EXPERIMENTAL MANIPULATION OF BROOD REDUCTION AND PARENTAL CARE IN COOPERATIVELY BREEDING WHITE-WINGED CHOUGHS

1. White-winged choughs, Corcorax melanorhamphos (Vieillot), are oblig ate cooperative breeders. Only very large groups routinely fledge all their brood of three to four chicks, while small groups usually lose y oung during the nestling period. Hatching asynchrony generates a weigh t hierarchy within the brood, and small, late-hatched chicks are most susceptible to mortality. 2, In order to examine the effects of food a vailability on parental care and brood reduction, we provided suppleme ntary food to groups during late incubation and the nestling phase. 3. Food supplementation increased the rate of food delivery to the nest by both breeders and helpers, leading to increased chick survival and fledging, and reduced variance in chick size at fledging. Helpers with supplemental food appeared more responsive to the need of chicks, inc reasing food delivery rates as the chicks grew older, and as brood siz e increased. 4. Control groups fed larger chicks preferentially, while supplemented groups favoured small chicks. This suggests that choughs deliberately manipulate the survival of individual young to maximize the fledging of healthy chicks, consistent with Lack's hypothesis for hatching asynchrony. 5. These data support the hypothesis that choughs must breed in groups because they cannot provide enough food to nestl ings without help. Hatching asynchrony and behavioural control over br ood reduction allow choughs to maximize offspring production according to group size and food availability.