Behavior of the Swallow-Tailed Gull of the Galápagos

Abstract

TWO species of gulls breed in the Galapagos, and both are peculiar to the archipelago. One of them, Larus fuliginosus, is apparently quite closely related to Larus modestus of the Peru-vian coast (Murphy 1936); but the other, the Swallow-tailed or Fork-tailed Gull, Creagmcs furcatus, differs strikingly from all other gulls in a number of ways, and its separation in a monotypic genus has been almost unanimously accepted. plumage (except dependent young) are seen at the breeding colonies. Creagrus is a rather large gull, with long pointed wings, long forked tail, and short tarsus, being in these respects more ternlike than other gulls (table 1). Unlike a tern, it has large webbed feet well adapted for swim-ming. Its beak is long and strong. The body plumage is white, with a pale gray mantle, edged with white along the scapulars. The wings have a conspicuous gray, white, and black pattern. In nuptial plumage the hood is dark gray and contrasts sharply with a con-spicuous patch of white feathers at the base of the beak, which is black with a pale-gray tip. The gape and eye-ring are vermilion, and the legs a more pinkish red. Swallow-tailed Gulls are social breeders, although the distance between nests varies widely. Eggs are laid in all months of the year, the reproductive cycle of the individual bird being usually 9 to 10 months (Snow and Snow 1967). A single egg is laid, and the in-cubation and fledging periods are unusually long (33-34 days and about 100 days, respec-tively). The nest site is typically on more-or-less broken cliffs, and Hailman (1965) has shown that the Swallow-tailed Gull shares with the Kittiwake, Ri.ssa tridactyla, some fea-tures that Cullen (1957) showed were adapta-tions to cliff-nesting. The calls and displays of Creagrus, which form the subject of this paper, are also un-usual. Some are so different from those of other gulls that in the following account they are given their own descriptive names. The terminology of Tinbergen (1959) has been used only for a few calls and movements that seem to be clearly homologous with those of other gulls. Creagrus is a pelagic, nocturnal feeder on small fish and squid. [A total of 49 food items were identified during the study, of which 37 were fishes (clupeoids) and 12 were squids (Symplectoteuthis ouulaniensis) . In Febru-ary-July, 34 of 37 food items were fishes, whereas in November, 9 of 11 were squids. Hence there may be important seasonal vari-ations in diet. Hailman (1964b) concluded that squid were the main food, but his obser-vations were made in November and Decem-ber only.] When it is not breeding it is totally pelagic, migrating eastward to the coasts of Ecuador and Peru. No birds in immature