How general is the central-periphery distribution among seabird colonies? Nest spatial pattern in the European Shag

The central-periphery distribution model of nest dispersion suggests that n ests located in the center of a colony are less accessible to predators and that birds breeding in the central area are of better physical quality and have greater reproductive success. Another hypothesis, the central-satelli te distribution model, suggests that low-quality birds build their nests ne ar those of hi.-h-quality pairs, which do not necessarily settle in the col ony's center. Advantages of this type of association include increasing the opportunity for extra-pair copulation by low-quality females and increasin g the potential for low-quality individuals to obtain a better breeding sit e or partner in the following season. Here we test these hypotheses on two colonies (Portelo and Faro) of the European Shag (Phalacrocorax aristotelis ) on the Cies Islands, Galicia, northwest Spain. Spatial distribution of pa irs differing in quality was analyzed using residuals of reproductive succe ss, eliminating the effect of physical quality of the nest site. A negative autocorrelation of these residual,,; between nests at short distances was detected for the Portelo colony. In Faro there was no evidence that nest di stribution differed from a random distribution. Occupation of nest sites (m easured as new nest or reoccupied nest) showed negative autocorrelation at short distances, implying that there was an association between new and reo ccupied sites. These results demonstrate that nest distribution of European Shag colonies does not fit the central-periphery model, but rather corresp onds to either the central-satellite model or to a random distribution.