Hierarchical Mechanisms of Fruit Selection by an Avian Frugivore

To examine behavioral mechanisms of fruit selection, American Robins (Turdus migratorius) were studied foraging on Crataegus monogyna fruits in western Or- egon. Robins faced a hierarchical decision-making process of fruit selection: (1) in which shrub to feed (decisions among shrubs); (2) which fruits to pick (visual decisions among fruits); and (3) which fruits to swallow once picked (within-bill decisions). Field observations of natural and experimentally manipulated populations of C. monogyna shrubs demon- strated that choices made by robins among shrubs were correlated with three plant traits (decision cues): fruit abundance, fruit size, and fruit pulpiness. These cues were themselves used hierarchically, so that when higher-ranking cues were held constant by manipulating shrubs, the next available cue became important. Once shrubs were selected, field and aviary work revealed that robins then made visual choices among fruits correlated with fruit size. However, no evidence was found to suggest that robins made within-bill decisions; once fruits were picked, additional discrimination did not occur. I suggest that a hierarchical process of fruit selection may have evolved to enable frugivores to cope with environmental uncertainty that is typical of fruiting systems. Such flexibility in foraging behavior may, in itself, promote variability within and among fruiting plants and therefore help to explain why so few general patterns have been identified in fruiting plant-frugivore relationships.