But just how common are gulls, really? And are they all the same? Depending on what taxonomic authority you want to follow, there are somewhere between 50 and 60 distinct species of gull recognized worldwide. Due to the propensity of many gull species to hybridize, gull taxonomy can be notoriously contentious. Below is a world map with the names of the 55 gull species that I recognize (I mostly follow the classification used by Olsen, 2018). I've placed the common species names in the approximate centre of their breeding range. This gives a decent picture of where these gulls make their homes, though a few species' ranges are distorted. For example, the Grey-headed Gull (Chroicocephalus cirrocephalus) breeds in both southern Africa and southern South America; the Kelp Gull (Larus dominicanus) breeds on all four continents in the southern hemisphere, the only gull known to nest in Antarctica.
the approximate centre of their respective breeding ranges.
The obvious answer seems to be that gulls simply don't like to nest in tropical climates. Indeed, this seems to be the case, but the question then becomes why not? The tern family (Sternidae) comprises the closest cousins of the gulls, yet they have no problem nesting in the tropics as you can clearly see from the map below (taxonomy taken from Bridge, Jones, & Baker, 2005).
the approximate centre of their respective breeding ranges. Unlike gulls, many species have
breeding ranges that circumnavigate the globe along wide swaths of latitude.
Yet, gulls have been around for long enough to disperse and speciate globally, about 5 to 10 million years (see Pons, Hassanin, & Crochet 2005). Gulls of course pass through the tropics during migration, and some will even overwinter there (e.g. the Laughing Gull (Leucophaeus atricilla) of the Caribbean). So why haven't they started breeding more in the tropics? There have been many hypotheses over the decades, but no satisfying answers.
The tropics provide many rich food sources and sustain plenty of other similar species, like many terns. The tropics should have no trouble supporting gulls too. Gulls are highly adaptable omnivores and will change their feeding habits to accommodate all kinds of short- and long-term changes in their food supply. Moreover, a favourite food source is the eggs and chicks of other birds (sometimes, even other gulls). The tropics are home to a variety of colonially breeding birds year-round, providing vast and stable food sources for gulls. Surely there would exist competition for these food sources with other species, but no one would accuse gulls of meekness. If gulls can bully eagles and ravens in the Holarctic, it's unlikely that they have been bullied out of the tropics themselves by competing species.
Clearly then, it's not lack of food or opportunity that is keeping gulls away.
One hypothesis that I find intriguing is that gulls don't usually breed in the tropics because their bodies just aren't well adapted to dealing with the heat stress. Like all animals, gulls possess ways of cooling themselves: they "pant", erect their feathers to capture a breeze, seek shade, bathe, and drink water. However, sitting on eggs and tending to young can make all but the first of these options difficult. You may notice that a nesting gull will often stand above their nest and pant when it is very hot out. This is no accident: gulls loose heat through their feet (Steen & Steen 1965) and standing also makes feather erection more effective at capturing a breeze. Standing can also prevent their eggs from getting overheated, a potentially fatal event for the developing chicks inside.
on a particularly hot day in Vancouver, BC. Note that chicks this young still cannot regulate their own
body temperature; they need their parents' help to stay warm enough and cool enough.
Now, terns share all these behaviours with gulls, but they also have a few more adaptations in their favour that would make tropical nesting more feasible. Terns, especially those that live in the tropics, are smaller and have proportionally narrower and longer bodies compared to gulls (on average). This means that terns have a large surface-area-to-volume ratio, while gulls have a relatively small one. A higher ratio is good for tropical living: the body has more surface area from which to dissipate heat. A lower ratio is good for colder climates: less surface area means less heat loss. This general principle is called Allen's Rule after the 19th Century zoologist Joel Allen.
Moreover, it has been experimentally shown that dark plumage can be more effective than light plumage at alleviating heat stress in a breeze (Walsberg, Campbell, & King 1978). There are 7 gull species that have non-white chests and/or bellies as adults, and all but one (the Dolphin Gull (Leucophaeus scoresbii)) breed in high heat conditions: these are the Heermann's, Swallow-tailed, Lava, Grey, Sooty, and White-eyed Gulls. If you look back at the gull breeding map above, you'll see that these gulls comprise all the "tropical" species. Note too that the Grey Gull (Leucophaeus modestus) nests in the Atacama Desert (!) of Chile, another high heat stress location. Dark plumage is far more common in the tern family (about 35% of all species). In fact, all the noddies have darkish plumage and all nest in tropical locations. Same for the terns of the Middle East and India.
grey chest and belly, along with the very dark wings.
These facts explain why gulls look the way they do given their environment, but they don't really tell us why evolution hasn't equipped more gulls with adaptations to breed successfully in a high temperature climate. Of course, the reason could be as simple as the positive selective pressures to colonize the tropics just aren't strong enough. But there is one other interesting possibility too.
Some species of gull are known to prey on the eggs and young of their conspecifics (i.e. members of the same species). This would be a maladaptive trait in a high heat environment: parents would require more trips to water sources to drink, bathe, and cool themselves off, more frequently leaving vulnerable eggs and young exposed to predation. The Grey and Swallow-tailed Gulls have adapted to their high heat environments by not attacking exposed eggs and young. Basically, they've evolved a kind of quid pro quo understanding with their neighbours. To my knowledge, all tern species have a similar arrangement: they do not prey on the eggs and young of conspecifics (e.g. see Dinsmore 1972). Such an arrangement allows nesting adults to leave their nests to cool off without fear of a raid. Moreover, their neighbours provide a deterrent to any non-conspecific predators.
This is an interesting hypothesis. From an evolutionary point of view, perhaps the advantages of colonizing more of the tropics are balanced out by the fitness advantages of preying on the eggs and young of conspecifics: you can't seem to have both at once. However, I think the prevalence of such cannibalistic behaviour has been overestimated in the gull family. It has been well-documented (see e.g. Tinbergen 1953, or Parsons 1971) in the European Herring Gull (Larus argentatus), undoubtedly the most thoroughly studied species of gull. Egg cannibalism has also been observed in Glaucous-winged Gull colonies. However, there are many other species of Holarctic gull that do not exhibit this behaviour (e.g. the Franklin's Gull (Leucophaeus pipixcan) - see Burger 1974), or for which such behaviour has never been observed, e.g. the smaller Mew (Larus brachyrhynchus) and Bonaparte's Gulls (Chroicocephalus philadelphia). Perhaps then a tendency toward egg cannibalism is just a characteristic of the large, white-headed gulls of the Holarctic. This only accounts, however, for about 30% of all gull species. So the fitness advantages of egg and young predation can only be part of the reason why the gull family has failed to colonize the tropics.
One final note: the gull habit of preying on the eggs and young of their neighbours is a characteristic of colonially nesting gulls. I have been studying non-colonially (urban) nesting Glaucous-winged Gulls for awhile now, and conspecific predation seems to be vanishingly rare. I am unaware of any literature mentioning if such behaviour is retained in non-colonial (urban) populations of the European Herring Gull, but I would like to find out. If non-colonial nesting is causally related to urban nesting (this is likely, given the structural realities of the urban environment), then might we predict that urban centres would be the easiest locations for new gulls to colonize in the tropics?
That's heavily speculative. One thing I can say for sure though is that until more gulls colonize the tropics, you won't find me living there!
Dinsmore, J.J. (1972). Sooty Tern behaviour. Bulletin of the Florida State Museum Biological Society, 16, 129-179.
Hand, J.L., Hunt Jr., G.L., & Warner, M. (1981). Thermal stress and predation: influences on the structure of a gull colony and possibly on breeding distributions. The Condor, 83, 193-203.
Olsen, K.M. (2018). Gulls of the World: A Photographic Guide. Princeton University Press.
Pasons, J. (1971). The Breeding Biology of the Herring Gull. Doctoral thesis. Durham University.
Pons, J.-M., Hassanin, A., & Crochet, P.-A. (2005). Phylogenetic relationships within the Laridae (Charadriiformes: Aves) inferred from mitochondrial markers. Molecular Phylogenetics and Evolution, 37, 686-699.
Steen, I. & Steen, J.B. (1965). The importance of the legs in the thermoregulation of birds. Acta Physiologica Scandinavia, 63, 285-291.
Tinbergen, N. (1953). The Herring Gull's World: A Study of the Social Behavior of Birds. Oxford, England: Frederick A. Prager, Inc.
Walsberg, G.E., Campbell, G.S., & King, J.R. (1978). Animal coat color and radiative heat gain: a re-evaluation. Journal of Computational Physiology, 126, 223-231.