HABITAT REQUIREMENTS AND SELECTION BY KELP GULLS (LARUS DOMINICANUS) IN CENTRAL AND NORTHERN PATAGONIA, ARGENTINA

Auk, The, Jan 2004 by Borboroglu, Pablo Garcia, Yorio, Pablo

ABSTRACT.-We analyzed habitat requirements and selection of Kelp Gulls (Larus dominicanus) breeding at 68 colonies along 2,500 km of coastline in Patagonia, Argentina. Kelp Gulls use a wide range of nesting habitats that include areas with shrub, grass, and herbs which provide different degrees of cover; open ground of various substrate types; and slopes of widely different gradients, confirming the plasticity of the species with respect to habitat recorded elsewhere in the Southern Hemisphere. Eighty percent of the colonies were on islands, the probability of occupation of which increased with distance from mainland and availability of vegetation. Vegetation cover was a key component of nesting habitat for Kelp Gulls in Argentina. Gulls occupied areas with higher cover than random areas; but within vegetated areas, they showed preference for sparser cover that increased visibility and facilitated escape. Kelp Gulls often nested with other species of seabirds and seals and had a positive association with the Olrog's Gull (L. atlanticus). Received 22 October 2002, accepted 3 November 2003.

RESUMEN. - Analizamos los requerimientos y la seleccion de habitat de gaviotas cocineras (Larus dominicanus) que reproducen en 68 colonias a Io largo de 2,500 km de costa en Patagonia, Argentina. La gaviota cocinera utiliza una amplia variedad de ambientes de nidificacion que incluyen areas con arbustos, pastos y hierbas que proporcionan diferentes grados de cobertura; sustratos de diferente composicion y pendiente. Esto confirma la gran plasticidad en cuanto a la seleccion de habitat observada en otras regiones del Hemisferio Sur. El 80% de las colonias estuvieron ubicadas en islas, cuya probabilidad de ocupacion aumento con la distancia al continente y la disponibilidad de vegetacion. La cobertura de la vegetation represente un componente clave del habitat de nidificacion para la gaviota cocinera en Argentina. Las gaviotas ocuparon areas con mayor cobertvira que las areas elegidas al azar, sin embargo, dentro de las areas con vegetacion, mostraron preferencia por coberturas intermedias que incrementan la visibilidad y facilitan el escape. La gaviota cocinera nidifico frecuentemente con otras especies de aves marinas y lobos marinos, y presente una asociacion positiva con la Gaviota de Olrog (Larus atlanticus).

HABITAT SELECTION is a hierarchical decisionmaking process that results in animals living in a restricted set of environmental conditions (Partridge 1978, Block and Brennan 1993). Animals make a series of species-consistent decisions on the basis of different sets of criteria for each scale (Orians and Wittenberger 1991, Bergin 1992). Birds with a wide geographic range, such as most seabirds (Buckley and Buckley 1980), may use either similar habitats in all regions or may select different sites from region to region (Cody 1985). In addition, birds may select similar physiognomic characteristics, despite differences in available vegetation species, vegetation cover, substrate types, or other physical characteristics (Burger and Gochfeld 1988).

Larus gulls are largely temperate-zone species that nest colonially, and many show great plasticity in their habitat requirements (Burger and Gochfeld 1996). Many studies of habitat selection in gulls analyzed only one or few colonies (e.g. Burger and Shisler 1978, Montevecchi 1978), which restricts the ability to identify the habitat that gulls use for colonies and nest habitats and makes it difficult to examine the flexibility in habitat choices (Burger and Gochfeld 1981). To determine how variability influences habitat choice requires studies that encompass a range of environments broad enough for the effects of variability to be evident (Orians and Wittenbereer 1991).

Kelp Gulls (Lams dominicanus) are widely distributed in the Southern Hemisphere, breeding in South America, southern Africa, Australia, New Zealand, subantarctic islands, and Antarctic Peninsula (Burger and Gochfeld 1996). In Argentina, that species is widely distributed and abundant, breeding along the Atlantic coast (Yorio et al. 1999). Its nesting habitat requirements have not been analyzed in Argentina, apart from nest-site selection at a single colony in central Patagonia (Yorio et al. 1995). Elsewhere, Kelp Gulls nest in varying habitat types (Fordham 1964, Lazo et al. 1992, Yorio et al. 1995, Simeone and Bernai 2000) with a preference for areas with vegetation cover (Burger and Gochfeld 1981, Quintana and Travaini 2000). Vegetative structure is frequently the primary proximate factor determining where and how many bird species use resources (Block and Brennan 1993).

Kelp Gull populations in Argentina have expanded during the last two decades, which may negatively affect other coastal species through predation, competition for breeding space, and kleptoparasitism, and their activity at or near cities may result in hazards to aircraft and threats to human health (Yorio et al. 1998a). Knowledge of their habitat requirements will contribute to management and conservation actions, particularly with respect to spatial interactions with other species. Here we describe the nesting habitat of Kelp Gulls breeding in coastal central and northern Patagonia, Argentina, compare characteristics across colonies to identify critical habitat selection factors, and examine the importance of vegetation cover.