Red Crossbill

Crossbills are perhaps the epitome of an irruptive bird, those species whose numbers vary year to year based on food supplies. While most such species show two- or three-year cycles, those of the Red Crossbill are highly unpredictable with gaps between invasions ranging from a few months to several years. The food supplies tracked by Red Crossbills are the seeds of conifers like pines and spruce, and the eponymous crossed bill is used to extract these seeds from between the cone scales. Once the bill is inserted the bird shifts the lower mandible to the side to spread the scales and obtain the seed. Crossbills can be either left or right “handed” depending on which way the two halves of the bill cross.

Recent research into variation in Red Crossbills has uncovered a dozen “call types” in North America alone. The calls in question are the “jip jip jip” sounds crossbills make in flight and are believed to serve to keep flocks together in search of food. Flight calls vary alongside differences in bill shape that are in turn related to the main type of cone that each population feeds upon. Careful listening and recording of flight calls can thus provide a clue to where the crossbills in an area originated. For example, “Type 2” specializes on the Ponderosa Pine of the Rocky Mountains, but when those are not available it may wander as far east as New England and feed on local species such as red and white pines. But if there’s not a good cone crop here the birds will go somewhere else. As a result, the distribution of Red Crossbills from one year to the next depends both on seed production in their core range and seed production outside of it, leading to no clear pattern of which type will show up where or when.

Because of their reliance on ephemeral but abundant food, Red Crossbills can settle down to nest at any time of year if there are seeds to feed their young. Here in New England, they typically arrive in late summer if there is a good cone crop and raise their young by mid-fall. Then they disappear back into parts unknown until food draws them back. In cases where pine seeds are exceptionally abundant and remain in cones through the winter, some of the birds that bred the previous summer – and their young – will return to breed again in early spring. But when the cones are finally depleted it may be several years before that call type returns, although a different one may wander in from somewhere else in North America to feed on a different species of conifer. Crossbills will always keep us guessing.