Studies from both tropical and temperate systems show that scatter-hoarding rodents selectively disperse larger seeds farther form their source than smaller seeds, and as a result, increase the probability of seedling establishment in these larger seeds. This dispersal pattern is particularly evident in some oaks (Quercus). Here, we predict the opposite pattern for small-seeded oaks dispersed by the Blue Jay (Caynocitta cristata). Blue jays are gape-limited and tend to disperse acorns of pin oaks (Q. palustris), but often carry several acorns in their crop during a single dispersal event. Thus, we predicted that smaller acorns would result in higher prey loading and greater dispersal distances than single acorns carried in the bill. To test this, we presented free-ranging blue jays with pin oak acorns of different sizes (1.8-3.5g) and then monitored the birds with both remote cameras and binoculars from a distance to determine number of acorns taken, whether they were eaten or cached, and the distance acorns were dispersed.
Results/Conclusions
In both years of the study, blue jays engaged in multiple-prey loading significantly more often with small pin oak acorns (48%-73% of trials) than larger acorns (< 20% of trials). Moreover, there was a negative correlation between seed size and dispersal distance indicating that smaller acorns may have a dispersal advantage. We suggest that jays and rodents may exert disruptive selection on acorn size, and in particular, favor dispersal and caching of smaller seeded acorns.
