Hummingbird isolegs in an experimental system

Optimal foraging theory was first looked upon as a tool to study the evolution of niches in community ecology. Isoleg theory is being developed to reestablish it as such a tool. Isoleg theories are maps of isolegs in a graph whose axes are population densities. There are two kinds of isolegs: some are lines of equal optimal behavior in the graph; others mark threshold combinations of densities past which sudden shifts in behavior should occur. A technique for determining whether isolegs exist is described and applied to hummingbird data. These data were collected experimentally in the field expressly to test one isoleg model. All three species of hummingbird exhibited at least one of the sudden-shift type of isoleg. Their behaviors map onto the density graph in the predicted portions of the graph with only one exception. The data also support the prediction that behavior in the field is disjunct, i.e. subject to substantial, abrupt, discontinuous changes produced by very small continuous changes in a control variable. Some evidence for continuous control was also found, but it is ambiguous. Theory predicts that the two forms of control should be found together in some optimal systems.