Sex‐differences in nest defence in Fieldfares Turdus pilaris in relation to their size and physical condition

Abstract

The nest defence of solitary‐breeding Fieldfares Turdus pilaris against a human intruder, a corvid and a hawk was studied in a mountain birch forest in central Norway in 1990–2002. Females defended their nests against a human intruder more vigorously than did males before hatching, and vice versa after hatching. Although both sexes increased their nest defence, only males showed a significant increase and showed a greater defence than females at the chick stage. Females with 1‐week‐old nestlings had longer wings, were heavier and in a better physical condition than those that failed to get nestlings to that stage. Females, but not males, lost condition during nesting. Birds in better condition were more aggressive. The pattern of nest defence by males and females against the corvid was similar to that shown against human intruders, whereas males took greater risks than females towards the raptor. There was weak evidence that birds that were more successful showed greater levels of nest defence. The sex‐role reversal in nest defence may be connected to differences in costs in relation to the renesting potential and to the physical condition of the sexes.