For decades, most gulls were classified under the genus Larus. This classification was based almost entirely on morphological, behavioral, and plumage characters suggested by several prominent ornithologists, notably Dwight (1925), Moynihan (1959) and Chu (1998).
In the earlier literature, Burch and Bonaparte felt that the gulls should be divided into two groups: the hooded and the non-hooded. This proved to be problematic since several known species at the time showed morphological characters that could be assigned to both groups (L. ichthyaetus is the classic example). Dr. Jonathan Dwight - the father of gull taxonomy and gull study - put forth the following in perhaps his greatest work contributed to ornithology, The Gulls of the World; Their Plumages, Moults, Variations, Relationships and Distributions:
"Accepting, then, the Laridae as a family, there are some forty or more species in a remarkably homogenous group that does not lend itself readily to subdivision. If on the one hand every minor difference or slight single character be recognized, as some authorities have advocated, then a multiplicity of monotypic genera hardly differing, if at all, from species will result and the value of such genera as taxonomic units will be completely nullified. If, on the other hand, ponderable differences are grouped in larger units, many slightly dissimilar species must be included in one genus".
Dwight gave the following solution to Burch and Bonaparte's two groups: Lareae and Xemeae. Under each group he suggested generic subgenera based on loosely allied species, although these subgenera seem to have never been advanced. It appears Dwight himself wasn't sold on the idea of various genera for the gulls:
"In fact, however we may arrange the species, some member of each group is bound to have some of the characters of others and rarely is there a species, or even a genus which has characters, or even one character, all its own".
Moyniham, more than others, held an inflexible position on splitting Larus into subgenera:
"Previous workers have recognized several different genera of gulls...None of this division is, however, really necessary or justifiable. All gulls are very similar to one another in fundamental morphological features...all the gulls must be in the single genus Larus" (Moynihan, 1959).
This grouping, however, seemed taxonomically uninformative to many modern ornithologists, including Crochet and Pons et al. who felt a genus as broad as Larus concealed the diversity of gulls. Surely one has to wonder why species such as Little Gull and Great Black-backed Gull, for example, would have ever been placed in the same genus, Larus.
In 2000, Crochet et al. proposed four general groups of gulls: masked, hooded, black-headed and white-headed gulls. This was certainly a step in the right direction, and in 2005, Pons et al. expounded on Crochet's modern work, providing an in-depth molecular phylogeny for all known larids. They identified 53 distinct species of gulls (two more than Burger and Gochfeld recognized in 1996). The two additional taxa that were recognized as legitimate species were smithsonianus and michahellis. For the sake of moving forward and away from a bulky genus, five former genera that were retired years ago were reassigned and several species were reshuffled into new clades. Pons et al. demonstrated that the genus Larus is not monophyletic, although the family of gulls is. Relying on convergent phenotypic attributes, they presented the following 10 genera:
Pons et al. argued that their genetic data was not only consistent with morphological and behavioral data, but also with voice and juvenile plumages as well - something Dwight and Moynihan are said to have overlooked, focusing mainly on adult plumages. Appreciably, Pons and company admitted that their work may very well be incomplete, and several clades remain unresolved until this day (2014).
A reoccurring question that I find myself asking is whether mtDNA evidence should outweigh the outward characters displayed by gulls. That is, what role should genetic data have on how we taxonomically classify the gulls? There are genetic studies that have suggested many gull "species", specifically large white-headed gulls from northern latitudes (LWHGs of near-arctic and arctic origin) are but one group of conspecific forms. Some have gone so far to suggest that species such as Glaucous (L. hyberboreous) and Herring (L. argentatus) are, genetically, more or less, the "same" species (see Bell et al. 2012). This, I've found to be very troubling, and I'd argue that as field observers, we should strive to categorize what we can see in the field, including plumages, voice, and breeding habitat preferences.
By no means is Pons' paper the last word in gull taxonomy, although dividing the large genus, Larus, seems to have moved Larinae in the right direction. There are still many questions at the species level and various field ornithologists and gull enthusiasts continue to adopt a few of their own unofficial divisions - some based strictly on mtDNA and some based solely on morphological differences.
Several groups still abound with questions including the subspecies of canus, fuscus and argentatus, not to mention the perplexing questions surrounding thayeri, kumlieni and glaucoides. Also, the rampant hybridizing of glaucescens with other large taxa in the North Pacific should be of much interest to anyone contemplating the concept of a species. Indeed, our biggest gap in gull study begins in the Bering Sea and west, including specifically, species that reside in Russia, Japan, and points west through the steepes of the Balkan region.
Baker et al. have recently found different results than Pons in their studies (2007); the differences are primarily in the relationships among certain clades. Pons et al. place the larger white-headed gulls of the genus Larus at the end of the sequence, implying this is the youngest evolutionary group.
Groupings based on Pons' phylogenetic tree:
Genus Creagrus (1)
- Swallow-tailed Gull, Creagrus furcatus
- Black-legged Kittiwake, Rissa tridactyla
- Red-legged Kittiwake, Rissa brevirostris
- Sabine's Gull, Xema sabini
- Ivory Gull, Pagophila eburnea
- Silver Gull, Chroicocephalus novaehollandiae
- Red-billed Gull, Chroicocephalus scopulinus
- Hartlaub's Gull, Chroicocephalus hartlaubii
- Brown-hooded Gull, Chroicocephalus maculipennis
- Gray-hooded Gull, Chroicocephalus cirrocephalus
- Andean Gull, Chroicocephalus serranus
- Black-billed Gull, Chroicocephalus bulleri
- Brown-headed Gull, Chroicocephalus brunnicephalus
- Black-headed Gull, Chroicocephalus ridibundus
- Slender-billed Gull, Chroicocephalus genei
- Bonaparte's Gull, Chroicocephalus philadelphia
- Saunders's Gull, Saundersilarus saundersi
- Ross's Gull, Rhodostethia rosea*
- Little Gull, Hydrocoloeus minutus
- Dolphin Gull, Leucophaeus scoresbii
- Laughing Gull, Leucophaeus atricilla
- Franklin's Gull, Leucophaeus pipixcan
- Lava Gull, Leucophaeus fuliginosus
- Gray Gull, Leucophaeus modestus
- White-eyed Gull, Ichthyaetus leucophthalmus
- Sooty Gull, Ichthyaetus hemprichii
- Great Black-headed or Pallas's Gull, Ichthyaetus ichthyaetus
- Audouin's Gull, Ichthyaetus audouinii
- Mediterranean Gull, Ichthyaetus melanocephalus
- Relict Gull, Ichthyaetus relictus
- Pacific Gull, Larus pacificus
- Belcher's Gull, Larus belcheri
- Olrog's Gull, Larus atlanticus
- Black-tailed Gull, Larus crassirostris
- Heermann's Gull, Larus heermanni
- Common Gull or Mew Gull, Larus canus
- Ring-billed Gull, Larus delawarensis
- California Gull, Larus californicus
- Great Black-backed Gull, Larus marinus
- Kelp Gull, Larus dominicanus
- Cape Gull, Larus dominicanus vetula
- Glaucous-winged Gull, Larus glaucescens
- Western Gull, Larus occidentalis
- Yellow-footed Gull, Larus livens
- Glaucous Gull, Larus hyperboreus
- Iceland Gull, Larus glaucoides
- Kumlien's Gull, Larus glaucoides kumlieni
- Thayer's Gull, Larus thayeri**
- European Herring Gull, Larus argentatus
- Heuglin's Gull, Larus heuglini
- American Herring or Smithsonian Gull, Larus smithsonianus***
- Yellow-legged Gull, Larus michahellis
- Caspian Gull, Larus cachinnans
- Vega Gull, Larus vegae****
- Armenian Gull, Larus armenicus
- Slaty-backed Gull, Larus schistisagus
- Lesser Black-backed Gull, Larus fuscus
*Note: The AOU adopted most of Pons's phylogenetic tree (Proposal 2007-B-3) but preferred to leave Ross's (genus Rhodosthetia) and Little (genus Hydrocoloeus) in separate genera; this leaves us with 11 gull genera in North America.
**Although Thayer's Gull remains a species in the eyes of the AOU, many maintain that it is the darkest subspecies of the Iceland Gull complex, including myself. There is insufficient evidence, it seems, to not include thayeri under the Iceland Gull. You can read more on this topic here.
***The AOU voted against splitting smithsonianus from argentatus (8 no and 2 yes votes).
**** Surprisingly, the AOU also voted this one down (6 no and 4 yes votes).