The Common Guillemot Uria aalge lays an egg that varies enormously in colour and in its degree of maculation between different females (Birkhead et al. 2021). This variation is an adaptation to high-density breeding (an average of 20 pairs per square metre) with no nest structures, and has evolved to enable parent Guillemots to identify and, if necessary, retrieve their own egg (Tschanz 1990). The vast majority of Guillemot eggs have a white, pale or dark blue or green ground colour and markings that can be spots, speckles, blotches or pencil-line-like squiggles (Birkhead et al. 2021; see plate 207 in Birkhead 2023). A particularly rare egg type is erythristic, usually referred to as being ‘red’ or port-wine coloured. These eggs, which lack the green pigment biliverdin, typically have a pink ground colour and reddish blotched or streaked maculation. In the past, such eggs were known to occur, albeit very rarely, at Bempton Cliffs, Yorkshire (Whitaker 1997), and on the Farne Islands, Northumberland (Perry 1940).
Guillemot eggs are among the most numerous eggs in museum egg collections. However, rare or unusual egg types are hugely over-represented in such cases, since collectors favoured these over more common egg types, and it is therefore not possible to use museum collections to assess the incidence of red Guillemot eggs. Five unbiased samples, observed or obtained from several different locations in the UK between 1973 and 2016, and comprising over 2,500 Guillemot eggs, contained no red eggs (see Birkhead & Montgomerie 2018). At Bempton Cliffs, where tens of thousands of eggs were collected annually in the late 1800s and through the twentieth century (Vaughan 1998), only one or two red eggs were ever found in any year (Whitaker 1997). Drane (1898–99) presented colour photographs of Guillemot and Razorbill Alca torda eggs from south Wales (probably including Skomer, Pembrokeshire), including some brown or reddish Guillemot eggs. However, on the basis of their shape, I think these are probably Razorbill eggs.
On 17th May 2023, together with wildlife-photographer Klaus Nigge, I was observing and photographing Guillemots on their breeding ledges at Skomer Island National Nature Reserve, Pembrokeshire. An incubating bird we were watching stood up briefly to reveal a red egg (plate 277). Among the several thousands of eggs observed during routine annual monitoring spanning a period of 50 years, this was the first red egg I have seen on Skomer. The day after we first saw and photographed the egg, another pair of Guillemots settled directly in front of this site preventing any further observations or photographs.
Birkhead, T. R. 2023. Fifty years of Common Guillemot studies on Skomer Island. Brit. Birds 116: 319–334.
—, & Montgomerie, R. 2018. Rare red eggs of the Common Guillemot (Uria aalge): birds, biology and people at Bempton in the early 1900s, Yorkshire. Archives of Natural History 45: 69–79.
—, Thompson, J. E., Cox, A. R., & Montgomerie, R. D. 2021. Exceptional variation in the appearance of Common Murre eggs reveals their potential as identity signals. Ornithology 138: 1–13.
Drane, R. 1898–99. Eggs of the Common Guillemot and Razorbill. Transactions of the Cardiff Naturalists’ Society 31: 52–53.
Perry, R. 1940. Lundy, Isle of Puffins. Lindsay Drummond, London.
Tschanz, B., 1990. Adaptations for breeding in Atlantic alcids. Netherlands J. Zool. 40: 688–710.
Vaughan, R. 1998. Seabird city: a guide to the breeding seabirds of the Flamborough headland. Smith Settle, Otley.
Whitaker, J. 1997. A Diary of Bempton Climbers. Peregrine Books, Leeds.
Tim Birkhead, Department of Biosciences, University of Sheffield, Sheffield S10 2TN; e-mail [email protected]