Nutritive quality of forage plants for greater snow goose goslings: when is it advantageous to feed on grazed plants?

Foraging herbivores often face a choice: move to ungrazed sites, where the biomass is high, or use grazed sites, where the biomass is lower but the quality of the regrown plants is often higher. Foraging strategies should therefore depend on the difference in biomass between grazed and ungrazed sites and the food quality in the two types of site, both variables that are likely to vary seasonally. We compared the nutritive quality (in terms of metabolisability of food, nitrogen, and energy, and rate of food intake) of grazed and ungrazed sites for early- and late-hatched greater snow goose (Chen caerulescens atlantica) goslings throughout the summer. We conducted field experiments with captive goslings feeding on grasses and sedges in the Arctic. Plant quality declined seasonally (nitrogen concentration decreased and fibre content increased) but, contrary to our initial expectations, grazed plants were not higher in quality or metabolisability than ungrazed ones. This was possibly a consequence of poor regrowth after grazing because of drought conditions that prevailed during this study. Consequently, rates of intake of food, metabolisable nitrogen, and metabolisable energy were always higher at ungrazed than at grazed sites. Rates of intake of metabolisable nitrogen and energy declined seasonally for both grazed and ungrazed plants, which suggests that goslings were not able to compensate for the seasonal decline in food quality by increasing their rate of food intake. No difference was found between early- and late-hatched goslings in food metabolisability, and both groups were exposed to the same seasonal effects. Weak regrowth after grazing leads to poor feeding conditions for goslings. Under these circumstances, ungrazed sites are of higher nutritive value for goslings than grazed sites.