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.
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.
During the summer of 1938 Crossbills, Pine Siskins and Cassin's Purple Finches were seed frequenting cliffs of andesite and pumice. In all instances the birds were seen to fly only to those parts of the cliffs coated with a soft, white to pink powdery crust of calcium salts not more than one-sixteenth of an inch in thickness. Certain parts of the rock exposed in the large road cut on the west side of Watchman Peak and the pumice cliffs exposed along Sand Creek are well supplied with such crusts. These crusted portions of the cliffs were regular forage grounds for the members of the finch family, apparently requiring salts in their diet. The crossbills visited the cliffs more than the other two species mentioned, and were more easily watched. Approaches were made to within twenty feet of the foraging birds their actions being observed for some time. Occasionally the flocks were diluted with one or two Siskins or Purple Finches, but because of their greater wariness comparisons of their salt-gathering habits were not made. Sometimes the crossbills would fly back and forth to the forage cliffs from a convenient perching tree nearby. At other times the flock would remain on the cliffs for the entire stay which would terminate with a flight out of the region. It may be possible, with additional observations, to correlate the differences of feeding habits with the nature of the cliff face and character of the salty crusts. Flights to and from rough cliffs which apparently afford the most suitable perching places occur less frequently. In all types of feeding the process seems to be a slow one with deliberate movements predominating.
On nearly vertical and relatively smooth cliff faces two types of feeding positions were observed. By far the most frequent position assumed was that resembling a woodpecker. The birds would fly directly to the cliff face and with some difficulty gain a foothold with the claws, and with depressed tail sustain a vertical position on or beside a white salt patch. Usually a pause of a few seconds was made prior to feeding and then the slow methodical movements were begun. Since they can pick up nothing with the tips of the bill the procedure was to place the head sidewards to the rock face, open the bill very wide and slowly move the tongue in and out, allowing the side of the tongue to lick off the salty crust. The tongue was distinctly reddish in bright sunlight and seemed unusually large for bird of this size. Such a use of the tongue is probably necessary in order to manipulate certain foods under the handicap of the crossed mandibles. Frequently the birds were seen to loosen the salt crusts by picking at them and follow with the usual licking process.
Another salt-foraging position observed as like that of a nuthatch, that is upside down. This position in all instances was seen to result from a pivot made from the regular woodpecker position. One bird in the nuthatch position was seen to have its lower mandible inserted in a notch in the rock to prevent slipping down head first, while all the time the fleshy tongue was at work.
Further study may indicate how much the location of suitable salt-forage grounds limits the range of this interesting species, and to what extent the physiology of the crossbill makes the feeding of salts imperative.
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