Salt-Feeding Habits
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.
