however, taking the advice of the Fish & Game and refraining from putting out bird feeders. It was a poor foraging year for bears, so they are still about, looking for something to eat. I don’t know what kind of foraging year it was for northern finches, but I guess we’ll soon find out.