Gulls. Professor John C. Coulson

with spots of a darker colour. They are laid in mid- to late April (the date of the first egg laid in different years at Ravenglass ranged between 12 April and 26 April), but even at this time the colony is still often deserted at night, with the adults moving to night roosts and leaving the early eggs unprotected. As clutches are completed and incubation begins, increasing numbers of birds remain in the colony throughout the night; it is at this point that predation by Foxes on incubating adults may occur.

      In Britain, the peak of laying in Black-headed Gulls is reached at the end of April or in early May, which is earlier than in other gull species breeding here. Fig. 36 shows the date distribution for the first eggs in each clutch recorded by Ian Patterson at Ravenglass (1965). There is considerable laying synchrony by the majority of pairs, but there is often a distinct tail or a secondary late peak of birds that re-lay after losing their first clutch or delay laying owing to inexperience.

      The typical clutch comprises three eggs, although two-egg clutches are common. Occasionally, four-egg clutches occur, but whether these are laid by only one female has not been investigated. Late-laying birds produce clutches of just one or two eggs. Average clutch size within a colony varies considerably, from 2.3 to 2.8 eggs. Nils Ytreberg (1956) recorded a high average of 2.9 eggs from 411 nests in Norway, but in a later year reported a smaller average of 2.62 eggs based on a sample of 100 nests (Ytreberg 1960). The average clutch size tends to be lower in years when laying starts late, such as in small colonies and those further north and in the uplands. Eggs are laid at variable times throughout the day and the interval between eggs is usually about two days.

      FIG 36. The spread of the laying season at Ravenglass, recorded by Ian Patterson in 1965. The secondary peak at the end of May/early June probably comprised repeat clutches produced by pairs that had lost their first clutch or by pairs of young, inexperienced birds.

      Incubation of eggs and feeding of young is shared by both members of the pair. The eggs are covered by an adult irregularly while the clutch is still being laid, with the first egg probably protected rather than incubated. Obviously, incubation does not take place at night on occasions when the adults desert the colony in the evening. Intensive incubation starts at variable stages during laying of the clutch and often before the third egg is laid, but the brood patches of incubating birds are not fully vascularised for the first few days and initially the covered eggs may not reach a high enough temperature to facilitate development. This results in variable incubation periods, and the early start of incubation is sometimes enough to cause the third egg to hatch a day or so after the first two. Ivan Goodbody (1955) recorded incubation periods from the laying of the last egg of between 23 and 26 days at different nests.

      Rearing young

      A study by Roland Brandl and Ingrid Nelsen (1988) reported that Black-headed Gulls feed their chicks at intervals of about 45 minutes during daylight (but not at night), and that chicks receive at least 20 feeds each day. Each parent made five to six feeding trips per day, and obviously retained food from each trip for at least two feeding bouts. The authors found that this rate was mainly independent of the nestlings’ age or brood size. Adults regurgitate food onto the ground for the chicks and often re-swallow any surplus.

      The newly hatched young have cryptically marked down, and as they grow, the feathers are patterned in shades of brown. The chicks remain in the nest for about a week if undisturbed and are brooded by their parents for a decreasing length of time each day as they grow older. Brooding at night often continues for a day or two longer after daytime brooding has ceased. Chicks eventually move a short distance away from the nest and shelter in nearby vegetation, where they receive some protection from both adverse weather and predators. One parent remains at the nest site most of the time, feeding the chicks by regurgitation and defending them against neighbouring adults and attacking wandering young from other pairs.

      Once chicks reach 20 days old, they develop an interest in searching the ground for items they can pick up and swallow. Although they probably find little that is edible this way, the behaviour seems to develop their ability to search for food. Young birds can fly when they are about 34 days old and leave the colony area soon after fledging, often accumulating in small groups in open areas nearby. There, they search for food, later joining feeding flocks of adults in fields or on the shore. There is no evidence that the young are accompanied or fed by their parents once they leave the colony, nor do they return to the nest site to be fed after their first flight.

      Breeding success

      Breeding success in Black-headed Gulls is highly variable and depends on whether the colony is protected from predators, including humans. American Mink, rats, Badgers (Meles meles), hedgehogs, Foxes and herons are major predators at some sites, and are suspected to have caused the desertion of several colonies. Intensive predation can result in very few young fledging from a colony. For example, in a sample area at Ravenglass studied in detail by Ian Patterson, 2,213 pairs each fledged an average of only 0.06 young in 1961, while in 1963, 2,290 pairs each fledged an average of 0.10 chicks (Patterson, 1965). This poor breeding success was mainly caused by Fox predation on eggs, young and adults late in the breeding season.

      At colonies where predation has been typically much lower than at Ravenglass, Black-headed Gulls probably fledge about 0.6–0.7 young per pair each breeding season. On its website, JNCC reports an average production of about 0.6 young per pair annually based on a few sample colonies studied between 1987 and 2015, but in the best year 1.2 chicks per pair fledged. Studies of several colonies over many years in France reported an average of 1.4 young fledged per pair (Péron et al., 2009). Once the last few young fledge, which is usually in late July or early August, colonies are rapidly deserted until the next breeding season.

      FOOD AND FEEDING

      Despite being small, the Black-headed Gull has been recorded feeding on a wide range of items, although in general these are smaller than those taken by the larger gulls. Animal material dominates, with invertebrates being by far the most frequently consumed food. Earthworms and the adult and larval stages of beetles and flies predominate in food obtained in fields. In coastal areas, marine crustaceans, molluscs and worms are frequently consumed when mudflats are exposed at low tide, or small food items are taken from the sea surface (Fig. 37). In addition to the consumption of animal matter, the gulls ingest a wide range of plant materials from time to time. Some of these are ingested incidentally with animal food, but seeds and grain are intentionally consumed – in autumn, for example, acorns are occasionally plucked from the tops of oak trees and Hawthorn (Crataegus monogyna) berries are picked off hedgerows by birds momentarily alighting on branches. A wide range of animal and vegetable materials, including bread, are taken around human habitation and at landfill sites. In some areas, bread and food scraps are regularly supplied by the public, particularly in parks and by lakes, and these form a major part of the winter diet in urban areas.

      The gulls look for potential food either by flying 1–2 m over the ground or water and landing to pick up items, or (more frequently) by landing, usually as a dense flock in a field, where they spread out by walking over the ground. This latter habit is used extensively just after dawn, as soon as the gulls arrive from their night roosts and when earthworms are still active on or near the ground surface. Small fish and riparian insects are captured and consumed on or near freshwater sources. Insects such as swarming winged ants are caught in flight on calm days in late summer. At times, adults bring large quantities of hairy caterpillars collected from moorland to feed to their young, but the hairs of some of these species are irritants and many chicks regurgitate them in numbers, such that they sometimes litter the ground within the colony. Occasional small mammals and amphibians are consumed, but these form a minute part of the diet.

      FIG 37. Black-headed Gulls feeding on small fish larvae. Note the unusual head position. (Norman Deans van Swelm)

Kleptoparasitism

      Kleptoparasitism is common in Black-headed Gulls,