Environmental regulation of growth in black brant

Abstract

Body size is an important determinant of life history traits such as survival and fecundity. There is a positive correlation between growth during the first summer and final body size in goose populations. I examined how environmental factors influence growth in Black Brant (Branta bernicla; hereafter brant) goslings. Growth declined seasonally and varied among brood-rearing areas. However, the pattern was not consistent among years. Models containing only environment and maternal effects explained 75% of variation in gosling mass, indicating that little of the observed variation in size is directly of genetic origin. Heritability did not differ from zero for both mother-daughter and father-daughter regressions. I also conducted an experiment to study the effect of gosling density on food abundance, feeding behavior, and development of brant goslings, in two habitat types important to brant: (1) Carex subspathacea grazing lawns and (2) slough levees which contain Triglochin palustris. Variation in grazing pressure was experimentally manipulated. Biomass and offtake of C. subspathacea was higher in lightly grazed plots than in heavily grazed plots even though goslings within heavily grazed plots spent more time feeding. Within slough levee habitat there were no differences between heavily and lightly grazed plots in either biomass or offtake of T. palustris. Peck rates were lower in slough levee habitat than in grazing lawns. Change in mass over an eight hour trial was positively correlated with the amount of forage biomass in the plot at the start of the trial. I found no variation in internal morphometrics or body composition among goslings. I also examined the relationship between forage available within a brood-rearing area, the number of birds using the area, and gosling growth. Annual variation in use of brood-rearing areas was correlated with forage availability. Gosling mass was negatively correlated with brood numbers when examined across all areas, however, within each brood-rearing area, the relationship between mass and numbers of birds was positive. I did not see a relationship between estimates of food availability (per m²) and brood numbers. Spatial variation in growth among habitats may result from habitats varying in quality and quantity of forage.