Foraging patterns of common crossbills (Loxia curvirostra) on spruces (Picea spp.) in Scotland

Abstract

Common crossbills forage on Sitka spruce and Norway spruce when the cones reach full size in late summer until the seeds have shed in the following spring. Sitka spruce start shedding seed in autumn, thereby reducing the food supply for crossbills through autumn to spring, whereas Norway spruce retain their seeds until spring. Crossbills forage on cones that are attached to the tree (more so for Norway spruces) or are removed from the tree by the crossbills before feeding. The latter cones are then dropped after seed extraction. The mean length of dropped Sitka spruce cones was 72 mm, close to the average length in a widespread survey. This suggests little size selection. The mean length of dropped Norway spruce cones was 90 mm, which was much smaller than the average length in the survey. Although used, large cones are probably left attached to trees because they are too heavy to handle if cut from the tree. The average percentage of seeds extracted before a cone was dropped was 45% for Sitka spruce and 42% for Norway spruce. There was a trend for fewer Sitka spruce seeds to be taken per cone across the period between autumn and spring. Incomplete extraction of seeds suggests that cones become unprofitable as the seed number is depleted in cones and that the threshold for switching cones changes as the number of seeds per cone declines.