Resource Use, Energetic Profitability, and Behavioral Decisions in Migrant Rufous Hummingbirds

Migrant Rufous Hummingbirds (Selasphorus rufus) defend nectar resources at stopover sites while replenishing fat reserves needed for migratory flights. During late summer in the Sandia Mountains, central New Mexico, they defend the wasp- and bee-pollinated Scrophularia montana from other hummingbirds. Both hummingbirds and hymenopterans exploit Scrophularia nectar during the early part of its flowering period. As summer colony growth increases the densities of the eusocial hymenopterans by 100-150%, their exploitation of Scrophularia nectar lowers its mean standing crop in flowers by 200-300%. Sometime during the summer, Rufous Hummingbirds abandon and do not further use this resource for the remaining 3-4 weeks of its flowering period. The abandonment always occurs when the mean standing crop of nectar is approximately 0.2-0.3 μL/ flower. This paper describes a model of Rufous Hummingbird energetics, that shows abandonment occurred 1-3 days after they passed the threshold at which the resource could have provided their minimum daily energy requirements. I suggest that constraints imposed by a highly competitive social environment severely reduced the options available to the hummingbirds, and caused them to continue to defend a resource that could no longer meet their energetic requirements.