Do scatter hoarders trade off increased predation risks for lower rates of cache pilferage?

Scatter hoarding is a common food-hoarding strategy of many granivores, known to significantly contribute to seed dispersal. Models of scatter hoarding typically hold that the spatial distribution of the scatter hoards made by birds and mammals result from a trade-off between the energetic cost of spacing caches and the increased risk of pilferage as cache densities increase. Here, we present evidence from 3 field experiments that eastern gray squirrels (Sciurus carolinensis) rely on an alternative strategy in which preferred food items are stored in open habitats, beyond tree crowns, where the probability of predation is higher but the risk of cache pilferage is also reduced (hereafter the habitat structure hypothesis). In the first 2 experiments, squirrels cached larger, more profitable acorns at significantly greater distances from canopy edges than smaller acorns, which were more often cached under trees. In the third experiment, in which we tested the effects of both tree canopy cover and cache density on pilferage rates, we found no effect of density on pilferage rates but significantly higher pilferage rates under canopy cover. Our results indicate that habitat heterogeneity, potentially as it relates to predation risks, influences the distribution of scatter-hoarded seeds, with more profitable seeds placed in more open vegetation beyond the shadow of parent plants. We suggest that this behavior may reflect a general strategy of other scatter-hoarding mammals and birds that likely increases the probability of dispersal and establishment of seeds.