Within- and between-Year Variation in the Presence of Individually Marked Ruff Calidris pugnax at a Stopover Site during Northward Migration

Ruffs Calidris pugnax migrate from wintering areas in West-Africa and Europe to breeding grounds in northern Eurasia, using stopover sites along the way. At one such stopover site in southwest Friesland (53°N, The Netherlands), we studied variation in the timing of individual stopover based on 6474 Ruffs colour-ringed in 2004–2012. 43% of males and 22% of females were recorded in the study area in March–May the years following marking. Minimal stopover duration of returning individuals showed substantial within-year heterogeneity. We distinguished two classes: (1) ‘transient’ individuals were observed only on a single day in the study area within a season (51% of observed males and 79% of females), and (2) ‘staging’ individuals were observed on multiple days. We observed two seasonal peaks in the presence of transient Ruffs, typically coinciding with the peak of arrival and departure of staging birds. Males known to winter in Europe were more likely to be observed in the study area and arrived earlier than males of unknown winter origin (3.1 days and 3.7 days earlier for transient and staging males, respectively), but departure was unrelated to winter origin. Staging and transient females arrived later than males. Between-year repeatability of individual behaviour was low, and individuals did not significantly advance their arrival date over the course of years, in contrast with a pattern of shifting arrival dates at the population level. The observation that a large proportion of Ruffs visit southwest Friesland for only a short stop suggests that many individuals rely on other sites for moulting and refuelling during spring migration. Resightings of marked individuals elsewhere in western Europe indicated that these sites are largely located between 51° and 54°N. Thus, during spring migration, Ruffs marked in southwest Friesland displayed high between- and within-individual variation in minimal stopover duration.