1. Phenological adjustments are an important aspect of a population’s response to climate change. Changes in phenology can occur through either individual plasticity or evolutionary change within populations. Few studies have investigated both these processes in Arctic environments. 2. Using 42 years of individual and pedigree data, we evaluated the contribution of plasticity and evolution to variation in breeding phenology at a colony of a high Arctic sea-ice obligate seabird, Mandt’s black guillemot (Cepphus grylle mandtii). Mean clutch initiation (first egg in a clutch) advanced 7.8 days, and both environmental (snowmelt) and demographic (years of breeding experience) factors varied among years. 3. Earlier phenology was associated with earlier snowmelt and experienced mothers. Females advanced phenology at different rates as they aged but at similar rates in response to variation in snowmelt. Heritability of clutch initiation was negligible, and there was no evidence of evolution contributing to phenological changes. 4. Earlier laying was associated with increased annual number of fledglings and annual adult survival at the individual level suggesting that the phenological changes are adaptive and are driven by phenotypic plasticity, but not genetic responses. 5. We propose that species with a constrained breeding season (like many Arctic species) may have a limited ability beyond existing plasticity to respond to changing environmental conditions.