Foraging habits of the Yellow-bellied Sapsucker, Sphyrapicus varius.

Abstract

The fitness of an organism is determined by its reproductive success. Proteins, carbohydrates, and fats are required components of all organisms' diets if they are to reproduce. Yellow-bellied Sapsuckers, Sphyrapicus varius, derive the bulk of their carbohydrates from sap they extract by drilling into the bark of trees. Sapsuckers derive the bulk of their proteins from insects they prey upon while in the air and while on the bark of trees. No known experiments have been performed to test whether Sapsuckers will prey upon insects at sapwells. Optimal foraging theory suggests that Sapsuckers should forage for insects at sapwells as the potential gains in nutrition outweigh the minimal costs in search and handling times. Our experiment addresses whether sapsuckers will forage for insects at sapwells, and if they do, whether they will show a preference among their prey. Insectivorous foraging at sapwells was tested by pinning eight different types of insects from the orders Coleoptera, Hymenoptera, and Lepidoptera at active sapwell sites at three different trees at the University of Michigan Biological Station. Prey preference by sapsuckers was tested by examining the disappearance rates of insects based upon total nitrogen content, percent nitrogen per unit weight, size (as a function of weight and length), coloration, and taxonomical order. Fifty-seven insects were taken from the sapwell sites; our analysis of prey preference relies upon the assumption that sapsuckers ate the missing insects. A chi-squared test of the insect disappearance rates revealed that significant sapsucker foraging preferences do exist. By running regression analyses on the data, we found positive correlations between disappearance rates, total nitrogen content, and weight of insects. The results of our experiment indicate that yellow-bellied sapsuckers will forage for insects at sapwells, and that the birds will show preferences for larger insects and insects with higher total nitrogen contents.