Nest Site Selection by Laughing Gulls: Comparison of Tropical Colonies (Culebra, Puerto Rico) with Temperate Colonies (New Jersey)

We examined nest site selection in Laughing Gulls (Larus atricilla) nesting on three islands in the Culebra National Wildlife Refuge, Puerto Rico, to determine if gull nest sites showed consistent characteristics differing from most of the available habitat, to compare nest site characteristics on islands with different vegetative cover, and to contrast nest site choices on these tropical island colonies with colonies that we have studied on the northern Atlantic coast of mainland United States. At Culebra, Laughing Gulls nested with Royal, Sandwich, and Bridled terns (Sterna maxima, S. sandvicensis, S. anaethetus), Brown Noddies (Anous stolidus), and Brown Boobies (Sula leucogaster), but their nearest neighbors were usually conspecifics. The gulls' nest sites generally differed from random points with respect to percent cover, vegetation height, visibility, and distance from clearings. Compared to the random points, gull nests were under denser vegetation of intermediate height with high visibility from their nests and with less cover around their nests. Their choice of nest sites provided protection and cover from predators and the sun. Dense and tall vegetation would hinder gulls from flying directly from their nests, and many nests were thus near rocks or clearings, which allowed the gulls to exit quickly. Laughing Gulls nesting on the Culebran cays generally nested in taller, denser vegetation than those nesting farther north along the Atlantic coast, which partially reflects temperature differences between the localities. The process of nest site selection involves choosing one site from those available within a bird's territory. Choice of site may be influenced by the proximity of aggressive neighbors, by a bird's previous nest site, and by physiognomic aspects of the habitat. Groundnesting birds may strongly prefer a specific substrate, plant species, or growth form, and may have a restricted range of acceptable slope, visibility, and cover. Alternatively, they may be extremely tolerant and scarcely prefer one spot over another. Thus, demonstration that nest site selection involves choices must contrast the actual nest sites with other sites that would be available. Larus gulls are largely temperate-zone species, and many species nest exclusively in north temperate and low arctic regions. The breeding range of a few species includes both temperate and tropical regions. Presumably, breeding adaptations, particularly those relating to nest site selection, vary in response to climatic variables. The need for protection against the heat of the noonday sun might cause gulls to choose sites in dense cover. On the other hand, potential predation in temperate as well as tropical zones should cause nests to be placed either so that adults can watch for predators and easily escape, or so that eggs and young are inconspicuous. Laughing Gulls (Larus atricilla) nest along t e Atlantic coast ofNorth America from Massachusetts to Florida, along the Gulf coast of Texas, and on some Caribbean islands (AOU 1983). They nest on sandy beaches or islands (Bent 1921, Nisbet 1971), as well as on Spartina salt marshes in New Jersey and New York (Bongiorno 1970, Burger and Shisler 1978, Buckley et al. 1978). Although nest and colony site selection by Laughing Gulls have been examined in salt marshes in New Jersey (Bongiorno 1970, Montevecchi 1978), few quantitative data exist from dry land colonies of Laughing Gulls. Physiognomically, Spartina salt marshes are relatively homogeneous, with little variation in the height and density of the grass. Laughing Gulls, however, use dry land habitats that vary from open sandy beaches with little grass to more densely vegetated habitats (Nisbet 1971, Di smore and Schreiber 1974). Variations in vegetation on the sites selected may relate to either predation pressures or to weather constraints (see papers in Cody, in press). We examined nest site selection in Laughing Gulls on three small cays off Culebra, Puerto