Factors affecting nest size in a population of Blackbirds Turdus merula

Abstract

Capsule: We examined intraspecific variation in the nest size of open cup nesting Blackbirds Turdus merula and found that of seven variables considered, nest size variation was determined by the length of the nest construction period, nest height and the experience of the building female. Aims: We tested seven hypotheses to investigate the influence of the following factors on nest size: (A) speed of construction: smaller nests are constructed faster; (B) nest height: nests placed higher up are smaller; (C) experience: inexperienced females construct smaller nests; (D) predation pressure: predation is less on smaller nests; (E) clutch size: larger nests contain larger clutches; (F) nest placement: nests constructed in open areas are larger than those situated in well-concealed sites and (G) incubation: larger and thicker nests are constructed during colder periods. Methods: We used data from 54 Blackbird females nesting (73 nests) in two urban parks. Results: Our data supported the speed, nest height and experience hypotheses. With respect to predation pressure, our data indicated that breeding failure occurred later in smaller nests. We found no evidence for the placement and incubation hypotheses. Conclusion: Nest size may be determined by predator avoidance and the energy used during building. The negative relationship between nest size and time of nest failure suggests that nest size parameters are adjusted to avoid predation.