The demographer collects data of birth and death rates so that he can predict population trends and changes. He needs to know the probabilities of surviving to different ages and the mean number of offspring born to individuals in each age class. Such data are very difficult to obtain except from human populations. Ideally, the probabilities of survival should be estimated by the proportions of survivors of a cohort of individuals who have been followed from birth until all have died.

Human populations consist of individuals of all ages. Reproduction is continuous. Many organisms reproduce annually at a particular time of the year. A population can then be divided into separate age classes. When the chicks have hatched in a population of birds, for example, individuals may be 0, 1, 2, … or x years old, having either just hatched, or hatched last year, or two years ago or, in general, x years ago. Birds are easily counted or caught for ringing in the breeding season. This is convenient because they are thus counted when almost exactly 0, 1, 2, … or x years old.

The demography of a population is usually described in terms of a standard set of symbols. The symbols l0, l1, l2, … lx, denote the probabilities of surviving for 0, 1, 2, … or x years. Obviously, l0, = 1, since the new-born have had no time to die.

Introduction to the theory of assortative mating

Assortative mating occurs when one or more phenotypes mate more often than at random with others like themselves. It is the tendency of like to mate with like. Disassortative mating, or negative assortative mating, is the tendency of unlike phenotypes to mate with each other.

Assortative mating has often been observed between phenotypes in polymorphic populations. It evolves as sub-populations become ethologically isolated. It can evolve without the isolation or sub-division of populations if mating preferences are selected in favour of particular phenotypes: the genes for the preferences increase in frequency in association with the genes for the preferred phenotypes; the association between the genes produces assortative mating (O'Donald, 1980a). Behavioural mechanisms such as imprinting on parental phenotypes produce assortative mating (Cooke, 1978). We should expect to observe and do observe, that assortative mating takes place in many polymorphic populations.

Disassortative mating has also been observed, though it does not seem to follow from simple evolutionary or behavioural mechanisms. The obvious example of disassortative mating is sex itself, which has still to be explained satisfactorily in terms of selective advantage (Maynard-Smith, 1978). Self-incompatibility systems are other examples. Disassortative mating between the sexes is of course complete: all fertile matings are males × females. Apart from these special mechanisms preventing asexual reproduction or inbreeding, a partial tendency to disassortment in matings between different phenotypes has been observed in certain polymorphic populations of birds.

Kleptoparasitism by Arctic Skuas in the breeding season and on migration

Arctic Skuas are pirates and predators. On the islands of the North Atlantic, they forage as air pirates, pursuing other seabirds in the air and forcing them to drop their food. This behaviour has become known as ‘kleptoparasitism’. Arctic Skuas are said to be ‘kleptoparasites’. On arctic tundra in northern Norway, they are predators as well as pirates, feeding on small birds, eggs, rodents, insects, and also on berries like the Crowberry (Empetrum nigrum). On Fair Isle, the chicks in particular eat considerable quantities of crowberries.

Auks (Puffins, Razorbills and Guillemots), Kittiwakes and terns (Arctic, Common and Sandwich Terns) are the main victims of the Arctic Skua's piracy. Several detailed studies have been made of the methods Arctic Skuas use in attack and the factors influencing their chances of success (Grant, 1971; Andersson, 1976; Arnason & Grant, 1978; Furness, 1978; Taylor, 1979). Arnason (1978) and Furness & Furness (1980) observed the chances of success in attacks by melanic and pale Arctic Skuas.

Grant (1971) and Arnason & Grant (1978) observed Arctic Skuas in the breeding season attacking Puffins. Taylor (1979) observed them on migration attacking terns. Arnason & Grant (1978) also analysed the factors that influence successful piracy.

Arctic Skuas usually hunt alone (Grant, 1971; Taylor, 1979). They search for victims among flocks of foraging Puffins, terns and Kittiwakes.

Surveys of breeding colonies

Like many birds, Arctic Skuas can be observed and counted fairly easily on their breeding grounds. Breeding birds have been counted at many different colonies in the last 50 years. After the breeding season is over, they can be counted at points along their routes of migration. Counts of numbers of migrating birds give rough estimates of the relative proportions migrating along different routes, but no estimates of population sizes. Even in a breeding colony, numbers are difficult to estimate accurately. There are several sources of error. Estimates are usually too low. The exact number of breeding birds in a population can be determined only by surveying and marking all the nests as they are found. The survey must be repeated every two or three days throughout the period when the birds are incubating their eggs.

On Fair Isle, the first eggs are usually laid on 16 May. The last eggs may be laid on about 25 June, 40 days later. If a survey were carried out at the end of May, little more than half the pairs would be found. If a survey were carried out on about 20 July, when the last eggs were hatching, about half the pairs would already have fledged their chicks. They would no longer be defending their territories or feigning injury at the approach of an intruder. Many of the pairs would then be very difficult to ascertain.

The Arctic Skua and its breeding grounds

The Arctic Skua is a circumpolar Arctic species that breeds as far north as Spitzbergen. At the southern end of its range, it breeds in small colonies in parts of the Hebrides and the north of Scotland. But in the British Isles it is common only in Shetland (59°N) where it is fairly widespread. Large colonies are found on the islands of Fair Isle, Foula, Bressay, Noss, Fetlar and Unst. Isolated pairs and loose colonies can be found throughout Shetland. Recently it has been giving ground to the Great Skua or Bonxie. In 1950, the largest Shetland colony of Arctic Skuas was to be found in the bird reserve of Hermaness on Unst, the most northerly point of the British Isles. This colony was completely over-whelmed by Bonxies as they rapidly increased in numbers between 1950 and 1975, completely taking over the bird reserve. The largest compact colonies of Arctic Skuas are now to be found on Fair Isle and Foula: about 130 pairs breed on Fair Isle and 300 pairs on Foula. Both colonies are under attack. The Foula colony is being pushed into a smaller and smaller area by the spread of the Bonxies. On Fair Isle, the predators are human. Unfortunately, Arctic Skuas are recorded as having been shot on Fair Isle–bodies with bullet holes having been found. In spite of such depredations about 120–140 pairs have bred since 1975.

Taxonomy

Shelducks are intermediate between the true ducks and the geese, although they are usually placed with the ducks in the family Anatidae, where they join the sheldgeese in the tribe Tadornini (Johnsgard, 1961). There are seven species of shelducks: the common or northern shelduck Tadorna tadorna (L.), the ruddy shelduck T. ferruginea (Pallas), the Cape or South African shelduck T. cana (Gmelin), the Australian shelduck or mountain duck T. tadornoides (Jardine & Selby), the New Zealand or paradise shelduck T. varietaga (Gmelin), the radjah shelduck or burdekin duck T. radjah (Garnot) and the crested or Korean shelduck T. cristata (Kuroda) (which may be extinct). All are large brightly coloured ducks with many goose-like features. The lack of a camouflaged female plumage, the persistence of the pair bond and the prolonged parental behaviour shown by the male all resemble the geese, whereas the general morphology, voice and the existence of an eclipse plumage are duck-like. Johnsgard (1978) considered that, like the radjah shelduck, the common shelduck represents a rather isolated offshoot from the rest of the group. Both species are adapted for dabbling and feed chiefly on molluscs and other invertebrates, whereas the rest of the group are principally vegetarian.

The common shelduck was originally called Anas tadorna by Linnaeus in 1758, but has had a variety of other names including Tadorna cornuta (Saunders, 1889) Tadorna vulpanser and Tadorna bellonii (Yarrell, 1843).

Having selected a nest site, the breeding bird must build a nest, lay a clutch of eggs and incubate them. The number of young hatched will depend on the size of clutch which can be produced and success in hatching them, in the face of many factors which might cause failure. Incubating birds must divide their time between the eggs and the need to spend some time feeding, the balance depending on the fat reserves which can be lost over the incubation period. In this chapter I will discuss these and other problems encountered by laying and incubating shelducks.

The timing of laying

The date on which the first egg of the clutch is laid can only rarely be determined by direct observation during the egg-laying period, due to the inaccessibility of the nests and the birds' intolerance of disturbance. Instead, a number of indirect methods must be used. In a few nests, observed before laying is complete, the laying date of the first egg can be back-dated since normally one egg is laid per day (Hori, 1964a). If the hatching date and clutch size are known, the laying date can be estimated using the incubation period of 29–31 days (Hori, 1964a; Young, 1964a), plus one day for each egg in the clutch. More commonly the laying date is back-dated from the first sighting of the brood of ducklings, adding a further day which the young spend in the nest after hatching (mean of four nests observed at hatching by Young, 1964a).

Parental behaviour normally involves a number of different activities: feeding the young, providing them with shelter and protecting them from predators and other dangers. By investing time and energy in this behaviour, even at some risk to their own chance of survival, the adults enhance their genetic fitness by increasing the survival of their progeny. Shelducks, like other parents of precocial young, do not feed their broods but merely accompany them while they feed themselves. The parents do, however, actively provide shelter and protection as I shall describe in this chapter.

Leaving the nest

Newly hatched young shelducks stay in the nest for some time, usually for at least 12 h and up to four days if there is a wide spread of hatching date within the clutch (Hori, 1964a). During this period, ducklings and female call frequently. Hori was able to approach very close to a nest inside a shed and described a monosyllabic ‘aarrk’ and a soft running ‘ugg ugg ugg’ given continuously for long periods by the female. I installed a microphone in a nest burrow over the hatching period and heard similar calls, along with piping responses from the ducklings. All of the calls are given very softly and can only be heard at very close range. It is likely that the ducklings become imprinted on the mother's voice while in the nest (Gottlieb, 1965). Since the ducklings' first hours are spent in the dark, it is obviously functional to use auditory rather than visual characteristics of the mother as a basis for imprinting.

An animal can be said to show territorial behaviour when it has some attachment to a site (or occasionally to a moving object) and when it is aggressive towards other animals which approach that place. The resulting territory, around the site of attachment, has been defined in a variety of ways. Pitelka (1959) and Schoener (1968) emphasised the owner's exclusive use of an area, usually with defined boundaries, whereas Davies (1978) recognised territories wherever interactions between individual animals led to their being spaced apart more than would be expected from random settlement. Between these two extremes, I prefer the simple definition of territory as ‘a defended area’ (Noble, 1939; Nice, 1941). This embodies the essential features of a special place, around which there is aggressive defence, without implying particular features such as exclusive use or rigid boundaries, or particular consequences such as spacing out of the individuals, which may occur in many but not necessarily in all cases.

Territorial behaviour raises some interesting questions. Since aggressive defence of an area requires the expenditure of time and effort, there should be some corresponding benefit to the owner's fitness which outweighs the cost of territoriality (Davies, 1978). The spacing effect of territorial aggression would be expected to influence the dispersion pattern and density of populations. These possibilities are of particular interest in the shelduck since territorial defence, particularly by both members of the pair, is not common among ducks.

Shelduck territories

Towards the end of winter, from February to April, shelduck pairs detach themselves from the winter flock and scatter widely over muddy shores, or freshwater pools and creeks near the coast.