Chick diet of Common Guillemots Uria aalge was studied by direct observation at Sumburgh Head, Shetland during 2007–09. Lesser Sandeels Ammodytes marinus comprised 55% of prey items, the remainder being Gadidae, including Whiting Merlangius merlangus and Saithe Pollachius virens, and a few Snake Pipefish Entelurus aequoreus. This contrasts with 1990–91 when chick were fed 80% sandeels, the remainder mostly being unidentified Gadidae. When this shift from a diet dominated by sandeels took place is unknown, but a similar shift in chick diet at Fair Isle (40 km southwest of Sumburgh Head) seems to have occurred some time during 2000–02. Since 2003, weights of chicks near fledging have been lower than during the 1990s, and average breeding success has been reduced.

Nothing has been published on Common Guillemot Uria aalge chick diet in Shetland since the late 1980s and early 1990s, when parent birds at two colonies fed their young almost exclusively Lesser Sandeels Ammodytes marinus (Harris & Riddiford 1989; Uttley et al. 1994). These studies have continued to be cited as indicative of chick diet in Shetland (Österblom & Olsen 2002; Swann et al. 2008), although the intervening years have seen considerable fluctuation in the breeding performance of Common Guillemots and other seabird species that are considered to need sandeels for successful breeding (Kunzlik 1989; Wright & Bailey 1993). This paper presents three years of observations at a Shetland colony, and compares chick diet and various parameters of breeding performance with a study at the same colony in 1990–91 (Uttley et al. 1994).

The seabird monitoring at Sumburgh Head has been carried out under contract to the Shetland Oil Terminal Environmental Advisory Group, funded by the Sullom Voe Association Ltd. Sarah Wanless provided the stimulus and the photographic identification key for the feeding watches, and Mike Harris and John Hislop measured and identified the fish collected in 2009. Dave Okill from the Shetland Ringing Group kindly provided chick measurements, and the manuscript was greatly improved by comments from Mike Harris and Sarah Wanless.

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