EFFICIENCY-MAXIMIZING FLIGHT SPEEDS IN PARENT BLACK TERNS

Optimal foraging theory assumes that individuals maximizing a given en ergy currency maximize their fitness. Though models with different cur rencies (and assumptions) have been successful at describing forager b ehavior, discriminating among currencies has proven difficult because models optimizing different currencies often make similar predictions. A field experiment was designed to use flight speeds of Black Terns ( Chlidonias nigra) feeding their young to test predictions from optimal foraging models that maximize (1) the net energy gained per unit of e nergy expended (efficiency; EFF), (2) the net rate of energy intake (N REI), and (3) the daily delivery rate (DR), respectively. Manipulating the distance an adult had to fly between a feeding enclosure and its nest enabled us to discriminate among the three currencies. Predicted speeds of the EFF model best matched the observed flight speeds. Maxim izing efficiency resulted in the lowest delivery rate to the nest, but it was also the only currency that predicted adult daily energy expen ditures within the theoretical limit (DEE(max)) estimated for Black Te rns (206 kJ/d; Kirkwood 1983). The feeding enclosure represented an un usually profitable food source for the terns, and this may have result ed in artificially high predictions for flight speed, particularly for the NREI and DR currencies. We therefore made another set of predicti ons from the three models using estimates of foraging success from par ents who had never foraged from the feeding enclosure, and compared th ese predictions with the measured airspeeds. The EFF model was apain t he best predictor of flight speeds even though none of the three model s predicted energy expenditures in excess of DEE(max). Though the DR m odel resulted in the highest total daily energy delivered to the nest, this represented only a small gain relative to the NREI and EFF model s. Black Tem parents may maximize EFF because this currency is least l ikely to result in daily energy expenditures that exceed their metabol ic limit.