Comparative Feeding Ecology of Wintering Abult and Juvenile Royal Terns (Aves: Laridae, Sterninae)

Comparison of the feeding behavior of adult and juvenile Royal Terns, Sterna maxima, showed the following: adults spent less time foraging over a given stretch of beach, making almost twice as many dives per minute as juveniles; while initial prey capture rates were the same on both age—classes, adult dived more frequently, and therefore caught more prey per unit time; both rarely dropped their prey without recovering it, but juveniles dropped their prey, subsequently recovering it, almost 14 times as often as adults, thereby expending considerably more energy per prey capture than adults. Analyses of counts of sitting and feeding terns confirmed predicted nonrandom associations: feeding adults avoided all other birds except their offspring, but, when resting, avoided even juveniles more often than expected; feeding juveniles occurred almost equally frequently alone, with adults, and in small flocks (adults never so occurred). An attempted time—budget analysis, limited by unmarked birds, suggests that adults feed more in the early morning hours, resting or foraging for their young later in the day; juveniles apparently do most of their solo fishing between 0900 and 1200 h, and in general spend less day time roosting than do adults. Analysis of data on recovered, banded terns indicate that significantly more juveniles than adults are caught on fishhooks and that adults' ability to recognize capturable prey is a prime factor in their fishing success. These results confirm Lack's and Ashmole's basic hypothesis that extended parental care and reduced feeding ability of young have played a significant role in the evolution of seabird clutch size. Considered in terms of the Holling—Griffiths—Watt "components of predation" analysis, our data suggest that these foraging differences must be incorporated in future predatory—prey models. The component analysis approach ought to be extended to other terms, especially over longer periods of time post—fledging. Salt and Willard's conclusions from their component analysis of the foraging behavior of Forster's Tern, reexamined in the light of our findings, lead us to believe that insufficient data are available to decide whether Forster's and Royal Terns better fit Holling—Griffiths—Watt or Lotka—Volterra predator—prey models, because these models fail to consider important aspects of the terns' biology and because certain data needed for their use were not obtained in either study.