Seed selection by crossbills Loxia spp. within cones of Scots Pine Pinus sylvestris

Abstract

Why foraging animals sometimes leave patches before consuming all available food items is a key question in behavioural ecology. Abandoning some food would appear to be disadvantageous, yet optimal foraging theory demonstrates that this sometimes optimises energy intake rates. Crossbills Loxia are specialist avian granivores that forage on seeds within the cones of many species of conifer in the northern hemisphere. They often abandon a few seeds within cones they have fed upon. We assessed whether seeds left within Scots Pine Pinus sylvestris cones by foraging crossbills were of lower mass or in particular positions within the cone. Crossbills foraged on long cones which have more seeds per cone and heavier seeds, but avoided the longest cones, which typically have thicker scales that require more energy to open. Cones dropped by foraging crossbills contained over four seeds per cone, representing approximately one fifth of those originally present. Crossbills left mostly small or empty seeds (< 2 mg), whereas the average mass of seeds from intact cones sampled from the canopy was 3.6 mg. Infrequently, single seeds of high mass (> 4 mg) were left behind, perhaps mistakenly overlooked during foraging. Such apparent preferential foraging on heavier seeds is probably advantageous, because of the higher energy reward per seed. To directly discriminate between seeds prior to extraction would reduce energy expenditure in foraging. This raises the question of how crossbills could attain this favourable outcome. While cones scales were closed an external cue would be required. After cone scale dehiscence, seeds would be visible to crossbills, allowing them to discriminate visually among seeds and selectively extract heavier seeds, leaving lighter seeds behind within the cone. Dropping cones when few seeds are encountered or as seed mass declines towards the distal scales could be additional components of crossbill foraging strategy.