Winter Fat Storage and Vertical Microenvironmental Gradients: Experimental Test of an Alternative Hypothesis

Cost-benefit optimal body mass models have become a cornerstone of behavioral ecology of the nonbreeding period of birds, and make the prediction that fat will increase with increasing deterioration of feeding conditions. Tests of this prediction have relied on comparing fat stores of birds along a vertical height gradient of resource unpredictability (greater snowfall nearer the ground), and lower fat levels in tree-feeders compared with ground-feeders supported the pre- diction in previous studies. Alternatively, as predation risk is often cited as a cost of fat storage, lower fat stores may be caused by greater predation risk higher in the vertical resource gradient compared with the ground microenvironment. Among three species of tree-feeding birds wintering in south-central Kansas, foraging birds frequently preferred a higher sunflower feeder over a similar lower one, with blind and microenvironmental effects considered indirectly. Interspecific dominance rank was significantly and positively correlated with body size. Social dominants frequently displaced subor- dinates from the higher to the lower feeder. Thus a minimum of fat in tree-feeding species that can be explained by pre- dictable resources (low snowfall), not high costs, underscoring the low benefit to fat in this winter foraging guild. Future resource-based tests of optimal fat models will need to measure both costs and benefits of fat in different winter foraging guilds.