Snowmelt predicts earlier breeding across the latitudinal range of an Arctic nesting seabird, the Little Auk (Alle alle).

Climate-driven temporal shifts in seasonal environments are altering some of the environmental cues that organisms use to time reproduction, potentially leading to trophic mismatches across ecosystems. In the Arctic, marine predators must balance conditions at sea with local terrestrial constraints at breeding sites, yet the relative importance of these cues for breeding phenology remains unclear. We used a crevice-nesting High-Arctic planktivorous seabird, the Little Auk (Alle alle), as a model species. Drawing on a unique multi-year dataset from four colonies with distinct climatic regimes, we tested whether breeding onset tracks the timing of snowmelt at breeding sites, a key terrestrial cue determining nest accessibility. Snowmelt timing is closely linked with hatching date, with earlier snowmelt enabling earlier access to nesting crevices and advancing hatching across all sites. Importantly, we detected no significant directional temporal trend in snowmelt timing over the study period (2000-2024), suggesting that this relationship reflects interannual variability. Within years, later hatching was associated with reduced chick growth and survival across colonies. However, interannual variation in mean hatching date was linked to chick survival in only one colony, indicating spatial heterogeneity in demographic consequences of breeding phenology. Future projections indicate that snowmelt timing will advance where little auks breed, potentially advancing breeding timing. However, other ongoing changes-such as borealization of zooplankton communities and the loss of summer sea ice-may alter future fitness consequences of breeding timing. Our study highlights the role of the terrestrial environment in shaping the breeding timing of high-latitude marine birds.