Deer have expanded their range and increased dramatically in abundance worldwide in recent decades. By foraging selectively, deer affect vegetation dynamics and succession. Cascading effects on other species also occur. We studied overabundant white-tailed deer (Odocoileus virginianus) introduced on Anticosti Island (Quebec, Canada). We assessed whether chronic browsing contributed to a decline of the quality of deer diet, and evaluated the impacts of reduced diet quality on body condition and reproduction. Rumen nitrogen content declined through time. Body mass of males and females also declined. The probability of conception increased through time, but litter size at ovulation declined, resulting in a similar total number of ovulations. We hypothesize that the density-dependent effects observed on body mass might have been exerted through habitat degradation. Our results suggest that following a decline in habitat quality, females modified their life-history strategies to maintain reproduction at the expense of growth. Such modifications may contribute to maintain high population density despite negative effects on habitat. Ecologists should actively participate in efforts to understand, monitor, and control deer impacts on ecosystems.