Foraging along a salinity gradient: The effect of tidal inundation on site choice by Dark-Bellied Brent Geese Branta bernicla and Barnacle Geese B. leucopsis

We report on the effects of salt deposition on food plants on the foraging preferences of Dark-bellied Brent Branta bernicla and Barnacle Geese B. leucopsis in a coastal staging habitat. Within one salt-marsh plant community, dominated by Red Fescue Festuca rubra, grazing pressure by geese increased with elevation (related to mean high tide), although both standing crop and nitrogen content of the forage were highest at plots lower on the salt marsh. Salt deposition on Festuca leaves decreased with increasing surface height along the elevational gradient. The, matching of census data for three spring seasons and data on soil surface elevation revealed that flocks of Barnacle Geese forage, on average, 0.4 m higher on the salt marsh than sympatric Dark-bellied Brent Geese. 84% of the Barnacle Goose flocks were encountered in areas characterised by an inundation frequency of less than 20 during the period from February through April, whereas this applied to only 44% of the Dark-bellied Brent Goose flocks. Barnacle Geese avoided foraging on salt-marsh vegetation experimentally sprayed with seawater, showing a significant preference for untreated control plots, whereas Dark-bellied Brent Geese did not react to the treatment. Dissection of nasal glands in carcasses of both species revealed that the metabolic mass-specific weight of the nasal glands of Dark-bellied Brent Geese exceeded that of Barnacle Geese by nearly a factor four. Within this species-pair of Dark-bellied Brent and Barnacle Geese, jointly using salt-marsh staging sites in spring, Barnacle Geese appear to be physiologically constrained in their choice of foraging sites within the salinity gradient of their habitat