Foraging Habitat Use and Profitability in Tropical Roseate Terns and Sandwich Terns

Abstract

I studied the foraging habitat use of Roseate Terns (Sterna dougallii) and Sand- wich Terns (S. sandvicensis) in southwestern Puerto Rico from 1991 to 1994, and determined foraging success of each species in three distinct habitat types: blue water, inshore shallows, and along reefs. The proportion of Roseate Tern foraging flocks differed significantly among the three habitat types, with over one-half of the flocks occurring in blue water. The pro- portion of Sandwich Tern flocks did not differ significantly among the three habitats. A higher proportion of Roseate Tern (84%) than Sandwich Tern (59%) foraging flocks occurred over schools of predatory fish, and mean flock size of Roseate Terns, but not of Sandwich Terns, was greater in the presence of predatory fish. Capture success (no. successful dives/ no. attempts) of Roseate Terns was highest in shallow-water habitats, when they pirated fish from Brown Pelicans (Pelecanus occidentalis) or used diving pelicans to scare fish to the surface. However, both capture rate (no. fish caught/min) and relative profitability (g fish caught/ min) were highest in blue-water flocks over predatory fish. Foraging success of Sandwich Terns was measured only for shallow-water habitats, and did not differ when foraging with or without pelicans. These results suggest that Roseate Terns in Puerto Rico forage primarily in habitats that result in the highest rate of prey capture. Moreover, Roseate Terns may rely heavily on predatory fish or pelicans to increase prey availability to them during the breeding season, whereas Sandwich Terns rely less on these sources to facilitate prey capture. Received 24 March 1995, accepted 20 June 1995.