We studied the summer ecology of snowy owls on Bylot Island (Nunavut) for over 20 years. Using data on growth rate of chicks and summer diet, we investigated the effects of the date of snowmelt and lemming density on the number of nests, egg laying date, clutch size and nesting success. Lemmings comprised more than 95% of the summer diet of snowy owls, and the number of nests and nesting success varied annually with lemming density. Mean egg laying date and clutch size varied annually, but were not affected by the timing of snowmelt or lemming density. While the density of lemmings appears to be the key factor triggering snowy owls to establish breeding territories on Bylot Island, the presence of a high density of lemmings in a given year was no guarantee that pairs would settle and breed.