Bill knob size and reproductive effort in Common Shelducks Tadorna tadorna

摘要

Male Common Shelducks Tadorna tadorna with large bill knobs were more effective at defending spring feeding areas in the Severn Estuary, southwest England, against other males than those with small bill knobs. Their female partners were consequently able to devote more time to feeding and less to walking and being alert. Most of the walking by pairs in which the males had small bill knobs was undertaken in order to avoid interactions with other pairs. All males, except those with the smallest knobs,made a significant investment in reproduction by spending more time being alert than females, allowing them to gather food resources for egg formation and incubation more quickly. By selecting male partners with large bill knobs, females thus increased their feeding time and decreased their energy expenditure. These observations are consistent with Patterson's (1982) contention that territorial behaviour has been selected for in Common Shelducks because it provides an undisturbed feeding area for the female.