THE REPRODUCTIVE SUCCESS OF BLACKBIRDS TURDUS-MERULA IN RELATION TO HABITAT STRUCTURE AND CHOICE OF NEST-SITE

The reproductive success of a population of Blackbirds Turdus merula o ccupying farmland and woodland was studied over 3 years to investigate the effects of habitat on breeding success, Territory distribution wa s patchy in both farmland and woodland; some areas were unoccupied, wh ile other areas were occupied at variable densities, Habitat structure appeared to influence occupation: the index of habitat complexity ('' cover score'') was higher in occupied areas than in unoccupied areas a nd high-density territories had higher cover scores than low-density t erritories, However, habitat structure had no significant effect on re productive success because the cover scores of territories where pairs were successful did not differ significantly from those of territorie s where there were no successful breeding attempts, There was no evide nce of differential mortality rates in adults according to habitat, Th e height, bulk and exposure of c. 430 nests were measured to determine the effect of nest and nest-site characteristics on reproductive succ ess, Nest exposure was the only feature that differed between successf ul and failed nests, successful nests being less exposed than failed n ests, The major cause of breeding failure was nest predation, but the effect of nest exposure operated only during the laying and incubation period and not during the nestling period. The significance of habita t structure for variation in population densities between habitats is discussed.