AVOCETTA - VOLUME 33 - N. 2
A widespread gull population in a complex wetland:habitat specifc methods to census breeding pairsSoldatini C., AlboresS-Barajas Y.V., Mainardi D., Torricelli P.
Abstract:The Mediterranean Yellow-legged Gull, Larus michahellis michahellis (Naumann 1840), has recently increased in numbers and begun to settle in urban habitat and nest on roofs. Gulls nesting on buildings are a common sight in coastal areas of Europe and North America. We censused a population of Mediterranean gulls in the city of Venice and its natural surroundings. The aim of this paper is to introduce and describe techniques to count and estimate population numbers in various habitats. In natural environments, an aerial survey can be used to identify Yellow-legged gull colonies and to count breeding pairs. In urban environments, however, different census techniques are needed in order to give a complete description of the gull population. We counted and monitored the urban breeding population by applying direct counting techniques from fxed observation points. By integrating different methods, we were able to describe the wintering and breeding populations of Yellow-legged Gull in the largest Italian wetland system. In recent times gull populations of some species have undergone a remarkable increase. The Mediterranean Yellow-legged Gull, Larus michahellis michahellis (Naumann 1840), is the protagonist of a recent phenomenon of general demographic increase and the settlement into urban habitat and roof nesting in many European coastal cities. The environmental problems caused by superabundant animal species, particularly “bird pests” have become increasingly acute during the past decades (Feare 1991). In particular, gulls are often found to be superabundant due to their adaptable, opportunistic and gregarious nature which makes them highly adapted to live in human-modifed habitats. Owing to their high ecological adaptability, their competitive behavior and their abundance, gulls are often considered pests (Vidal et al. 1998). Some close relatives of this species, such as the herring gull, Larus argentatus, and the black-backed gull, Larus fuscus, started settling into towns in the 1950’s (Goethe 1960). Since then gulls nesting on buildings are a common sight in coastal and, more recently, inland areas of Europe (Monaghan and Coulson 1977, Vincent 1987) and North America (Vermeer et al. 1988). Usually nesting gulls cause disturbance to the inhabitants of buildings due to noise, fouling and the aggression of adult gulls in defense of their young, and can also damage the façade of the building. Consequently, the spread of gulls into urban areas is a matter of growing concern and thus it is necessary to monitor population numbers and distribution. We studied the Yellow-legged Gull population in the Venice lagoon in order to monitor the population in and around the city of Venice. As city habitat is different than natural habitat, we developed a habitat-specifc counting system with various existing methods. In many cases study areas are heterogeneous, e.g. consist of various habitats and so it is necessary to choose the right counting technique for each of these habitats and circumstances to perform wildlife counts. In particular this is the case of the Yellow-legged Gull in the Venice lagoon, where estimates have been done based on partial surveys but a complete description of the population is still lacking due to the extent of the lagoon, the complex habitat composition and the diffcult access to some private areas. Thus, it is necessary to use an integrative approach to study the Yellow-legged Gull in the Venice lagoon. We censused both the urban and the natural Yellow-legged Gull populations living in the Venice lagoon, combining multiple census techniques in order to obtain a complex but realistic overview of the species’ population present in the lagoon.