R.D. Wooller

Long-term population studies of seabirds

Long-term studies of seabirds, some now 30-40 years old, have begun to reveal significant age-related changes in the survival and reproduction o f these long-lived animals. Evidence for density-dependent regulation of seabird numbers, however, remains sparse whereas unpredictable, disastrous breeding years may be an important influence. Critical evaluation will require better data on (1) the extent of movements of seabirds between colonies, (2) the characteristics of those individuals that contribute disproportionately to the next generation, and (3) the importance of year and/or cohort effects on population processes.

Age-Dependent Survival of Breeding Short-Tailed Shearwaters Puffinus tenuirostris

(1) A small population of short-tailed shearwaters has been monitored each year since 1947. Survival rates have been calculated from the disappearance of marked individuals and related to breeding experience. (2) Shearwaters breeding for the first time had a relatively high rate of mortality. This rate then decreased considerably and remained low for about ten years before increasing substantially in older birds. (3) The sex of a shearwater, the period when it was active in the colony and whether or not it had been born in the colony had no major impact on the length of its breeding lifetime. (4) Increasing offspring production was associated with a decreasing mortality rate early in a shearwaters breeding career but not in mid-life. Later in its career, higher offspring production was associated with an increasing rate of mortality. (5) The log proportional hazard regression model and GLIM algorithm used attempted to avoid some of the problems inherent in analysing age-dependent survival in long-lived species.

The effect of parental condition on egg-size and reproductive success in short-tailed shearwaters Puffinus tenuirostris

Eggs were exchanged between 50 pairs of shorttailed shearwaters Puffinus tenuirostris on Great Dog Island, Bass Strait, Australia, in an attempt to distinguish the intrinsic effects of egg-size from any effects stemming from differential quality of parental care. At 64 “experimental” nests, large and small eggs were exchanged whereas at 36 “control” nests, eggs of equivalent, medium, size were exchanged. Egg-size appeared independent of maternal effects. In both groups, hatching and fledging success were independent both of eggsize and of the body condition of the attending parents. This suggests that breeding success in these birds may be more closely related to the behavioural traits of parents than to physiological factors.

Digging and soil turnover by a mycophagous marsupial

The woylie Bettongia penicillata is a small (1 kg) kangaroo-like marsupial that digs to obtain the fruiting bodies of fungi. The number of woylies in a 60 ha area of sclerophyll woodland in south-western Australia was estimated using mark-recapture at 3 month intervals over 3 successive years. The number of new diggings by woylies, determined at the same intervals, allowed an assessment of the rate of digging per individual. This varied three-fold from 38 to 114 diggings per individual per night, with no consistent seasonality. On average, each woylie displaced 4.8 tonnes of soil annually.

The Relationship of Pair-Bond Formation and Duration to Reproductive Success in Short-Tailed Shearwaters Puffinus tenuirostris

A substantial correlation exists between the breeding ages of short-tailed shear-waters in pair-bonds. Much of this correlation can be explained by prolonged pair-bonds between the same mates, and the predominant availability of unpaired, inexperienced birds. Using Monte Carlo simulation, a comparison of the differences in age observed between pair members with those produced assuming random pairing of available birds, indicated some weak assortative mating on the basis of breeding age. The probability of breeding success depended both upon the breeding ages of the partners and upon the length of time they had bred together; the latter appeared the stronger effect. In the first year of a pair-bond, the breeding age of the female had a much stronger effect than that of the male in determining the probability of reproductive success.

Comparative foraging ecology of five sympatric terns at a sub-tropical island in the eastern Indian Ocean

Over one million pairs of seabirds breed annually on the Houtman Abrolhos island group, off the mid-western coast of Australia, the largest seabird breeding station in the eastern Indian Ocean. Eight of the 13 species that breed annually on Pelsaert Island are terns. Dietary samples (regurgitates) were collected from the five most numerous tern species from 1993 to 1999. The prey items identified were related to observations of foraging seabirds around the island. The largest species studied, the crested tern Sterna bergii, foraged mainly for reef fish over shallow reef flats near the breeding islands, as well as for schooling clupeid fish over coastal shelf waters. The smallest species, the roseate tern S. dougallii, also foraged within sight of its colonies, but over deeper waters than crested terns. Three larval fish species characteristic of continental shelf waters (a gonorhynchid, a goatfish and a bellowfish) figured prominently in the diets of roseate terns and three other tern species studied. Of these, only the sooty tern S. fuscata also ate squid and lanternfishes, foraging farthest (480–600 km) from its breeding island, off the shelf edge. The lesser noddy Anous tenuirostris and brown noddy A. stolidus took very similar taxa of prey and also overlapped considerably in their foraging ranges, about 180 km from their colonies, between the islands and the continental shelf edge. However, the lesser noddy took smaller fish than did the larger brown noddy. Although these five abundant tropical tern species, that bred sympatrically, showed some segregation in their diets and, more clearly, in their foraging ranges, considerable overlap of both aspects remained.

The relationship between animal foraging and nutrient patchiness in south-west Australian woodland soils

The woylie (Bettongia penicillata) was once common and abundant over the southern third of the Australian continent. Since European settlement the range of this rat-kangaroo has become reduced by more than 97%, and until the early 1990s, only 3 small natural populations remained, all in south-western Australia. These medium-sized (c. 1 kg) marsupials create a large number of diggings as they forage for the hypogeous fruiting bodies of ectomycorrhizal fungi upon which they feed. The effect of such foraging activity on the availability of plant nutrients in the vicinity of such diggings was evaluated in simulated digging experiments. Available nitrate, ammonium, and sulfur decreased significantly 3 years after diggings were constructed and had filled in, whereas phosphorus, potassium, iron, and organic carbon remained unchanged. The results suggest that preferential water infiltration via woylie diggings leads to a decrease in those soil nutrients that are susceptible to leaching and indicates that digging vertebrates may influence the distribution of surface soil nutrients. SR at nd s di n M rk et Additional keywords: ectomycorrhizal fungi, biopedturbation, mycophagy, soil heterogeneity, woodlands, soil water repellency.

Nestling obesity in procellariiform seabirds: temporal and stochastic variation in provisioning and growth of short-tailed shearwaters Puffinus tenuirostris

Abstract Procellariiform seabirds such as short-tailed shearwaters Puffinus tenuirostris accumulate large quantities of lipid during the nestling period. The functional significance of this pattern of development remains unclear, but has been related both to temporal variation in feeding conditions around the colony and to stochastic variation in the foraging success of individual parents. This paper examines temporal and age-specific variation in the pattern of food delivery to nestling short-tailed shearwaters, which have one of the lowest provisioning rates of any procellariiforms and are known to experience occasional long intervals between feeds. We assess whether variation in the provisioning rates of chicks was associated primarily with temporal variation in food delivery at the level of the colony or with stochastic variation in food delivery at the level of the individual. We then discuss this variability in the context of nestling obesity. For all but the youngest chick age-classes, individual meals delivered by adults averaged 141 g, which was 25% of adult body mass. The proportion of chicks fed each night was low (49%) and highly variable (coefficient of variation = 82%), which means that occasional long intervals between feeds would be expected to arise simply by chance. In keeping with this, intervals between feeding events for individual chicks followed a negative exponential distribution with a mean of 2 nights and a maximum interval of 13 nights. There was significant temporal variation in food delivery, but deviations from expected values for both feeding frequency and meal size were restricted to a small number of nights, included values both higher and lower than expected and did not persist for more than 2 nights in succession. These data suggest that even among those species with very low feeding frequencies and occasional long intervals between feeds, nestling obesity in Procellariiformes should be regarded as a response to chronic stochastic variability in food delivery at the level of the individual chick rather than as insurance against sporadic temporal variation at the level of the colony.

Opportunistic breeding in the polyandrous honey possum, Tarsipes rostratus

Honey possums, Tarsipes rostratus, tiny (7-12 g) flower-dependent marsupials, were trapped in three areas of south coastal heathland in Western Australia on 5-8 occasions each year from 1984 to 1995. Mark-recapture estimated annual mortality at 86%, with only a few individuals living for more than one year. Most females breed for the first time while not yet fully grown and may produce up to four litters in a year. Maximal litter size is four, but usually only two or three young are reared. The small litter size and relatively slow growth of pouch young is attributed to the time needed for the mothers to harvest pollen, upon which T. rostratus relies for its nitrogen requirements. Females with pouch-young were recorded in all months, but with a higher frequency over winter when nectar was most abundant, and at a lower frequency (in some years, none) when food was scarce in autumn. Young are in the pouch for about 60 days and some females give birth to the next litter soon after pouch exit, presumably from delayed blastocysts. We suggest that T. rostratus females are polyandrous and that the smaller males compete by searching for females in oestrus. The multiple paternity of several litters, confirmed by single-locus microsatellite profiling, supports this model.

Surface Nesting by Little Penguins on Penguin Island, Western Australia

The Little Penguin Eudyptula minor reaches the northem limit of its breeding range in Western Australia on Penguin Island, 42 km south-west of Perth (Blakers et al. 1984), where up to 500 breed annually. Members of this isolated population breed from April to November, rather than in spring and summer as is more common elsewhere (Dunlop & Wooller 1986; Stahel & Gales 1987). Penguin Island is a 12.5 ha limestone island, thinly covered in places by Holocene aeolian sands bearing low bushes and shrubs (Chape 1984). Lack of a cohesive substrate results in few penguins digging burrows and most nest under dense bushes and shrubs (Dunlop et al. 1988). The island lies 600 m offshore, close to a metropolitan area, making these surface-nesting penguins potentially vulnerable to disturbance by visitors. This paper examines the use of Little Penguin nests in relation to vegetation and human activity and explores possible links between surface nesting and the winter breeding regime of the species.