Contrasting spring arrival dates of two shorebirds in N Norway

In the Northern Hemisphere, rising temperatures have led to advances in the onset of spring that, in turn, have been regarded as a major determinant of arrival times of migratory birds. The return of spring migrants to N Norway has been studied since 1980 using data collected by local ornithologists and the general public. This study focuses on two species that are true migrants but with contrasting timing and destination: European Oystercatchers Haematopus ostralegus that arrive in March to breed locally and Red Knots Calidris canutus that arrive two months later to refuel for 2–3 weeks on their way to their breeding grounds in Greenland and NE Canada. Oystercatchers advanced their arrival dates by two weeks between 1980 and 2016. Apparent arrival dates of Red Knots showed a parabolic trend with year, becoming later during 1984–1996 and earlier between 1996 and 2016. This might not reflect changes in phenology; rather it might result from a decrease and then increase in the number of Knots stopping over in Balsfjord. Although much less than other early migrants, Oystercatchers vary their date of arrival by 3–4 weeks, partly in response to air temperatures in March in the breeding area. The knots arrive within a 10-day window in May and are thus much more consistent in their arrival dates. This consistency, which is independent of a slight increase in May temperatures, may be related to factors outside Norway.