Effects of pair migratory behaviour on breeding phenology and success in a partially migratory shorebird population

In migratory systems, variation in individual phenology can arise through differences in individual migratory behaviours, and this may be particularly apparent in partial migrant systems, where migrant and resident individuals are present within the same population. Links between breeding phenology and migratory behaviour or success are generally investigated at the individual level. However, for breeding phenology in particular, the migratory behaviours of each member of the pair may need to be considered simultaneously, as breeding phenology will likely be constrained by timing of the pair member that arrives last, and carry-over effects on breeding success may vary depending on whether pair members share the same migratory behaviour or not. We used tracking of marked individuals and monitoring of breeding success from a partially migrant population of Eurasian oystercatchers (Haematopus ostralegus) breeding in Iceland to test whether (a) breeding phenology varied with pair migratory behaviour; (b) within-pair consistency in timing of laying differed among pair migratory behaviours; and (c) reproductive performance varied with pair migratory behaviour, timing of laying and year. We found that annual variation in timing of laying differed among pair migratory behaviours, with resident pairs being more consistent than migrant and mixed pairs, and migrant/mixed pairs breeding earlier than residents in most years but later in one (unusually cold) year. Pairs that laid early were more likely to replace their clutch after nest loss, had higher productivity and higher fledging success, independent of pair migratory behaviour. Our study suggests that the links between individual migratory behaviour and reproductive success can vary over time and, to a much lesser extent, with mate migratory behaviour and can be mediated by differences in laying dates. Understanding these cascading effects of pair phenology on breeding success is likely to be key to predicting the impact of changing environmental conditions on migratory species.