Intraspecific variation in size and density of Avocet colonies: effects ofnest-distances on hatching and breeding success

In many colonial bird species there is considerable intraspecific variation in colony size and inter-nest distance (colony density). Possible causes o f this variation and its effects on hatching success (survival of eggs) and breeding success (probability of a pair raising chicks) were studied in 48 Avocet Recurvirostra avosetta colonies in Schleswig-Holstein (Germany) bet ween 1991 and 1996. Colony density was influenced by time of year and habit at (categories: island or mainland. close to or Far from feeding grounds). Colonies on islands had the highest densities. When all available space at. a colony site was used, colonies became very dense (mean nearest-neighbour nest distance less than 1 m). Colony size (number of clutches) was influen ced by time of year, but not by habitat. Hatching success was low in high d ensity colonies and in very low density 'colonies' (single nests) and high over a broad range of intermediate nest densities. The low success rate of single nests was caused by a Very high predation rate, whereas the low succ ess rate in very dense colonies was caused by a high rate of nest abandonme nt. Nest abandonment in very dense colonies was associated with a high leve l of aggressiveness among Avocets during the egg-laying period. Due to terr itorial behaviour, Avocets seemed to be expelled from the densest breeding sites. In very dense colonies, high frequencies of clutches of unusual size occurred due to conspecific nest parasitism. The number of Avocets taking part in attacks on potential egg predators was small and (in colonies of mo re than one clutch) depended neither on colony size nor on colony density. Despite a low hatching success in very dense colonies, individuals breeding in the densest colonies had significantly better chances of raising chicks than Avocets breeding in less dense colonies. Coloniality seemed to be obl igatory for Avocets in order to ensure hatching success. The size and densi ty of colonies seemed to be associated with the availability of suitable ne sting habitats (islands).