Ecological Correlates of Residence and Non-Residence in a Mediterranean Passerine Bird Community

Estacion Biol6gica de Donana, Sevilla-12, Spain SUMMARY (1) A southern Spanish passerine bird community was studied throughout a year, collecting information on species composition and abundance, as well as foraging behaviour and feeding site utilization of component species. In this paper, comparisons of behavioural, morphological and feeding site niche characteristics between resident and non-resident species are made. (2) For all species, within-species foraging diversity is significantly greater for non-resident than for resident species, indicating that individual resident species have a narrower repertoire of foraging tactics than non-resident ones. On a seasonal basis, between-species foraging diversity for non-residents is higher than that for residents during spring and early summer, but it falls well below the figures for resident species in winter. (3) Resident species are morphologically more diversified and less densely packed in the morphological space than non-resident ones. Resident species tend to show a wider range of bill length, and to be long-billed, with respect to non-residents. (4) Resident species exhibited feeding site niche shifts in response to its own population density, higher densities promoting wider niches as theoretically expected. Non-residents did not show comparable shifts, although some degree of response to density was noted. (5) A canonical correlation analysis was able to show that resident and non-resident species adjust themselves to the changing environment in different ways. Whereas residents do it by means of changes in density, niche breadth and between-species foraging diversity, non-residents species achieve the same end by means of changes in morphological specialization, behavioural specialization and diversity of foraging tactics, all of which take place through seasonal replacements of species. (6) The seasonal organization of the study community is discussed in relation to other temperate and non-temperate communities. It is proposed that the observed great dominance of residents with respect to number of individuals may be associated with a relatively low degree of seasonality in food supply to birds. The reliability and predominating abundance throughout the annual cycle of the food types exploited by residents, are likely to explain the maintenance of the high behavioural and morphological specialization by individual resident species. The wide spacing on the morphological space among these species may be related to the regular occurrence of periodic phases of food