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 qu ality of the regrown plants is often higher. Foraging strategies should the refore depend on the difference in biomass between grazed and ungrazed site s and the food quality in the two types of site, both variables that are li kely to vary seasonally. We compared the nutritive quality (in terms of met abolisability of food, nitrogen, and energy, and rate of food intake) of gr azed and ungrazed sites for early- and late-hatched greater snow goose (Che n caerulescens atlantica) goslings throughout the summer. We conducted fiel d experiments with captive goslings feeding on grasses and sedges in the Ar ctic. Plant quality declined seasonally (nitrogen concentration decreased a nd fibre content increased) but, contrary to our initial expectations, graz ed 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 o f intake of food, metabolisable nitrogen, and metabolisable energy were alw ays higher at ungrazed than at grazed sites. Rates of intake of metabolisab le nitrogen and energy declined seasonally for both grazed and ungrazed pla nts, which suggests that goslings were not able to compensate for the seaso nal decline in food quality by increasing their rate of food intake. No dif ference was found between early- and late-hatched goslings in food metaboli sability, and both groups were exposed to the same seasonal effects. Weak r egrowth after grazing leads to poor feeding conditions for goslings. Under these circumstances, ungrazed sites are of higher nutritive value for gosli ngs than grazed sites.