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Density dependent processes affecting foraging strategies may in turn influence vital rates and population regulation in large herbivores. Increased competition may lower both forage availability and quality, but whether the main activity constraint at high density is increased searching time or increased digestion time is poorly investigated. In a fully replicated landscape-scale experiment, we used long-term data (2003–2009) from domestic sheep grazing at high and low density (80 and 25 sheep km–2, respectively) on alpine summer ranges to test density dependence in allocation of time to feeding (moving) vs digestion (resting) activities and how this in turn affected body growth. Sheep at high density spent more time actively feeding than sheep at low density, but sheep moved shorter distances while foraging at high density. Increased activity levels at high density suggest that the main activity constraint at high density was availability of high-quality food increasing searching time and possibly reducing intake rates. Increased movement distances at low density is consistent with a higher selection for more productive vegetation types since high-quality patches are dispersed in the landscape. The alternative hypothesis, that food processing time increased at high density was not supported as it would have reduced overall activity levels. Individual activity levels increased body growth, but this was not sufficient to fully compensate for lower habitat quality leading to an overall reduced body growth at high density. Our experiment clearly documents changes in activity budgets and movement distances of a large herbivore at high population density, providing one potential behavioural mechanism of density dependent responses observed in vital rates.