Striking rusty-brown neck collars in ruffs: Plumage polymorphism or staining?

Among Ruffs Calidris pugnax migrating through the province of Friesland in The Netherlands in spring, some individuals have a remarkably plover-like, rustybrown neck collar. In this paper we explore the frequency of occurrence of this neck collar in 2,395 Ruffs in which the presence or absence was scored in yearround catches in Friesland between 2006 and 2019. Additionally, 49 nesting reeves in northern Finland and 73 skins of females (mostly in breeding plumage) in the Moscow Zoological Museum were checked for neck collars. The rustybrown neck collar was found in 395 Ruffs migrating through The Netherlands in spring. Among birds of known sex, age and morph (n = 2,098), the collar occurred in 14% of females, in 3.5% of faeders, and in 40% of satellite and 20% of independent males. However, among males, the rate dropped to zero by late April after the start of moult of the ruff and tuft feathers, and the occurrence was lower among second-calendar-year males (11%); no age effect was detected among females. Rusty-brown neck collars were rare among breeding females, with no cases in Finland, one possible case in the museum collection and one other case observed in Medusa Bay, north-central Siberia, Russia. The collar was also rare (0.4%) in non-breeding Ruffs caught in The Netherlands between June and February. The likely cause of the neck collar is staining by ferric oxide (Fe₂O₃) or ferrous oxide (FeO) as collar feathers tested positive for iron, while regular brown feathers tested negative. The presence in spring, but not after moult into the supplemental plumage or later, suggests that the birds acquire the ironbased collar in the wintering areas in West Africa or at stopover sites during northward migration.