Abstract
House sparrow (Passer domesticus) numbers have declined rapidly in both rural and urban habitats across Western Europe over the last 30 years, leading to their inclusion on the UK conservation red list. The decline in farmland has been linked to a reduction in winter survival caused by reduced food supply. This reduction in food supply is associated with agricultural intensification that has led to the loss of seed-rich winter stubble and access to spilt grain. However, urban house sparrows have also declined, suggesting that reduced food supply in farmland is not the sole reason for the decline. Here, we show that changes in house sparrow mass and thus fat reserves are not regulated to minimize starvation risk, as would be expected if limited winter food were the only cause of population decline. Instead, the species appears to be responding to mass-dependent predation risk, with starvation risk and predation risk traded-off such that house sparrows may be particularly vulnerable to environmental change that reduces the predictability of the food supply.