The population dynamics of the Eider Duck Somateria mollissima and evidence of extensive non-breeding by adult ducks

The dynamics of an Eider Duck population have been investigated over 25 years, using census and capture-mark-recapture methods. During the study the population increased two and a half fold, with two periods of major increases in numbers, giving a stepped growth pattern. Mean clutch size showed considerable annual variation, the extremes being 5.40 and 3.78 eggs. The variation in clutch size was greater than that recorded in the Netherlands. Adult female Eiders had a high annual survival rate, averaging 0.895, and varying between 0.75 and 1.00 in individual years. The survival rate decreased markedly in the old ducks. There was no indication of any change in the survival rate during the study. Recruitment of ducks to the breeding group was irregular, with most years showing little recruitment and a few years high recruitment. However, recruitment, associated with good duckling survival, appears to have been the main factor associated with increase in the population. In many years, an appreciable proportion of the surviving ducks, which had already bred in a previous year, failed to nest. The extent of non-breeding increased during the study and in one year, 1973, this reached 65%. Lower clutch size and adult survival were associated with years of high non-breeding. The ‘red-tides’ in 1968 and 1975 appeared to have little effect on the Eider. It is suggested that the Eider missed breeding in years in which its survival was potentially poor, in order to maximize its reproductive output during its life span. This is supported by the smaller clutch size laid by those females which nest in years when many females fail to breed. It is suggested that young ducks may also miss breeding in the year after first nesting but this is not associated with the non-breeding in older ducks, although it may he related to body condition. It is suggested that non-breeding by adults of long-lived birds may be widespread. This has important implications in survey work based on nest counts.     

Nonbreeding and nests without eggs in the Lesser Black-backed Gull Larus fuscus

Estimates of the number of breeding Lesser Black-backed Gulls Larus fuscus at the Tarnbrook Fell gullery, Lancashire, have been made from annual counts of nests since 1981. During all of these surveys, the mean percentage of nests which did not contain eggs has been remarkably constant from year to year (54%± 1.9%). A study of specific nests at the colony in 1992 showed that the great majority of these "empty" nests did not subsequently receive eggs. in 1993, a study of a sample of territory-holding gulls was made on a 0.41-ha study plot to investigate and to quantify the occurrence of empty nests. Of the 62 pairs that defended a territory and constructed a complete nest, 27% subsequently failed to produce eggs. These birds attended their territory as a pair significantly less frequently than pairs that produced clutches and achieved significantly fewer apparently successful mountings. Sixty percent of pairs constructed more than one nest, and 58% of all nests built received no eggs. The construction of empty nests was not found to be related to the density of nesting gulls. Since 1981, estimates of the number of gulls in the colony have been based on the assumption that one nest represents one pair of gulls. This has been shown not to be the case, and a correction multiplier of 0.61 must be applied to counts of nests at the colony to obtain an estimate of the number of breeding gulls.     

Habitat selection by the swamp wallaby (Wallabia bicolor) in relation to diel period,food and shelter

Patterns of resource selection by animals may be influenced by sex, and often change over a 24‐h period. We used a dry sclerophyll landscape managed for commercial timber production to investigate the effects of sex and diel period on habitat selection by the swamp wallaby (Wallabia bicolor). We predicted that selection would be (i) affected by both sex and diel period; and (ii) positively related to lateral cover during the day, but to food resources at night. Non‐metric multidimentional scaling indicated that some of the available habitats differed markedly with respect to visibility (an indicator of lateral cover), fern cover, forb cover, wallaby density and a forage quality index, providing the basis for non‐random habitat selection. At the landscape scale, wallabies showed strong selection for 5‐year‐old regenerating sites, selected against 10‐year‐old regenerating sites and unharvested forest, and avoided recently harvested (3–10 months post‐harvest) sites completely. At the scale of individual home ranges, a pooled male and female sample demonstrated selection for unharvested forest over recently harvested sites during both diurnal and nocturnal periods. A separate analysis showed that both sex and diel period influenced the selection of 5‐ and 10‐year‐old sites and the surrounding unharvested forest. Using a novel approach, we demonstrated that diurnal habitat selection by both sexes was negatively correlated with visibility, representing stronger selection for areas with more lateral cover. Nocturnal selection by females was positively correlated with values of a forage quality index, but this was not the case for males. We hypothesise that the observed patterns of selection were driven by the need to find food and avoid predators, but were also affected by the different reproductive strategies of males and females. Our results demonstrate the importance of incorporating factors such as sex and diel period into analyses of habitat selection.