Abstract

Using museum specimens and field observations we examined the occurrence and timing of rump moult in House Martins Delichon urbicum in the non-breeding season. Individuals with brown rumps, some of which were particularly dark, were widespread in sub-Saharan Africa and occasionally occurred in Europe. The extent of brown feathers in the rump also varied between individuals in a given time period within the same area.During the breeding season, House Martins Delichon urbicum have white underparts and rumps, with females and juveniles having a grey wash below varying from a pale buff-grey to dark greyish brown. After breeding, they undertake a moult of the body feathers, after which the white areas of the plumage become greyer and the rump of both adults and first-winters becomes brown. This body moult usually starts on the breeding areas in Europe and can continue during migration, whereas the flight feathers are typically not moulted until the bird reaches sub-Saharan Africa (Jenni & Winkler 2020). The brown coloration on the rump lasts until shortly before spring migration, at which point House Martins undergo another moult of the body feathers; at this point, the brown rump feathers are replaced by white ones (Jenni & Winkler 2020). The brown non-breeding plumage is well described in the larger European handbooks such as Witherby et al. (1938) and Cramp & Simmons (1988), yet most African field guides and handbooks, as well as some European field guides, depict the House Martin as having a white or whitish rump year-round, and none illustrate or discuss in full the (sometimes surprisingly dark) brown rump (e.g. Bannerman 1939, Roberts 1948, Keith et al. 1992, Borrow & Demey 2001). Our aim in this paper is to provide a photographic record of this under-reported variation in non-breeding plumage of House Martins during winter and investigate when and where on the non-breeding grounds such a plumage can be expected to be seen in the field. To do this, we looked at the rump colour of House Martin specimens in museums and from limited observational records in sub-Saharan Africa.