Female weight predicts the timing of forced copulation attempts in stitchbirds, Notiomystis cincta

Male birds can often accurately gauge the fertile status of females. As is found in other species, the stitchbird manifests this ability through increased mate guarding and attempted extrapair copulations centred on the female's peak fertile period. Males are thought to use various behavioural. cues to assess female fertility, including (1) within-pair copulation, (2) mate-guarding intensity, (3) female solicitation, (4) female flight behaviour, (5) nest building, (6) egg laying and (7) paired male song intensity. Using a correlational approach, I examined which behavioural cues were closely linked with the fertile status of the female stitchbird, as well as which cues accounted for patterns of extrapair male behaviour. Female stitchbirds increased their body weight by an average of 31% (maximum 43%) in the 3 weeks before laying. Extrapair male interest rose sharply as the female's weight increased from 31 g to a peak weight of approximately 41 g, 2 days before the first egg was laid. Thus, increasing female weight appears to be the primary fertility indicator in this species, and it is likely that males evaluate this through changes in her flight behaviour. Because of potential costs associated with forced copulation, female stitchbirds may attempt to limit the availability of information regarding their fertile state by burying eggs within the lining of their nest. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.