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      FIG 9. A typical wing-tip pattern of a Herring Gull (Larus argentatus) breeding in Scotland, with the 10th primary still growing. (John Coulson)

      Differences caused by hybrids

      One definition of a biological species is that the individuals can form a group of interbreeding or potentially interbreeding organisms that produce viable offspring. If hybrids occur between two species, such as in the classical case of a horse and a donkey, the hybrid offspring of the two (a mule or hinny) are usually sterile, probably because the two parent species have different numbers of chromosomes. In the case of gulls, hybrids are not uncommon and are reported far more frequently than in major taxa, such as terns. Studies have revealed that Larus gulls have the same number of chromosomes (72) and as a result, hybrids are usually fertile and they have been reported breeding successfully. There are now numerous records of gulls of different species and even different genera pairing and rearing hybrid offspring, as listed below:

      Mediterranean Gull × Black-headed Gull

      Herring Gull × Lesser Black-backed Gull

      Herring Gull × Yellow-legged Gull

      Lesser Black-backed Gull × Yellow-legged Gull

      Western Gull (Larus occidentalis) × Glaucous-winged Gull

      Great Black-backed Gull × American Herring Gull

      Herring Gull × Glaucous Gull

      Glaucous-winged Gull × Glaucous Gull

      American Herring Gull × Kelp Gull

      Iceland Gull × Thayer’s Gull

      Common Gull × Ring-billed Gull

      Mediterranean Gull × Common Gull

      Laughing Gull × Black-headed Gull

      Laughing Gull × Ring-billed Gull

      Herring Gull × Caspian Gull

      In most cases, adults that are believed to be hybrids have been recognised by the intermediate nature of their plumage and the colouring of their legs and bill, but in only a very few instances has the plumage been described for adults that are known to be hybrids and were ringed as such before they fledged. It is usually believed that hybrid gulls show intermediate characters of their parents in terms of plumage, bill colour and leg colour, but this is not always the case, and in several instances they display minor characteristics not evident in either parent.

      There is little doubt that some hybrids can share similarities with, and resemble, other gull species. As a result, it is sometimes difficult to accept a new sight record of a species from a geographical area where it has not been previously or convincingly been recorded before, and to confirm that it is not a hybrid between species that breed nearby. Rarity committees have a particularly difficult job with gulls, and ideally need DNA samples obtained from feathers of the presumed rarity to be certain of the record.

      Some hybrid gulls, when adult, have been known to pair and mate with an individual of one of their parent species, producing offspring known as back-crosses. Even less is known about the plumage of these offspring, but it is likely that they differ both from the original species and from the hybrid parent. Breeding between pairs of hybrid gulls has not been recorded. However, hybridisation and subsequent breeding is likely to produce at least three different types of individuals, all of which vary in some respect from the original parent species as well as from each other. The immature plumages of hybrid gulls are poorly known and in many cases their origin has been assumed only because of their intermediate characteristics.

      Eventually, after several generations of breeding, a particular gene can be transferred via the offspring of a hybrid from one of the parent species to the other. This has been recorded in the American Herring Gull, which appears to have acquired a gene from the Great Black-backed Gull in North America, presumably through hybrids between the two species. To date, this gene has not been recorded in the European Herring Gull.

      In Belgium and the Netherlands, mixed pairs of Yellow-legged Gulls and either Lesser Black-backed Gulls or Herring Gulls have occurred particularly frequently. For example, more than 15 mixed pairs were reported in Rotterdam annually from 1986 to 1998 (van Swelm, 1998) and more in more recent years, and others have been frequently identified in at least five other colonies in the Netherlands and Belgium. Hybrid individuals that have reached adulthood and that are presumed to be crosses between Yellow-legged and Lesser Black-backed gulls have also been recorded in Belgium breeding with Lesser Black-backed Gulls, producing back-crosses.

      Inter-species breeding is more frequent when one of the gull species is rare and spreading into the main range of the other. For example, when Lesser Black-backed Gulls first started to breed in the Netherlands in the 1930s, a few individuals joined large colonies of Herring Gulls and several mixed breeding pairs were recorded. Despite the fact that both species are now numerous and breed in the same colonies, hybrid pairs still occur, although they are infrequent. Very few pairings between these two species have been reported in Britain, except when experimentally induced (see below).

      When Herring Gulls spread to Iceland in the 1920s, individuals formed mixed pairs with Glaucous Gulls, and by 1966 about half of the adults there were considered to be hybrids. These were distinctive in showing small but variable amounts of dark pigment on the tips of the primaries (Ingolfsson, 1970).

      When Mediterranean Gulls first started to breed in Britain, early pioneers frequently paired with Black-headed Gulls (as they have done so elsewhere). In fact, this is ongoing, as a few individuals continue to spread north from the south coast of England. The recent arrival of a few adult Yellow-legged Gulls in Britain has seen them join both Herring Gull and Lesser Black-backed Gull colonies. Again, they have formed mixed pairs that on some occasions have managed to fledge hybrid chicks. Perhaps this inter-species breeding occurs because individuals arriving in new areas are mainly of the same sex and fail to find a mate of the same species.

      In a study carried out on the island of Skokholm in south-west Wales, Mike Harris (1970) switched large numbers of eggs between Lesser Black-backed Gull and Herring Gull nests. The chicks that subsequently hatched imprinted on their foster parents and apparently considered that they were the same species, so that when they matured they chose a mate of that species, forming a series of mixed-species pairs. The young produced and reared by these mixed pairs were hybrids between the parent species and usually (but not always) showed plumage and leg colour intermediate between the two. At least 40 of these hybrids later returned to breed on Skokholm and on nearby Skomer, and most paired with adult Herring or Lesser Black-backed gulls. The chicks they produced were back-crosses and, when adult, were more similar to one of the parent species than the first generation of hybrids. While some of these hybrids reared chicks, it is not known whether they and their offspring were less viable. However, as each generation was produced, presumably both parent species incorporated small amounts of the genetic material belonging to the other species into their make-up despite appearing to be ‘pure’ Herring

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