During recent decades, gull populations have increased dramatically in Europe, north America and Australia, probably as a result of an increase in food availability derived from human activities (Blokpoel & Spaans 1991; Furness, Ensor & Hudson 1992; Pons 1992; Smith & Carlile 1993). In some cases, large numbers of gulls can have adverse effects on other bird populations through predation or nesting competition (Bradley 1986; Hario 1994; Guillemette & Brousseau, in press), and on humans through contamination of domestic water supplies, bird strikes at airports and nuisance in towns or disruption of commercial operations (Benton et al. 1983; Monaghan 1983; Burger 1985; Furness & Monaghan 1987; Blokpoel & Tessier 1992). To minimize such effects, wildlife agencies have implemented several management measures.
Measures that reduce food availability have proved to be effective in reducing gull numbers in the long term (Pons 1992; Pons & Migot 1995). However, these measures have been little used because they are often expensive (Spaans & Blokpoel 1991). In breeding colonies, periodic scaring, alteration of nesting habitat, and sterilizing or removing the eggs have been used to reduce numbers (Thomas 1972; Christens & Blokpoel 1991; Blokpoel & Tessier 1992; Wanless et al. 1996). In addition, a rapid reduction in numbers of breeding gulls has frequently been achieved by killing adults (Thomas 1972; Duncan 1978; Coulson, Duncan & Thomas 1982; Wanless & Langslow 1983; Smith & Carlile 1993).
However, several factors associated with the population dynamics of gull colonies are density-dependent, such as fecundity and recruitment (i.e. the entry of new individuals into the breeding population). On the Isle of May (Scotland), increases in egg size, body size and body condition of adults, and a reduction in the age at recruitment, were observed following culls of breeding adults (Duncan 1978; Coulson, Duncan & Thomas 1982). In addition, culling a single gullery has unpredictable effects at the metapopulation level because it may influence immigration and emigration rates between colonies (Coulson 1991; Defous du Rau 1995). Culling may thus be less effective than planned and its effects may be neutralized once it is discontinued (Spaans et al. 1991).
Most studies dealing with the effects of gull control have analysed variations in both the number of pairs and nest density (Thomas 1972; Wanless & Langslow 1983; Skira & Wapstra 1990; Alvarez 1992), but few have examined factors associated with the population dynamics of the colony (Spaans, de Wit & Van Vlaardingen 1987; Coulson 1991; Wanless et al. 1996). This paper reports the effects of a 5-year culling programme on several ecological parameters in a large colony of yellow-legged gulls Larus cachinnans Pallas in the north-western Mediterranean. Culls are being performed in many parts of their range (e.g. Spain, France, Portugal; Blondel 1963; Aguilar 1991; Alvarez 1992; Morais, Santos & Vicente 1998; Vidal, Medail & Tatoni 1998) but little is known about the effects and the effectiveness of this control measure in the Mediterranean region. In our study, the effect of reducing numbers of breeding adults on nest density, breeding performance, interspecific predation, body condition of adults and diet was analysed throughout the years of the culls. Survival and recruitment were also estimated using capture–recapture techniques, to model the effects of culling on the population dynamics of this colony (Lebreton & Clobert 1991). Finally, the suitability of culling as a measure to reduce the presence of gulls is discussed.