The magnitude of climate change has been greatest in the Arctic, accelerating climate-induced shifts in phenology, but wildlife responses vary. Variation may be due to the relative importance of phenotypic plasticity or phenotypic selection.

Here, we examine and contrast the environmental drivers of plasticity in breeding phenology of two circumpolar seabirds at their receding summer range limit using unique datasets of marked individuals covering 25 and 30 years. Based on prior knowledge of the local ecosystems, we predicted that climate would generate opposing patterns of plasticity in the two populations.

Laying phenology of kittiwakes in the Gulf of Alaska was associated with a large-scale climate oscillation (Pacific Decadal Oscillation) while the Arctic-breeding murres adjusted laying to sea-ice conditions. Kittiwakes laid earlier after experiencing colder climate about two years prior and laying dates did not advance over the study, but murres laid earlier when warmer climate led to earlier spring sea-ice break-up, and murre laying dates advanced by one week since 1990. Selection favoured earlier laying in both species.

Both populations adjusted breeding phenology to environmental variation, but we anticipate opposing effects on phenology with continued climate change. Ice-constrained species can likely adapt to some extent because plasticity can provide the necessary shift to this physical barrier, although individuals were only able to adjust by ~one week while ice conditions advanced by over a month. In more temperate regions, where phenology is driven by bottom-up effects, plasticity and selection counteract one another leading to limited adaptability. We provide insights into the likely adjustments by Arctic marine animals to an increasingly warmer and ice-less summer.

• Black-legged kittiwake laying phenology and breeding success data were collected on Middleton Island, Alaska
• Thick-billed murre laying phenology and breeding success data were collected on Coats Island, Nunavut
• Middleton Island climate/environmental data, collated to monthly resolution
• Coats Island climate/environmental data, collated to monthly resolution

Detailed methods are described in: Whelan, S., Hatch, S.A., Gaston, A.J., Gilchrist, H.G., & K.H. Elliott. Opposite, but insufficient, phenological responses to climate in two circumpolar seabirds: relative roles of phenotypic plasticity and selection. Functional Ecology.

• For both species, we included complete data for first breeding attempts only
• Each excel workbork includes a sheet that describes each variable
• Code used for analysis is also included; the code relies on the variable names used in the workbook

Detailed methods are described in: Whelan, S., Hatch, S.A., Gaston, A.J., Gilchrist, H.G., & K.H. Elliott. Opposite, but insufficient, phenological responses to climate in two circumpolar seabirds: relative roles of phenotypic plasticity and selection. Functional Ecology.