EFFECTS OF NEST-SITE AVAILABILITY AND DISTRIBUTION ON DENSITY-DEPENDENT CLUTCH SIZE AND LAYING DATE IN THE CHOUGH PYRRHOCORAX-PYRRHOCORAX

We compared the breeding phenology and clutch size of Choughs Pyrrhoco rax pyrrhocorax in three Spanish areas that differed in feeding habita t, breeding environmental. conditions and the distribution and availab ility of different nesting sites (cliffs or artefacts), The variation in the timing of laying seemed to be related to differences in climati c conditions determined by the altitude, The variation in clutch size was associated with the different use that Choughs made of the feeding areas because of the contrasting availability and distribution of the ir nesting sites, Within southeastern Madrid, clutch size of the Choug h decreased and its variance increased at high nest densities on cliff sites, suggesting that density dependence in fecundity arises from ha bitat heterogeneity rather than by interference, At high densities, in dividuals, territories, nest sites or a combination of these may diffe r in quality, thus promoting differences and increasing the variance i n the initial investment of reproductive effort. The syncronization of the onset of laying at increased breeding densities suggests that soc ial influences arising from communal foraging or avoidance of predatio n by early warning and predator swamping may be acting also, We sugges t that nest-site availability and distribution have a major influence on the social organization of Choughs through their breeding and forag ing strategies.