Migratory behavior can differ markedly amongst individuals within populations or species. Understanding the factors influencing this variation is key to understanding how current environmental changes might influence migratory propensity and the distribution and abundance of migratory species across their range. Here, we investigate variation in migratory behavior of the partially migratory Eurasian oystercatcher (Haematopus ostralegus) population breeding in Iceland. We use resightings of color-ringed adults and stable isotopes to determine whether individuals migrate or remain in Iceland during winter and test whether individual migratory strategies vary in relation to sex, body size and breeding location. We also explore individual consistency in migratory strategy and test whether assortative mating with respect to strategy occurs in this population. The proportion of migrants and residents varied greatly across breeding locations, but not with respect to sex or body size. Individuals were consistent in migratory strategy between years and there was no evidence of assortative mating by migratory strategy. We use these findings to explore factors underlying the evolution and maintenance of partial migration at high latitudes.

The study took place from the south to the north-west of Iceland during the summers 2013-2017 and was expanded to the north-east and east in 2017. Incubating oystercatchers were captured on the nest using a spring trap, and individually marked with unique color-ring combinations. For each individual we measured tarsus + middle toe length to the nearest millimeter and collected a sample of 4-5 chest feathers (grown on wintering sites during the pre-nuptial moult) for isotopic analysis. For a subset of birds, we also collected a sample of blood for molecular sex determination.

Molecular sex determination 

DNA was extracted from blood samples using a standard ammonium acetate technique (dx.doi.org/10.17504/protocols.io.knycvfw) and diluted to a working concentration of 10–50 ng/μL. The sex of individuals was then determined using the molecular methods set out in Fridolfsson & Ellegren (1999).

Stable isotope analysis

Prior to isotopic analysis, feathers were washed in a 2:1 chloroform/methanol solution, left to dry overnight in a fume cupboard and cut into small fragments. Each sample was then weighed (from 0.45 to 0.55 mg) with a microbalance, packed in tin capsules, and loaded into a combustion Costech elemental analyser coupled to a Thermo Scientific Delta XP continuous flow isotopic ratio mass spectrometer and a Conflo III interface. Carbon and nitrogen stable isotope ratios are reported as δvalues in parts per thousand(‰) deviations from the international standards Vienna Pee Dee Belemnite (V-PDB) for δ¹³C and AIR N₂for δ¹⁵N. Replicate measurements of the internal laboratory standard (collagen) indicated measurement errors of 0.2‰ and 0.1‰ for δ¹³C and δ¹⁵N, respectively.