The national censuses of gulls and other seabirds have been incredibly important and informative, and at last we have a sound knowledge of both the distribution and numbers of adults. However, we do not have a census value for numbers of immature individuals that have never bred for any of the gull species. Since immature birds may include up to five year classes (varying according to species), the numbers involved are appreciable and can be estimated only from a life table formed from survival rates obtained from detailed marking studies. Table 1 gives rough estimates of the proportion of immature gulls of the six commonest species occurring in Britain and Ireland. The figures are approximations but indicate that, by autumn, there is a large proportion (probably about 40 per cent) of individuals of every species listed that have not yet matured and bred. The proportions of immature individuals will have decreased by early spring because the mortality rate of young birds in their first year of life is usually markedly higher than that of adults, but they will still form an appreciable minority of the numbers of each species.
Gulls recorded in Britain and Ireland
| Genus Hydrocoloeus | |
| Little Gull (H. minutus) | Regular visitor, very occasional breeder |
| Ross’s Gull (H. roseus) | Vagrant |
| Genus Xema | |
| Sabine’s Gull (X. sabini) | Regular visitor, usually in small numbers |
| Genus Pagophila | |
| Ivory Gull (P. eburnea) | Vagrant |
| Genus Chroicocephalus | |
| Slender-billed Gull (C. genei) | Rare vagrant |
| Bonaparte’s Gull (C. philadelphia) | Vagrant |
| Black-headed Gull (C. ridibundus) | Abundant breeder and winter visitor |
| Genus Larus | |
| Common Gull (L. canus) | Common breeder and winter visitor |
| Ring-billed Gull (L. delawarensis) | Vagrant |
| Great Black-backed Gull (L. marinus) | Common breeder and winter visitor |
| Glaucous-winged Gull (L. glaucescens) | Vagrant |
| Glaucous Gull (L. hyperboreus) | Regular winter visitor |
| Iceland Gull (L. glaucoides) | Regular winter visitor |
| Kumlien’s Gull (L. glaucoides kumlieni) | Vagrant subspecies |
| Thayer’s Gull (L. thayeri) | Vagrant |
| European Herring Gull (L. argentatus) | Abundant breeder and winter visitor |
| American Herring Gull (L. smithsonianus) | Vagrant |
| Caspian Gull (L. cachinnans) | Currently vagrant but increasingly recorded |
| Yellow-legged Gull (L. michahellis) | Visitor and now an occasional breeder |
| Lesser Black-backed Gull (L. fuscus) | Abundant breeder |
| Slaty-backed Gull (L. schistisagus) | Vagrant |
| Genus Ichthyaetus | |
| Great Black-headed Gull or Pallas’s Gull (I. ichthyaetus) | Vagrant |
| Mediterranean Gull (I. melanocephalus) | Rapidly increasing breeder |
| Audouin’s Gull (I. audouinii) | Vagrant |
| Genus Leucophaeus | |
| Laughing Gull (L. atricilla) | Vagrant |
| Franklin’s Gull (L. pipixcan) | Vagrant |
| Genus Rissa | |
| Black-legged Kittiwake (R. tridactyla) | Abundant breeder |
FIG 3. A phylogenetic tree of the gull species that occur in Britain and Ireland. In general, the shorter the line leading from each species, the more recently that species is presumed to have arisen. This does not apply to the large genus Larus, where more work is necessary to elucidate the affinities of the different species. This diagram includes all of the genera in the world, with the exception of the genus Saundersilarus (where Saunders’s Gull, S. saundersi, is the only species) and the genus Creagrus (where the Swallow-tailed Gull, C. furcatus, is the only species), neither of which occur in Europe. The Vega Gull (L. vegae) has not been included, and its presence in Britain has yet to be confirmed, while Kumlein’s Gull is regarded as a subspecies of the Iceland Gull. Based in part on the work of Pons et al. (2005).
Table 1 also shows estimates of numbers of the six most abundant gulls in Britain and Ireland in about 2000. Again, these figures are estimates, but they indicate that for all six species combined there are about 3.2 million gulls of all ages in Britain and Ireland in the early autumn. This total is probably about 3.3 million in winter, when the numbers of departing Kittiwakes are replaced by immigrant Black-headed, Herring and Common gulls. By late spring, the winter visitors have departed and Kittiwakes have returned to their colonies, maintaining numbers at about 2.6 million just before breeding begins.
SUBSPECIES IN WESTERN EUROPE
There are only a few western European gull species that are represented by more than one subspecies. As mentioned earlier in the chapter, the Herring Gull was previously split into several named subspecies, some of which have now been elevated to species (Yellow-legged Gull, Caspian Gull). Two subspecies occur in Britain and Ireland: Larus argentatus argenteus, which breeds here; and the larger, darker L. a. argentatus, which breeds in northern Scandinavia and north-west Russia, and winters in Britain and the North Sea region. The nominate Common Gull subspecies, L. canus canus, occurs widely in western Europe, while the larger, darker subspecies L. c. heinei, which breeds in Russia, probably (based on ringing data) occurs occasionally in Britain, but is difficult to identify. In the Atlantic, the Kittiwake is represented by a single subspecies (Rissa tridactyla tridactyla); individuals are progressively larger towards the north of its range, but this gradual change does not justify separate subspecies status and is described as a cline. There are probably several other clines among gulls that are yet to be recognised, including the Black-headed Gull, Glaucous Gull and Lesser Black-backed Gull.
SURVIVAL AND LONGEVITY
Gulls are medium- to long-lived birds, with an average