Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [14]
is a skill for which all the guide books, all the geographical check lists and life histories, and all the learned volumes on ornithology are of little help. The nest finder must go out into the fields and woods with his wits sharpened to a razor’s edge, with all his senses tuned to their highest pitch, and with his mind free from the distractions and preoccupations that burden the society he has temporarily left behind. His consciousness must be focused on the world outside himself, in which he must move without self-awareness. If he succeeds in attaining this rapport with nature, all creatures, as Thoreau said, will rush to make their report to him. He will learn who his companions are, where they are, and what they are about. All their activities will be as shouted declarations, and no secrets will be kept from him.
My expectations are modest. I go with the flow, but the start of my journey, as in any of scientific discovery, starts from precedent. I had seen the birds previously in December and then again in February, and I hoped to see them again now, in March. In December they were still traveling and feeding in flocks, so nesting had not yet started, but by mid-February the raspberry pink males had left their flocks and were singing their loud, musical warbles and trills, while the golden-brown females hopped unobtrusively in the spruce branches nearby. There were mutual chases; the flocks had disbanded and breeding was about to begin. However, I saw no nest-building, and the presence of females out of the nest meant that eggs had not yet been laid. Given that it takes about a week to build a nest, and three or four days to lay three to four eggs, plus about two weeks of incubation, I calculated that now, in mid-March, they should be close to finishing with incubation.
Crossbills raise their young when the seeds of spruce or pine cones are most plentifully available. This often requires them to lay their eggs in the winter. Nests with eggs have been found in New Brunswick, Canada, in the middle of January, and in February near Calais, Maine (Smith 1949). Crossbills are reputed to breed at almost any time of the year depending on any of a variety of different kinds of cone seeds they may find. In contrast, the crossbill’s relatives in the family of finches to which they belong, our goldfinches as well as European goldfinches, are the latest-breeding birds; they delay breeding until August, when their seeds (thistle seeds) are ripening.
Crossbill prying apart cone bracts to reach seeds.
Finches are strikingly colorful birds, but perhaps even more amazing is their unusual and varied bill morphology, adapted for extracting thistle seed, cracking cherry pits, or prying seeds from under stiff cone bracts. Crossbills’ bills look misshapen, as from some developmental defect. Their long and slender (for a finch) two-centimeter-long upper bill crosses over a one-half-centimeter-shorter lower bill. By inserting their partially open bill under a cone bract and then closing the bill, the bill-tips separate by about 3 millimeters, applying strong leverage laterally so that the bract is pried away from the cone. The seeds under it can then be reached with the tongue. Given their unique bill structure, adapted for extracting the seeds from pine and spruce cones; their wide wanderings over the continent in search of seeding conifers; and their specific timing as to when they nest, crossbills are consummate conifer specialists