The effect of within-year variation in acorn crop size on seed harvesting by avian hoarders

Abstract

Spatial and temporal variation in resource distribution affect the movement and foraging behavior of many animals. In the case of animal-dispersed trees, numerous studies have addressed masting—the synchronized variation in seed production between years—but the fitness consequences of spatial variation in seed production within a year are unclear. We investigated the effects of variable acorn production in a population of valley oaks (Quercus lobata) on the composition and behavior of the avian-disperser community. We found that western scrub-jays (Aphelocoma californica), high-quality dispersers that store seeds in the ground, were attracted to, and exhibited increased per capita dispersal rates from, trees with large acorn crops. In contrast, acorn woodpeckers (Melanerpes formicivorus), low-quality dispersers that store acorns in trees where they are unlikely to germinate, increased per capita hoarding rates but did not attend trees with large seed crops in higher numbers, suggesting that the two species responded to resources on different spatial scales. Antagonistic interactions within and between species increased with the number of birds attending a tree, resulting in a potential cost for foraging birds, but did not reduce dispersal rates. Using a simulation model, we estimated that trees with large initial crops experienced a greater proportion (77 %) of high-quality seed dispersal events than trees with small crops (62 %). Our findings provide support for a mechanistic link between seed production and foraging behavior of seed dispersers as predicted by the predator dispersal hypothesis for the functional consequences of variable seed production in hoarder-dispersed trees.