Nest predation: The relative effects of nest characteristics, clutch size and parental behaviour

The relative contribution of parental behaviour to nest predation rates in blackbirds, Turdus merula, was estimated by comparing the mortality of natural nests (with parents) with the mortality of the same nests immediately after the breeding season with artificial clutches (but without parents). Nest predation rates during the egg stage were equally high (ca 5% per day) in both experimental and natural nests. Nest predation rates were independent of clutch size for natural nests, but clutch size was lowest in nests that were further from the edge of nest 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 nests, but for experimental nests probability of nest failure was dependent on the nest's detectability, and to a lesser extent height. There was no relationship between the outcome, or the duration before predation, of a natural nest, versus the same nest used with an experimental clutch. The results suggest that nesting blackbirds altered their nest defence to compensate for the predation risk associated with a nest's location.