Resident and Migratory Falcons' Breeding Phenology and Productivity Respond Differently to Weather and Climate Change Across the Arctic.

Climate change is disproportionately warming the Arctic, leading to changes in weather patterns that affect breeding phenology for many Arctic species. Despite extensive research on non-predatory birds, few studies have examined the impacts of climate change on higher trophic levels, especially those comparing breeding phenology and productivity between migratory and resident species. Our study used long-term monitoring data from 11 sites monitoring migratory peregrine falcons (Falco peregrinus) and 9 sites monitoring resident gyrfalcons (Falco rusticolus) across the circumpolar Arctic. We used remotely sensed environmental data to explore how environmental conditions affect breeding phenology and productivity for both species. We analyzed 5546 peregrine and 2241 gyrfalcon hatch records and observed earlier hatch dates at 5 of 9 gyrfalcon sites and 4 of 11 peregrine sites. In the Low Arctic, peregrines advanced hatch dates by 0.41 days/decade and gyrfalcons by 1.62 days/decade, whereas in the Sub Arctic, we observed a trend toward later hatch dates for gyrfalcons. Greater spring temperatures and vegetation greenness consistently resulted in earlier hatch dates for both species. Earlier snowmelt was linked to earlier hatch dates for peregrines and higher productivity for both species. Our data suggest that long-distance migrating peregrines advance their phenology less than resident gyrfalcons, aligning with other studies showing that long-distance migrants possess more fixed annual schedules and are less likely to align with variations in spring green-up compared to residents. Contrary to our hypothesis, greater summer temperatures reduced productivity in both species. Given our findings, we recommend more detailed research on how summer temperatures affect productivity, as nestlings could suffer higher mortality from heat prostration than previously anticipated. Our findings indicate that while climate change affects breeding timing and productivity, the impact varies, suggesting increased weather variability could lead to more unpredictable responses in falcon breeding ecology in the Arctic.