Notes on the Sierra Crossbill
By Elmer C. Aldrich, Ranger Naturalist, 1938
During the latter part of July or the first part of August the highly vagarious Sierra Crossbill (Loxia curvirostra bendirei) are conspicuous in Crater Lake National Park. They are to be heard or seen in great numbers nearly everywhere within the boundaries. The occurrence of these birds at the rim of Crater Lake seems to be coincident with the production of mature cones of the White-bark Pines and Mountain Hemlocks.
Though they are usually seen in small flocks up to about fifteen in number, occasionally a single individual may be seen flying, giving with each undulation of its finch-like flight a pair of staccato notes (chup-chup). When flying in flocks the notes are given as when flying singly, but seemingly not so loud. Undoubtedly, at this time of the year, there is a desire in the crossbills to flock, and the staccato notes may serve either to attract other individuals to join the flight or serve to keep the already formed flock together. Flocks may be seen to start off without any provocation visible to the observer, continue loosely in an aimless, erratic course, and either fly out of sight or suddenly circle above a group of trees, and then settle as quickly as the flight began. The stay may last several minutes while feeding ensues, or the flock may stop only momentarily and then individuals strike up a chatter consisting of the ordinary notes but given in faster succession. This seems to be the signal for another flight of the same type. When a flock has descended into a tall coniferous tree, the individuals can be seen only with difficulty because the birds take up positions which are usually well within the foliage. Frequently, however, one of the flock may remain perched on the topmost branch and continue giving the notes as though on guard. Usually the others are less noisy while foraging. Though there is little evidence that the crossbills nest in the park, the writer is of the opinion that they nest fairly commonly but are not obvious due to their remaining singly or in pairs, and not giving the staccato flocking note which brings them to the observer’s attention later in the season. On one occasion the writer (August 26) observed a yellowish crossbill high in a tree feeding a young one that was streaked and that could fly excellently.
Description
Sierra Crossbills, if approached cautiously, are confiding and can be observed easily, especially if they are feeding or drinking. One will immediately notice that members of a flock vary greatly in coloration and markings. One has some difficulty in finding two individuals very much alike. The young of the year may look totally gray at a distance but on closer examination they are found to be streaked with dirty white and gray, making them appear similar to the immature and female Cassin’s Purple Finch which are so common in this region. The streaking of the crossbills is finer and more irregular than that of the purple finch which gives the former a more variegated appearance. The adults are more brilliantly colored on the head and rump than the young, and the colors of these parts may be seen as lemon-yellow, greenish-yellow, yellow-green, orange, rose-red and scarlet. Some ornithologists have attributed these variations in color of individuals to differences in age, but this fact is yet to be proven.
In form, the crossbill, about six inches in length, presents a stockier build than that of the purple finch, having a relatively shorter tail that is more deeply forked. The head is relatively larger, probably serving to support the massive, specialized bill structure which certainly is the most distinct feature of the crossbill. The tips of the mandibles do not meet but cross near the ends, not always in the same direction, but seemingly indiscriminately in the different individuals as observed in flocks. Though it is generally believed that the bill structure is advantageous since the crossbill lives on a diet of sees obtained from the cones of conifers, from observations it seems apparent that their ability to grasp small objects with the tips of the bill has some disadvantages. It is this crossing character of the bill in relation to what are apparently peculiar feeding habits that prompted observations recorded in these notes.
