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.