Dominance, brood size and foraging behaviour during brood-rearing in the lesser snow goose: an experimental study.

We investigated the relationship between brood size and social dominance during the brood-rearing period in Lesser Snow Geese (Anser caerulescensc aerulescens) by experimentally manipulating food availability to create high-biomass food patches. A total of 128 social interactions were subsequently observed in experimental areas; the rate of interactions was significantly higher in experimental high-biomass plots (9.6 hr-1) than in control, low-biomass, areas (0.4 hr-1). During social interactions families (pairs with one or more goslings) were always dominant over pairs without goslings. However, there was no clear dominance hierarchy among families in relation to brood size. Neither aggressiveness (the number of interactions initiated) nor the proportion of successful interactions varied consistently with brood size. We conclude that, during brood rearing, dominance ranking is determined more by individual variation in aggressiveness of adult (parent) birds, rather than by any "motivational" effect of offspring or by brood size per se. Geese fed longer in the high biomass plots (mean 19.2 min per visit) than in control plots (2.9 min), and birds "defended" high biomass areas: 32% of all interactions involved a social unit inside the experimental plot driving off a social unit which was trying to enter the plot from outside. This suggests that geese derived benefits from monopolization of good quality food patches. The behavior of foraging geese varied in relation to food availability: birds took fewer steps per minute during both feeding and non-feeding bouts in the experimental plots and females, but not males, had shorter feeding bouts in experimental plots, i.e., they adopted the vigilant head-up posture more frequently. We suggest that the benefits of utilizing high biomass food patches during brood-rearing include higher intake rates, decreased energetic costs of foraging and reduced predation risk through increased vigilance behavior by parents and greater cohesion of the family unit.