EVOLUTION OF GULLS
The Charadriiformes constitutes a single large and distinctive lineage of modern birds, and includes waders, skuas, auks, terns, gulls and a few apparently aberrant species such as jacanas. Although the skuas appear to be similar to gulls, current evidence – including DNA studies – suggests that their ancestry may be nearer to the auks than to gulls.
The lightly built bones of birds associated with flight are fragile and therefore do not often produce good fossil remains. As a result, the evolution of present-day birds is poorly known, and much less so than that of reptiles, fish or mammals. Fossil remains attributable to gulls are particularly scarce. Many of those that have been found have not or cannot be attributed to the presently recognised genera, and certainly not to present-day species. More recently, several fossil bones initially attributed to gulls have been found to belong to other avian groups.
Fossil bones attributed to gulls and possibly members of the genus Larus have been reported both in Europe and the USA from deposits from the Middle Miocene, 20–15 million years ago. The relationship of these fossils to modern-day gulls is unclear; fossilised bird bones are often, and perhaps uncritically, given different specific names to those of currently existing species, ignoring the fact that the bones of a present-day species vary considerably in size according to sex and locality.
TAXONOMY OF GULLS
Initially, gull species were separated and identified on the basis of plumage, skeletal structure and size. In many species, specimens from different geographical areas held in museums and private collections were often named and given subspecies status based on minor differences in size and plumage, but all too frequently this relied on small numbers collected from only a few localities. Many of these named subspecies are still used today and, while the majority are probably justified, others that were described and named many years ago should be re-evaluated using modern techniques and larger samples. Some subspecies have already been rejected on this basis, and it is likely that others will not stand critical re-examination and will also be rejected.
In other cases, some existing subspecies have been promoted to the status of a full species, as has occurred recently within the Herring Gull complex in Europe and Asia. Still others may show only gradual changes in size, structure or plumage shades over their geographical range, a concept not recognised by early taxonomists until Julian Huxley applied the term clines to these groups in 1942. Such clines have already been demonstrated for the Black-legged Kittiwake (Rissa tridactyla), the Puffin (Fratercula arctica) and the Common Guillemot (Uria aalge) breeding in the North Atlantic. Questionable subspecies of gulls still exist, and some are discussed in more detail later in this chapter.
Initially, the eighteenth-century taxonomist Carl Linnaeus placed all gulls in the genus Larus, and most species remained there in what became a very large taxon. Eventually, the two species of kittiwake were removed from Larus and placed in the genus Rissa, while the Ivory Gull was moved to the genus Pagophila (P. eburnea), Sabine’s Gull was transferred to the genus Xema (X. sabini), and Ross’s Gull was placed in the genus Rhodostethia and then, more recently, to Hydrocoloeus (H. rosea), alongside the Little Gull (H. minutus). The Swallow-tailed Gull became the sole species in the genus Creagrus (C. furcatus). These separations were not unreservedly accepted, however, and as late as 1998, Philip Chu proposed returning all gulls to a single genus, Larus. Seven years later, the intensive study of the mitochondrial DNA of many gull species made by Jean-Marc Pons, Alexandre Hassanin and Pierre-Andre Crochet (2005) moved in the opposite direction and separated gulls into nine genera, and in so doing created the new genera Chroicocephalus (with 10 species worldwide), Hydrocoloeus (with two species) and Saundersilarus (comprising only Saunders’ Gull, S. saundersi, in China). Worldwide, at least 24 gull species, especially those with white heads in the breeding season, are still retained in the large genus Larus. Aside from Saundersilarus, three other genera are composed of only one species: Creagrus, containing the Galapagos Islands’ Swallow-tailed Gull; Xema, containing the High Arctic Sabine’s Gull; and Pagophila, including the Ivory Gull, also breeding in the High Arctic. Like Hydrocoloeus, the genus Rissa also contains two species.
One of the major findings made during the in-depth investigation by Pons et al. (2005) was that the gulls that had dark heads as adults did not form a single taxonomic group, as had been suggested by studies made in the second half of the twentieth century, but were composed of three distinct groups of species. These groups were called the ‘black-headed gulls’ and placed in the genus Ichthyaetus, while ‘hooded gulls’ were separated into another new genus, Leucophaeus. The third group, including the Black-headed Gull, were called ‘masked gulls’ and were placed in the genus Chroicocephalus. To an extent, this separation of gulls with dark heads in the breeding season is supported by similar courtship behaviour within each group, as originally suggested by Niko Tinbergen and his co-workers in the 1950s and supported by more extensive recent studies.
GULL SPECIES WORLDWIDE
Currently, there are about 50 species of gulls in the world. This total has increased in recent years and will probably be increased further as improved molecular techniques are used to revise their status; even the definition of a species may be modified or revised. The uncertainty about the precise number of species reflects the fact that the gulls as a group present a taxonomic nightmare, and this has resulted in years of confusion and disagreement. For example, the American Ornithologists’ Union (AOU) considers the Herring Gull breeding in North America to be a subspecies of the European Herring Gull (Larus argentatus smithsonianus), while the British Ornithologists’ Union (BOU) regards it as a separate species, Larus smithsonianus. There is still much confusion within the extensive Herring Gull and Lesser Black-backed Gull complex of species and subspecies, particularly those occurring in Asia (see box).
Speciation concepts
The decision as to whether and under what circumstances two populations of animals that occur in different geographical areas can be considered distinct species remains a taxonomic problem, because the level of genetic difference between the two that justifies specific status is often an arbitrary decision and one that is not always universally accepted.
One major taxonomic problem relates to the Herring Gull and Lesser Black-backed Gull complex of subspecies. In 1942, the evolutionary biologist Ernst Mayr suggested that the subspecies formed a chain around the northern hemisphere, starting with the Lesser Black-backed Gull in Europe, and then further subspecies occurring eastwards through Asia, each having progressively lighter-coloured wings and leading to the American Herring Gull in North America, and finally completing the ring with the Herring Gull of western Europe. The theory is that, by the time this chain of subspecies has spread eastwards around the northern hemisphere and the ends meet up again in Europe, the Lesser Black-backed Gull and the Herring Gull are obviously separate species and interbreed only very rarely. This beautiful explanation of a series of subspecies first spreading, then part of each becoming isolated, allowing the formation of further subspecies around the northern hemisphere and eventually producing two distinct species, was widely acclaimed and has been frequently quoted in books, scientific papers and lectures on genetics and speciation.
However, the recent development and application of mitochondrial DNA techniques has shown that the American