Gulls

Less is known of the survival of immature gulls, but there is usually a lower survival rate in the 12 months following fledging, after which the survival rate approaches that of the adults.

The longevity records based on birds ringed as nestlings and living under natural conditions are given below, although several individuals are known to have lived longer in captivity.

The species with the longest recorded lifespans are mainly those that have been ringed in large numbers, and therefore have more chance of an exceptional record. It should be kept in mind that these lifespans are reached by exceptional individuals – perhaps one in several thousand – and so it is very likely that the maximum known age of many gull species in the wild will increase in future years as more recoveries of marked birds accumulate.

SIZE DIFFERENCES BETWEEN SPECIES

Gull species vary considerably in size. The Little Gull is the smallest and weighs about 100 g (the weight of an Arctic Tern, Sterna paradisaea), while the largest is the Great Black-backed Gull, with males averaging 1,800 g and some individuals exceeding 2,000 g. Fig. 4 shows the average weights of adult females of 19 gull species on the British list. The weights of females of nine of these species are less than 400 g on average and overlap with terns, of which the adult females of all except one species on the British breeding list weigh under 400 g. The distribution chart for male weights is similar, but is shifted to the right because of their slightly greater size.

FIG 4. The average weight of the females of 19 gull species on the British list.

INDIVIDUAL VARIATION

Like all animals, individuals of each gull species show variation in many characters, including size, colour and age at first breeding. Males are larger than females and tend to have a more substantial bill, and size within a species can also vary geographically.

Variation in the immature plumage is widespread in gulls of the same age and this is frequently overlooked in the field identification of species, particularly within the genus Larus. The occurrence of hybrid individuals further adds to plumage variation and typical examples of hybrids can often be identified in the field by experienced observers; the characteristics used often overlap with those of other species. Consequently a proportion of immature and even adult birds that are infrequently recorded in Britain and Ireland may fail to be identified because of potential confusion with other species.

Immature plumages

The plumage, leg and bill colours of recently fledged chicks are very different from those of their parents, to such an extent that, many years ago, a first-year Kittiwake was claimed as a species new to science, despite the adult having already been described and named some years earlier. The first plumage of the young of most gull species is made up of feathers of varying shades of brown and grey, producing a cryptic pattern that helps to conceal them in vegetation in a colony and also appears to reduce aggression from adults. In the smaller gull species, the plumage is replaced by one that resembles the adult at the first annual moult; that is, when the bird is 13 months or so old. In the larger species, all feathers are replaced each year, but only some of the new ones resemble those of the adults and the full adult plumage pattern is not achieved until four years after hatching. These progressive changes in plumage at successive annual moults can vary between individuals and produce a series of different plumage patterns that make it a challenge to identify both the species and the age of immature individuals.

The slow and progressive acquisition of the adult plumage through successive moults contrasts with the rapid growth of bones and wing feathers, which reach full size within a few weeks after hatching because they are necessary before flight can be achieved. Why the acquisition of the adult plumage takes longer in the larger species of gull than in the smaller ones is not clear. The mechanism determining plumage patterns is obviously controlled by hormones and is linked with the greater length of immaturity in the larger gull species, but it is not evident why the large species delay reaching maturity for so long.

Differences between the sexes

The sexes of gulls have identical plumage and differ only in that females are usually smaller and tend to have slightly less substantial bills. Fig. 5 shows the relationship between the differences in weight of male and female gulls of seven species, using data from different parts of their geographical ranges where available. The extent of the difference between sexes is not constant from species to species, but increases with the weight of the species, ranging from a 5 per cent increase in the Black-headed Gull to more than 20–25 per cent in large species.

Standard measurements of wing length also tend to be longer in males than in females, but the magnitude of the difference is much smaller, ranging from 1 per cent in some small species to 6 per cent in the largest species (Fig. 6). Even when this difference is converted to wing area, it still results in the wing loading being higher in the large gulls, which explains why these species typically have a more laboured flight, with a slower, more powerful wing-beat. The small gull species, which are of similar weight to many tern species, have a characteristic buoyant flight similar to that of terns.

FIG 5. The percentage by which male gulls of several species are heavier than females, based on data for seven well-studied species. It is evident that there is a much greater difference between the size of males and females in the larger species of gulls.

FIG 6. The relationship between adult weight and the extent to which the male has a longer wing than females. The percentage difference in wing length between the sexes increases in heavier species, but is much less than the difference of body weight shown in Fig. 5.

The reason why there is a greater size difference between the sexes in large gull species is not known, and currently it is possible only to speculate. Perhaps there is a greater need in the large species to reduce competition for food between the sexes, or perhaps the dimorphism is related to the greater need for males of large species to defend nesting territories. The reader might speculate further, bearing in mind that in the skuas, females are invariably larger than males, while male terns are only 1–3 per cent heavier than females.

Despite the average size differences, there is an overlap in the range of sizes of male and female gulls. Niko Tinbergen claimed that, despite the overlap in size between the sexes of Herring Gulls, invariably the male is larger in every pair. Because of the average difference in size between the sexes, by chance the male will be larger than the female in many pairs, and more so in the larger gull species, but I have not found evidence that the male is invariably larger than the female. Size, and particularly the size of the bill, may play a part in individual birds recognising the sex of other gulls, but it is more likely that behaviour – particularly during courtship – plays the major role in sex recognition in gulls, especially in smaller species.

Sexing gulls

As male and female gulls have identical plumage features, distinguishing them in the field can be very difficult. The most reliable way to determine the sex of an individual bird – without killing it and then dissecting it to examine the gonads – is by carrying out a DNA analysis on samples obtained from feathers or blood. While this method is highly efficient, it is time consuming and it is expensive when large numbers of birds are being studied. In the field, biometric measurements taken while a bird is temporarily captured for ringing can also be used for sexing the individual. I found that the best measure was the head and bill length (from the back of the head to the bill tip), which also had the advantage of showing the highest degree of consistency when measured by different people. Further, the proportionate difference in head and bill length between the sexes is almost twice that for wing length (for example, 9.6 per cent compared to 5 per cent in the Great Black-backed Gull). The only disadvantage of this measure is that in some museum specimens part of the back of the skull was removed during preparation, which prevents it being used in these cases. As shown in Table 2, the head and bill measurement is satisfactory in sexing 92–98 per cent of individuals of several gull species. Including two other body measurements (wing length and bill depth) in a discriminant analysis increased the accuracy of sexing only by less than 2 per cent points.

TABLE 2. The head and bill lengths of adults of six species of gulls in Britain, the measure separating the sexes and the proportions sexed correctly by this single measurement. The data are based on samples of at least 80 individuals of each sex breeding in Britain, except for Common Gulls (Larus canus), which were captured in winter and so were from unknown breeding areas. Based on Coulson et al. (1983a) and additional data.

When a group of breeding gulls is being studied, the behaviour of marked individuals can be used as a reliable method of sexing. Copulation is totally reliable in this respect, as is courtship feeding of the female by the male and intensive food begging by the female. More details on the methodology used to sex gulls are given in Chapter 12.

Adult plumages

There is considerable variation in the shade of grey on the wing and mantle in adult gulls of the same species, which is evident in birds nesting in the same colony. This is illustrated in Fig. 7, which shows the extent of such variation in Herring Gulls breeding on the Isle of May in Scotland (subspecies Larus argentatus argenteus) and in northern Norway (subspecies L. a. argentatus). Because of the variation, there is overlap in wing shades between the two subspecies of Herring Gulls and most, but not all, individuals can be identified on this basis alone (see also box). Even using more measurements of body size does not completely separate all argenteus males from argentatus females.

In Lesser Black-backed Gulls breeding in the Netherlands (Fig. 7), there is also considerable variation in wing shade, with the palest approaching the darkest shade of Herring Gulls breeding in northern Norway. The darkest shade reported in Lesser Black-backs in the Netherlands is said to fall within the shade range of the subspecies Larus fuscus fuscus, which breeds in eastern Scandinavia and typically has a black mantle and wings very similar to those of the Great Black-backed Gull. The majority of Lesser Black-backed Gulls breeding in the Netherlands have a range of shades found in both the subspecies L. f. intermedius (breeding in north-west Europe) and L. f. graellsii (breeding in Britain).

Identification of the Lesser Black-backed Gull subspecies intermedius and graellsii in the field is further complicated by whether the individual is seen in bright sunlight or under dull conditions, and also by the direction of the light, all of which affect the apparent shade of grey of the same individual recorded in photographs or observed in the field. Reliable records of shade need to be measured with the bird in the hand, using standard lighting and comparing it against a reliable shade chart, but even this would not identify the subspecies of all individuals.

FIG 7. The shade of the wings of adult (a) Herring Gulls (Larus argentatus) breeding in Britain (n = 1,591) and (b) northern Norway (n = 140), and (c) Lesser Black-backed Gulls (L. fuscus) breeding in the Netherlands (n = 899). The shades increase in darkness from left to right and correspond approximately to shades of grey, which range from 1 (white) to 20 (black). The data for Lesser Black-backed Gulls are taken from Muusse et al. (2011).

Differences in wing pigmentation

A major source of plumage variation within gull species is the pattern of black or brown pigmentation on the wings of immature individuals. These show progressive changes at each annual moult, until adult plumage is eventually achieved. In addition, this patterning varies appreciably between individuals of the same age, even within a single colony. For example, fully adult Herring Gulls in the same colony showed a range in the number of primaries that are tipped with black pigment (Table 3), and they also showed variation in the extent of white on the tip of the longest (10th) primary. Part of this variation is linked to the age of the birds (Table 4), with change continuing for several years after individuals reach breeding age, but the patterning is not related to gender.

TABLE 3. The percentage of fully adult Herring Gulls (Larus argentatus) examined in breeding colonies with different numbers of black-tipped outer primaries. Dutch data from Muusse et al. (2011), Norwegian data mainly from Barth (1975).

TABLE 4. The wing-tip pattern in Herring Gulls (Larus argentatus) of known age breeding in colonies in Britain. The extent of white on the tip of the 10th primary tends to increase with age.

Part of the variation is also geographical, as seen in adult Herring Gulls breeding on the east and west sides of England and Scotland (Table 5). This difference between east and west in the black-and-white patterning on the primaries is maintained in the winter, presumably because the north–south dispersive movements of British Herring Gulls mainly follow either the eastern or the western coastlines, with relatively few individuals crossing between the two coasts. Figs 8 and 9 illustrate the variation in two adult Herring Gulls captured in north-east England, with one showing the thayeri-type pattern on the ninth primary (second from left), where the black does not spread across the whole of the width of the feather.

TABLE 5. Comparison of wing-tip patterns of adult British Herring Gulls (Larus argentatus) breeding on the east and west coasts of England and Scotland. The differences in both characters are significant.

FIG 8. The wing-tip pattern of an adult Herring Gull (Larus argentatus) captured in north-east England, showing what is known as the thayeri-type pattern on the ninth primary. The bird was subsequently found breeding in northern Norway. (John Coulson)

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 or Lesser Black-backed gulls (as discussed above for the American Herring and Great Black-backed gulls in North America).

BREEDING

Gulls are monogamous, although a few cases of male Kittiwakes breeding simultaneously with two females at different nest sites have been recorded. Pairs of gulls produce only one brood each breeding season, but if their eggs are lost, many will lay a replacement clutch. While most gulls breed annually during a well-defined breeding season, some individuals skip breeding for a year. The exception is the Swallow-tailed Gull on the Galapagos Islands, which does not have a clear-cut breeding season and nests throughout the year, with individuals breeding at nine- to 10-month intervals.

Breeding sites

Gulls typically favour bare ground and areas with short vegetation for nesting, or floating vegetation on lakes or marshes. The main exceptions are Bonaparte’s Gull, which regularly nests in trees; Common, Black-headed and American Herring Gulls, which occasionally nest in low trees at a small number of localities; Kittiwakes, which favour narrow ledges on steep sea cliffs; and Herring, Glaucous and Ivory gulls, which sometimes use larger cliff ledges.

Ground nesting makes gulls particularly susceptible to mammalian predators, and most species nest only at sites where these predators are usually unable to reach the colonies, such as small islands or isolated peninsulas. Gulls vary in their ability to deter avian predators. Adults will attack birds of prey and corvids, but in parts of northern Scandinavia White-tailed Eagles (Haliaeetus albicilla) are having an increasing impact on breeding gulls – this is a future risk for Britain, since the species has been reintroduced here and its numbers are increasing. In addition, adult gulls at breeding sites suffer occasional predation from Peregrine Falcons (Falco peregrinus). Ravens and crows are a problem for some smaller gulls, but in general they are attacked and prevented from entering dense gull colonies. Individual Herring, Lesser Black-backed and Great Black-backed gulls, as well as Great Skuas (Stercorarius skua), have developed the ability to reach and prey on eggs and young gulls at otherwise well-protected nesting sites (sometimes even attacking their own species).

Mammalian predators such as Red Foxes (Vulpes vulpes) and Badgers (Meles meles) have now reached some gull colonies in Britain after being absent for many years, and these and the spread of American Mink (Neovison vison) has often resulted in the sudden desertion of sites used by breeding gulls. This desertion may be immediate, while in other cases the decreases in numbers of adults are spread over several years, apparently because new recruits to the colony are deterred by the presence and activity of predators. In particular, Foxes have become very much more abundant in Britain in recent years, and have captured and killed many incubating gulls at night. Six gull species now also nest on buildings in urban areas (here); these have the same characteristics as natural sites, in that mammalian predators cannot normally reach them and they are generally given public protection.

Humans entering gull colonies are not usually attacked by the smaller gulls, which instead tend to fly overhead giving alarm calls. Large gulls do frequently dive at human intruders, however, usually from behind. While they pass closely overhead, they seldom actually strike. I have been struck only once by a large gull, although one of my students was knocked to the ground by a particularly aggressive Lesser Black-backed Gull defending its unfledged chicks.

Colony and nest site fidelity

Adults gulls are highly site faithful, provided that the nesting site remains safe and is not subject to high levels of predation on eggs, young or adults. Males – particularly those that were successful in rearing young in the previous breeding season – tend to return to the same nesting sites. In contrast, young birds are much less likely to return to the place where they were reared as chicks. A proportion – usually dominated by males – does so, and these birds often return to the same part of a large colony in which they were reared. This behaviour is called philopatry, the extent of which is influenced by many factors, including competition for nesting sites and food availability. In the past, the proportion of birds moving elsewhere has often been underestimated because of the much greater difficulty in locating those that have moved. In some species, the majority of the young that survive to breed move to other colonies, with some moving 100 km or more away. In several species, young individuals have been recorded visiting a number of colonies while approaching maturity, including their natal colony, only then to move and breed elsewhere. Such movements are, of course, necessary to form new colonies.

Colonial breeding

Colonial breeding is widespread among gulls, and only a few species regularly breed both as isolated pairs and in colonies. Some gull colonies are composed of mixed species, and the smaller species frequently nest in or alongside colonies of terns. In many gull species, it appears that single pairs cannot breed in isolation and the presence of a group of gulls of the same or even different species is necessary before egg-laying is possible, thus making colonial breeding essential. The main exceptions are Common and Great Black-backed gulls, where a proportion of breeding pairs nest in isolation from others. It is obvious that colonial nesting is not forced upon gull species as a result of shortage of suitable nesting sites, and it is usually regarded that there is an advantage to breeding close together. An obvious reason is that it improves defence of eggs and unfledged young against predation, but while this is evident in deterring Ravens (Corvus corax) and crows, it is less evident that colonial nesting prevents mammalian predators from raiding colonies and consuming eggs, young and even adults.

The reliance upon a group to ensure breeding suggests that stimulation from other individuals is necessary. This was first suggested in 1938 by Frank Fraser Darling, who noted that the display from neighbours within the colony stimulated pairs to breed and in larger groups or colonies to lay eggs earlier, and that it resulted in greater synchrony with neighbouring pairs. He suggested that the effect of this synchrony was that eggs or chicks were available over a shorter time period, and therefore fewer were predated and breeding success was enhanced. At the time, Fraser Darling’s idea appealed to some, but others were critical of the concept. Unfortunately, his original data on Herring and Lesser Black-backed gulls only hinted at this effect, and others later showed that the variations he found between colonies of different sizes could have been produced by chance and were not statistically significant.