NEST PREDATION - THE RELATIVE EFFECTS OF NEST CHARACTERISTICS, CLUTCHSIZE AND PARENTAL BEHAVIOR

The relative contribution of parental behaviour to nest predation rate s in blackbirds, Turdus merula, was estimated by comparing the mortali ty of natural nests (with parents) with the mortality of the same nest s immediately after the breeding season with artificial clutches (but without parents). Nest predation rates during the egg stage were equal ly high (ca 5% per day) in both experimental and natural nests. Nest p redation rates were independent of clutch size for natural nests, but clutch size was lowest in nests that were further from the edge of nes t bushes. Nest predation rates of experimental nests were greater for nests that were less than 10 m apart. The probability of nest failure was independent of all measured physical characteristics for natural n ests, but for experimental nests probability of nest failure was depen dent on the nest's detectability, and to a lesser extent height. There was no relationship between the outcome, or the duration before preda tion, of a natural nest, versus the same nest used with an experimenta l clutch. The results suggest that nesting blackbirds altered their ne st defence to compensate for the predation risk associated with a nest 's location. (C) 1997 The Association for the Study of Animal Behaviou r.