In counts of gulls, terns, wildfowl and waders breeding on about 100 small islands along the mainland coast of west Scotland during 1996–2009, 123 clutches containing the eggs of two species (‘mixed clutches’) were found among 69,775 clutches of 20 species (0.18%). The annual percentage varied between 0.09% and 0.33%. Most (89%) mixed clutches contained only one egg of one of the species. The 123 mixed clutches involved 13 species, 19 species-pairs and at least 31 permutations of guest and host species. The most frequent species-pairs were Common Gull Larus canus and Eurasian Oystercatcher Haematopus ostralegus, Herring Gull Larus argentatus and Common Eider Somateria mollissima, and Common Gull and Black-headed Gull Larus ridibundus, these together accounting for 54% of cases. Sixteen species-pairs had been recorded in an earlier study in 1990–95, and a further six were found in 1996–2009: Common Eider and Mallard Anas platyrhynchos (4 mixed clutches), Herring Gull and Great Black-backed Gull Larus marinus (3), Common Tern Sterna hirundo and Arctic Tern Sterna paradisaea (2), Canada Goose Branta canadensis and Common Eider (2), Common Eider and Greylag Goose Anser anser (1), and Canada Goose and Greylag Goose (1). The appearance of goose eggs in mixed clutches reflected large increases in breeding numbers of both species in the area. The most frequent guest species were Eurasian Oystercatcher and Common Eider and the most frequent hosts were Common Gull and Herring Gull. Mixed clutches probably arise from attempted egg parasitism. There are records of chicks from guest eggs being reared successfully in some closely related species-pairs, but other species-pairs seem incompatible.

Nests are sometimes found which contain the eggs of two bird species (hereafter ‘mixed clutches’). They may be related to two kinds of nest parasitism. Intraspecific egg parasitism or ‘egg dumping’ has been recorded in at least 236 species of 43 families (Yom-Tov 2001). The more specialised interspecific brood parasitism, found in birds such as cuckoos and cowbirds, is confined to a smaller number of species and families; for example, 100 brood-parasitic species in six families were listed by Davies (2000).

Mixed clutches are more frequent than might be supposed but have received little study, probably because reports are scattered in journals with restricted distri- bution, such as local bird reports, and usually involve single nests encountered by chance. This paper describes mixed clutches found during systematic annual counts of nests at seabird colonies along the mainland coast of west Scotland in 1996–2009. It follows similar work in the same area in 1990–95 (Craik 1997), but presents data for a greater number of species and a much larger number of clutches. I also identify as closely as possible the species in whose nest each mixed clutch occurred (the host species), which the earlier work made little attempt to do. Finally, I consider how mixed clutches may arise among these species and give some evidence that may be relevant.

I am grateful to many people for help with fieldwork, particularly to Rob, Audrey and Niall Lightfoot, and to Nick Davies, Mike Harris, Martin Heubeck, Sarah Wanless, an anonymous referee and others for helpful comments on and improvements to an early draft.

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