Pre-breeding moult in adult Black Guillemots Cepphus grylle

Abstract

The progression of pre-breeding moult in adult Black Guillemots Cepphus grylle was observed at Bangor, Co. Down. Moult began as soon as early November, which is much earlier than more northerly populations in the UK. Moult was completed within ten weeks. Early return to the breeding colony and early moult may help individuals to obtain breeding sites.

The breeding (summer) and non-breeding (winter) plumages of Black Guillemots Cepphus grylle are very different: birds are predominantly black in summer and white in winter. Brief summaries of the annual moult cycle are shown by, eg, Asbirk (1980), Ginn & Melville (1983) and Baker (1993), whilst greater details of the timing and sequence of moult are provided elsewhere (eg Storer 1952, Cramp 1985, del Hoyo et al 1996, Gaston & Jones 1998, Butler & Buckley 2002). All of these summaries are based, to a lesser or greater extent, upon the detailed studies of Salomonsen (1944) and Asbirk (1979). Salomonsen (1944) used museum skins for his account of populations of Black Guillemots inhabiting the Baltic and Scandinavian coasts, whilst Asbirk (1979) also predominantly used museum skins but from Denmark. The only British study has been that of Ewins (1988) in Shetland; his study was used in a summary of moult in British auks by Harris & Wanless (1990). Ewins (1988) concluded that the timing of moult was similar to that of Black Guillemots from western Scandinavia. Pre-breeding body moult was observed by mid January, with the first bird in full breeding plumage seen by the end of February, and virtually all birds were in full breeding plumage by early April.

The aim of this research was to determine the pre-breeding moult period at a Northern Irish site. My study site was the marina at Bangor, Co. Down (Irish grid: J5082); a description of the site was given by Greenwood (2010). Early morning visits were made weekly (mostly) to the marina from early October 2011 until all adults had attained full breeding plumage the following spring (2012). Salomonsen (1944) used an eight-point scale to describe the sequence of pre-breeding moult based upon 33 museum specimens. The same eight-point scale was used by Asbirk (1979) to describe the moult sequence in his 57 museum skins. Ewins (1988) made over 3,500 observations in the field; by necessity a much simpler moult-scoring scale of four points was used (winter plumage, white with some black, black with some white, summer plumage). I used an eight-point scale: winter plumage; neck in moult; neck and head in moult; neck, head and body in moult; moult about half completed; moult about three-quarters completed; moult nearly completed; and summer plumage. The moult score of individual birds seen in the marina was determined using 10 x 42 binoculars at no more than 100 m.

Two hundred and seventeen observations were made, with samples ranging from three to 25 (Fig 1). The first individuals in moult were seen in early November. The duration of moult was estimated in three ways: the period between the observation of the first birds in moult and the first birds in complete breeding plumage; the period between the observation of the last birds not in moult and the last in moult; and the period between the observation when all birds were in moult and when all birds were in breeding plumage. In each of these three cases, the duration of moult was ten weeks: a little longer than the two months observed in Shetland (Ewins 1988), Denmark (Asbirk 1979) and Scandinavia (Salomonsen 1944). Moult began about two months earlier, however, than in other European populations. Storer (1952) stated that birds inhabiting cold waters moulted later in the spring compared to those birds inhabiting warmer waters and Ewins (1988) suggested that the similarity of moult regime between Shetland and western Scandinavia was due to similar winter sea-surface temperatures. Such temperatures are different between the seas around northern Britain and those around southwestern Britain and eastern Ireland (Defra 2010), being about 3°C cooler in the north. Thus, if Ewins' (1988) assumption is correct, earlier pre-breeding moult in more southerly populations should be expected.

Figure 1. Progression of pre-breeding moult in adult Black Guillemots at Bangor, Co. Down.

However, there is a distinct behavioural difference between Black Guillemots in Bangor compared to Shetland. Ewins (1985) has shown that, although Black Guillemots in Shetland were present on the sea around their nesting sites from October, the first landings on shore were not until March. Black Guillemots at Bangor behave very differently; arriving on the water adjacent to the breeding sites from the end of September and coming ashore from early October (Greenwood 1987, 1991). In southwest Ireland, the first visits ashore by Black Guillemots are usually in March, although on calm autumn days they may be seen flying up to the cliffs in the first half of October (Sharrock 1973). Unfortunately there are no observations of pre-breeding-season visits elsewhere in Ireland.

I have previously suggested that Black Guillemots that return to their colonies early do so to ensure a breeding site for the forthcoming season (Greenwood 1987). Although the number of nesting sites has increased over the last 20 years with the provision of nestboxes (Greenwood 2010), there were many fewer holes for breeding prior to that (Greenwood 1987). In the summer of 1986 for instance, there were at least 37 adult birds at the colony, but only about 15 holes available (Greenwood 1987). What is the point of attending the breeding colony in the autumn for courtship and gaining a nest site if one does not look the part? There must be strong selection pressure to attain breeding plumage early in such situations. Early colony return has been observed in both Common Guillemots Uria aalge (Birkhead & Taylor 1977) and Razorbills Alca torda (Greenwood 1972). Indeed Birkhead & Taylor (1977) stated that many Common Guillemots spent only a short period each year in their winter plumage, either because there must be a strong selection pressure to get into breeding plumage in winter for courtship or breeding-site defence, or because the unknown advantages of being in winter plumage may have weakened. Harris et al (2006) found that Common Guillemots on the Isle of May (Scotland) arrived in October consistently from 1962 through to 2005. In Shetland, Harris et al (2006) showed variation, with birds arriving in the spring in the early 1960s, then gradually moving to autumn arrival for about 20 years from the late 1960s before gradually reverting to spring arrival. Harris et al (2006) concluded that competition for nest-sites was the most likely reason for returning to colonies at different seasons. The same reason must surely apply to Black Guillemots breeding in Bangor. As Harris & Wanless (1990) say, if winter colony visiting by Black Guillemots and Razorbills spreads, it will be interesting to see whether the timing of moult will change until breeding-plumaged auks become a regular feature of British (and Irish) seabird colonies in winter.

It would be intriguing to discover whether other groups of Black Guillemots in southerly breeding sites attend their colonies in autumn and moult earlier than northern populations, particularly in the light of recent ring recoveries (Wernham et al 2002) that show dispersal movements of young Black Guillemots around the Irish coast and across the Irish Sea.

Acknowledgements

I am grateful to Sarah Wanless, Mike Harris and Jeremy Greenwood for critically reading an earlier draft of this paper.