LITMIR.BIZ

Скачать книгу

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.

      In the 1950s, Edward White and I collected information on the timing of breeding in a series of colonies of Kittiwakes. We found that the laying season in large colonies was more spread out than in small ones, the opposite of Fraser Darling’s contention. While some pairs in the larger colonies started laying earlier than those in the smaller ones, pairs of late-breeding birds (which we now know were mainly young, first-time breeders) were recorded in all the colonies studied.

      However, we did discover that breeding in smaller groups of 20–50 Kittiwake pairs within a single colony was more synchronous than in the colony as a whole, and that the average breeding date of each of these groups was earlier as the density of pairs increased. This finding suggested that social stimulation was occurring at the more local level, among groups of breeding pairs, and not in the colony as a whole.

      I know of no single pair of Kittiwakes that has been recorded breeding in isolation. As a result, it is difficult for members of the species to form new colonies because this requires a group of individuals to be attracted to, and display at, a new potential breeding site before egg-laying can occur. Up to 20 pairs will collect at the site of a potential new colony for a year or more, and before a few of the pairs succeed in building a nest and laying eggs. However, in subsequent years the numbers breeding increase rapidly. In contrast, Herring Gulls sometimes nest in isolation, but these are birds that have bred before. Young Herring Gulls attempting to breed for the first time do not seem to be able to breed without joining a group already breeding together.

      Colonial nesting allows gull species to benefit from the stimulation of their neighbours engaging in courtship activity, and probably encourages earlier nesting in some individuals. This was clearly evident in Kittiwakes when observing a pair reuniting at the colony after one member had been away on a fishing trip (Coulson & Dixon, 1979; Coulson, 2001). The birds engaged in a vocal display of calling, which immediately stimulated similar mutual calling in neighbouring pairs. Where Kittiwake nests were denser, the frequency of this

Скачать книгу