Sex differences in defence of eggs and nestlings by northern mockingbirds, Mimus polyglottos

Northern mockingbirds, Mimus polyglottos, actively defended eggs and nestlings from humans visiting nests. There was a significant positive correlation between levels of defence by mates within pairs. Pairs in which males strongly attacked intruders displayed greater nesting success than pairs in which males did not. By several measures, males defended offspring more than females did: males approached closer when defending nestlings but not eggs, and engaged in more attacks on the intruder during both egg and nestling stages. Males, but not females, increased their frequency of attacks from the egg to the nestling stage. More males than females struck the intruder when defending nestlings. When defending nestlings, males followed the intruder to the edge of the territory more often than did the females and vocalized more frequently during encounters with intruders. Both sexes hovered over the intruder infrequently during the egg stage. During the nestling stage, males hovered more than females, but the sexes hovered at similar distances overhead. Birds tending to hover over the intruder were mated to birds that likewise hovered. Males, but not females, approached intruders more closely as nestlings aged. A decrease in the distance of approach to the intruder with age of the eggs was related to nest revisitation. A similar decrease in distance of approach with the ageing of nestlings was not explained as a nest revisitation effect. A male-biased sex ratio of adults in this population may favour the condition where males defend offspring more strongly than do females.