Hatching Asynchrony and Brood Reduction in Cooperatively Breeding White-winged Choughs Corcorax melanorhamphos

Abstract

White-winged Choughs Corcorax rnelanorharnphos are large insectivorous birds of eucalypt woodlands that are totally dependent on helpers for reproduction. Pairs without helpers fail to fledge any young and success increases with each additional helper up to group sizes of at least 14. Clutch size (usually four eggs) and degree of hatching asynchrony do not vary with group size. The eggs usually hatch one day apart over four days creating a size hierarchy within the brood. I relate the higher fledging success of large groups to the effect of extra food brought to the nest by helpers. The total amount of food delivered to the nest increases with group size and as the nestlings grow older. Early hatched young generally get more food than later hatched young but all nestlings get more food in larger groups. Surviving younger nestlings weigh as much as their older siblings by the time they fledge and young from large groups are generally heavier at fledging. These data suggest that asynchronous hatching of eggs serves to match both brood size and nestling quality with the number of helpers.