Stable isotopes of carbon reveal flexible pairing strategies in a migratory Arctic bird

Many birds change their partners every year and pairing may occur before arrival on the breeding grounds. Early pairing strategies can benefit mates by strengthening pair-bonds and increasing the rate of pre-breeding resource acquisition, leading to increased reproductive output and success, especially for migratory species breeding in seasonally-constrained environments like the Arctic. Despite the theorized and documented advantages of early pairing, we know rather little about pairing phenology in many species. Here, we test the use of a stable isotope (carbon δ¹³C) method to assign geographic origin of paired birds to examine pairing phenology in Arctic-breeding Common Eiders (Somateria mollissima borealis). During two consecutive years, we captured paired individuals upon their arrival at breeding grounds approximately 2–3 weeks before laying. Pairs with similar δ¹³C in their claws indicates that they paired during winter, while similar blood values (with no similarity in claws) would reveal pairs formed much later, during the pre-breeding period near or on the breeding grounds. While a large proportion of pairs (43%) appeared to pair on wintering grounds, an almost equal number (52%) likely paired within 1 month prior to arrival on the breeding grounds. The remaining 5% did not have an obvious pairing time. Despite this variability in pairing phenology, we found no significant differences in body condition between females or males which paired in winter or spring. In the year characterized with more challenging winter conditions, pairs formed in spring tended to have a higher breeding propensity than those formed in winter, although there were no detectable links to body condition. Delaying pairing until spring may be advantageous for Arctic-breeding eiders, although a specific mechanism is unknown. Future research focusing on the energetic costs and benefits for male eiders during these periods would help further understand pairing phenology and potential impacts on males of female breeding decisions.