Competition and Predation: Herring Gulls versus Laughing Gulls

Abstract

Gulls of the genus Larus generally nest colonially, often in mixed-species assemblages. For example, Herring Gulls (L. argentatus) in Europe nest with Lesser Blackbacked Gulls (L. fuscus; Brown 1967, MacRoberts and MacRoberts 1972) and Black-headed Gulls (L. ridibundus; Greenhalgh 1974); and in North America Ringbilled Gulls (L. delawarensis) nest with California Gulls (L. californicus; Vermeer 1970) and Herring Gulls (Southern 1970). It is often difficult to determine whether such assemblages are actually mixed, or if monospecific colonies simply exist side by side. When assemblages are mixed it is often impossible to ascertain whether they are so by choice or because of a lack of suitable nest sites. The rate of competition and predation by one or both species has not been investigated. In this paper I examine competitive interactions and predation between Laughing Gulls (L. atricilla) and Herring Gulls on Clam Island, New Jersey, from 1976 through 1978. Laughing Gulls on the east coast of the United States traditionally nest in Spartina salt marshes (Bent 1921), although at the limits of their range they frequently nest on dry land (Nisbet 1971, Dinsmore and Schreiber 1974). Thus, when Herring Gulls expanded southward in the early 1900's into Maine and Massachusetts, they came into direct conflict with Laughing Gulls (Nisbet 1971, Drury and Kadlec 1974). As early as 1943, Herring Gulls nested with Laughing Gulls in a sand dune colony at Muskeget Island, Massachusetts (Noble and Wurm 1943). Despite initial differences in habitat, the Laughing Gulls disappeared (Nisbet, pers. comm.). Increases in Herring Gulls in the northeast also have been associated with decreases in Common Terns (Sterna hirundo; Nisbet 1973). In the 1950's and 1960's Herring Gulls again expanded their breeding range, nesting as far south as North Carolina with Laughing Gulls (Hailman 1963, Parnell and Soots 1975). They nested on the upper parts of domes on man-made islands while Laughing Gulls occupied low swales between the domes. Where Herring Gulls were absent, Laughing Gulls nested farther up the domes (Parnell and Soots 1975). When Herring Gulls began breeding in more southern areas, they chose to nest in salt marshes, the customary nesting areas of the large and stable Laughing Gull colonies (Burger 1977). Although differences in nest site selection were noted initially, these differences decreased as Herring Gull colonies expanded. Overlap in colony and nest site characteristics is obvious in several New Jersey colonies (Burger and Shisler 1978). Since the first report of Herring Gulls on Clam Island (Rogers 1965), the colony has increased from 50 to 1,200 pairs whereas the Laughing Gull colony of 4,000 to 6,000 pairs remained about the same size through 1976. My primary objectives in this paper are to examine how these species interact temporally and behaviorally, and to determine the result of these interactions in terms of nesting space allocation and reproductive suc-