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.

On reaching adulthood the young animal faces the problem of settling into the patch of habitat which may be its home for the rest of its life. The selection of an area is crucial to the individual's genetic fitness since it may affect both survival and breeding success and thus the total number of progeny produced in the animal's lifetime. Many individuals settle preferentially in their natal area, but others emigrate and face a choice between different regions for settlement. Selection of an area may depend mainly on assessment of the resources available but should also take into account the degree of competition for them. The response of individuals to increasing competition may limit the total number of young which settle in a local population and this may be important in the limitation of density.

In many territorial species, recruitment occurs in two stages, firstly to the non-territorial flock and then from there to the attainment of a breeding territory, sometimes several years later (Charles, 1972). In this chapter I will describe the return of young adult shelducks to the flock, the factors affecting settlement and the possible consequences for population density.

Return of the young

After dispersing for their first winter, many young shelducks return to their natal area, mostly when about one year old. Of 60 Ythan-reared ducklings (ringed before fledging) which returned, 45 per cent were first seen as yearlings, 40 per cent as two-year-olds and 15 per cent at three years old (Patterson et al., in press b).