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.

Individual tracking of Icelandic oystercatchers

Since 2013, incubating oystercatchers in the south, west and north-west Iceland have been captured, measured and individually marked with coloured leg-rings. Adults were caught on the nest using a spring-trap and feather samples were collected for stable isotope analysis (see Méndez et al. 2020 for details). Through a network of volunteer observers reporting sightings of marked individuals throughout the wintering range, the migratory behaviour (resident or migrant) of 186 of the 537 marked individuals has been identified. For the remaining 351 individuals, migratory behaviour has been determined using a discriminant function analysis of stable isotope ratios (δ13C and δ15N), after calibration using the isotopic signatures of those individuals that were observed during winter within or outside Iceland (Méndez et al., 2020).

Nest monitoring and breeding data collection

From mid-April each year (2015-2018), we surveyed study areas every 2-3 days to search for returning colour-marked individuals, and to find and monitor pairs and nests until hatching or clutch loss. Laying date (the date when the first egg was laid) was estimated by back-calculating from hatching dates (assuming one egg is laid per day and 28 days of incubation, starting when last egg is laid) or incubation stage, using the egg flotation method (Liebezeit et al., 2007). For each breeding attempt (including replacement clutches following clutch loss), we recorded clutch size and the outcome (successful if at least one chick hatched or failed if predated, trampled or abandoned).

Oystercatchers remain in the vicinity of the nest after hatching their chicks and feed them throughout the growing period. Chicks were metal-ringed just after hatching and individually-marked with colour rings at around two weeks old (when tarsus length was sufficient to fit the rings). Families were monitored every 3-4 days until all chicks were fledged or lost, allowing productivity (number of chicks fledged per pair) and fledging success (number of chicks fledged in nests where at least one egg hatched) to be recorded.

References

Liebezeit, J. R., Smith, P. A., Lanctot, R. B., Schekkerman, H., Tulp, I., Kendall, S. J., … Zack, S. W. (2007). Assessing the development of shorebird eggs using the flotation method: species-specific and generalized regression models. Condor, 109, 32–47.

Méndez, V., Alves, J. A., Þórisson, B., Marca, A., Gunnarsson, T. G., & Gill, J. A. (2020). Individual variation in migratory behaviour in a sub-arctic partial migrant shorebird. Behavioral Ecology. doi: 10.1093/beheco/araa010