Gulls. Professor John C. Coulson

of their paler background in comparison to the eggs of most other gulls (see here). Fig. 11 shows the relationship between female weight and egg weight for a series of species. The relationship is curvilinear, but the Ivory Gull and Kittiwake sit almost exactly on the curve, fitting the data based on the other gull species.

      Each egg is laid at about two-day intervals. Usually, the last egg in the clutch is the smallest and tends to differ in shape, being longer and narrower than the rest. The infrequent cases of four eggs in a clutch are probably the result of a second female contributing one or more eggs. This can occur when female–female pairs are formed, caused by a shortage of males. Incubation lasts for more than three weeks and is longer in the larger species. Both members of the pair contribute to the incubation and feeding of the young.

      Incubation period

      Ideally, the precise measure of the incubation period is the length of time that the eggs are maintained at a temperature just below the adult’s body temperature of about 40 °C. There is a lower temperature (which may be as high as 25 °C) below which the development of the embryo ceases; in temperate regions, this is usually above the environmental temperature. As a result, development soon stops when the eggs are not covered by an adult and the temperature falls below the critical temperature for development. If the incubation of the eggs is frequently interrupted, the time taken for the eggs to complete development to hatching is lengthened.

      In practical terms, the incubation period is usually measured as the number of days taken from the start of incubation to the time the egg hatches, but hatching takes several hours and confusion has existed as to whether the end of incubation is when the shell is first pierced by the chick or when it becomes entirely free of the shell. Covering the eggs by an adult gull often takes place before the final egg of the clutch is laid, but in some cases the vascularisation of the brood patches on the adult may not have become fully developed with supplementary blood vessels and the optimal temperature for embryo development may not be reached. Consequently, there may be a delay of a day or so until the eggs reach the critical temperature for development. As a result, the recorded incubation period for a species will vary. The incubation period of birds, and in gulls in particular, is slightly longer in the larger species, but the differences are small and the variation recorded between individual pairs is considerable (Table 6).

      In gulls, the incubation of the eggs is often initiated by the female, but overall the sexes tend to share incubation equally in a shift system. Either sex may incubate overnight. In most cases, the incubating individual remains on the eggs until relieved by its partner, so that once incubation starts, the eggs are covered by an adult for about 90–95 per cent of the time.

TABLE 6. Estimates of the incubation period in eight well-studied gull species, presented in order of adult size (smallest first). Data mainly from Cramp & Simmons (1983), Fisher & Lockley (1954) and other sources where appropriate, including personal data for the Black-legged Kittiwake.

      Periodically – perhaps five to eight times a day – the incubating adult will stand up and roll the eggs. Egg-turning is believed to facilitate the absorption of the albumen by the embryo and prevents the embryo from adhering to the shell membrane. In domestic hens, the failure to roll eggs in an incubator does not necessarily prevent chicks from hatching, although the success rate is reduced and the chicks often have a lower weight. Egg-turning is unique to birds and the reason why reptile eggs do not need to be rolled is as yet unclear.

      In gulls that lay three-egg clutches, it is common for the third egg – which is laid two or three days later than the second egg – to hatch one or sometimes two days after the first two. This asynchronous hatching suggests that effective incubation in these cases starts during the normal two-day period between the laying of the second and third eggs.

      Chicks

      At hatching, chicks of most gull species have a patterned down that offers some cryptic protection; this is replaced by a pattern of dark feathers that helps to conceal the young birds when they attempt to hide under vegetation. The dark patterning at hatching is missing in Kittiwake chicks, which hatch with a pale grey down and then acquire white, grey and black feathering when older, all of which could also be regarded as aiding camouflage among the whitewashed nests and rocks on the cliffs where they nest.

      An effect of the relationship between egg size and adult weight is that, at hatching, the chicks of larger gulls are smaller in proportion to the adults than those of small gull species, and they therefore must achieve more growth in order to reach their adult size and weight. This is presumably one of several reasons why the period of chick growth (and the fledging period) tends to be longer in large gull species than in small ones.

      The time between the chick hatching and first flying is known as the fledging period and also shows much individual variation (Table 7). There is a weak trend for it to be longer in the larger species of gulls. The fledging period of the Kittiwake tends to be longer than in other gulls of a similar size. In the case of the Kittiwake, the first flight has to be totally successful from the cliff-nesting site and so is an all-or-nothing event, with the young bird either flying well enough to reach and return to the nest, or to achieve sustained flight immediately, leave the colony and start an independent life.

      Fledging is often a gradual process and tends to take longer in the larger species. It is longer in Kittiwakes, where the young cannot fly until they are at least five weeks old and many are at least a week older before their first flight. Unlike most terns, parental feeding of young gulls often ceases at the time of fledging, but sometimes family parties of large gulls stay together for some weeks. During this time they visit feeding sites together and make loud contact calls while moving between areas, and they may even return to the colony in the evening. Exceptionally, a Herring Gull has been observed feeding chicks three months after they fledged.

TABLE 7. Estimates of the fledging period in eight well-studied gull species, presented in order of adult size (smallest first). Data mainly taken from Cramp & Simmons (1983), Fisher & Lockley (1954) and modified from other sources where appropriate. Note that the considerable variation in several species is genuine, and depends on the rate of growth and development of the chicks and whether there had been a stimulus to fly, for example, by the presence of a predator or the observer.

      Breeding success

      Under favourable conditions, breeding success of gulls is high, with about half the eggs laid producing young that fledged. However, lower success rates occur when there are food shortages or predation occurs. No gulls are known to breed as one-year-olds and they do so only when they have acquired almost adult plumage.

      WALKING AND FLYING

      Gulls have relatively short legs, and this is particularly true of Kittiwakes and Ivory Gulls. Gulls typically walk or run only short distances, and rapid movement beyond a few metres is usually achieved by flying. The exception to this occurs when gulls are searching for food on grassland and mudflats, when walking – with stops to pick up food – is the norm. Of all gull species, Kittiwakes walk less than any other. During an appreciable part of the year they are oceanic, either flying over or floating on the sea. Immature Kittiwakes that visit land during moult typically remain where they alight, and the adults fly directly onto their nests on precipitous sea cliffs without needing to take more than a step or two as they land. This behaviour is associated with the species having short legs (an adaptation to produce a low centre of gravity to enable the birds to withstand strong gusts of wind while nesting on sea cliffs), while the Kittiwake’s hind toe, which in other birds – including gulls – assists with walking, is reduced to a small protrusion.

      The long, narrow wings of gulls, petrels, albatrosses and shearwaters are an adaptation for flight over water,