DOMINANCE, BROOD SIZE AND FORAGING BEHAVIOR DURING BROOD-REARING IN THE LESSER SNOW GOOSE - AN EXPERIMENTAL-STUDY

We investigated the relationship between brood size and social dominan ce during the brood-rearing period in Lesser Snow Geese (Anser caerule scens caerulescens) by experimentally manipulating food availability t o create high-biomass food patches. A total of 128 social interactions were subsequently observed in experimental areas; the rate of interac tions was significantly higher in experimental high-biomass plots (9.6 hr(-1)) than in control, low-biomass, areas (0.4 hr(-1)). During soci al interactions families (pairs with one or more goslings) were always dominant over pairs without goslings. However, there was no clear dom inance hierarchy among families in relation to brood size. Neither agg ressiveness (the number of interactions initiated) nor the proportion of successful interactions varied consistently with brood size. We con clude that, during brood rearing, dominance ranking is determined more by individual variation in aggressiveness of adult (parent) birds, ra ther 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 b iomass areas: 32% of all interactions involved a social unit inside th e experimental plot driving off a social unit which was trying to ente r the plot from outside. This suggests that geese derived benefits fro m monopolization of good quality food patches. The behavior of foragin g geese varied in relation to food availability: birds took fewer step s per minute during both feeding and non-feeding bouts in the experime ntal plots and females, but not males, had shorter feeding bouts in ex perimental plots, i.e., they adopted the vigilant head-up posture more frequently. We suggest that the benefits of utilizing high biomass fo od patches during brood-rearing include higher intake rates, decreased energetic costs of foraging and reduced predation risk through increa sed vigilance behavior by parents and greater cohesion of the family u nit.