Kathleen M.C. Tjørve

Influence of Disturbance and Predation on Breeding Success of the African Black Oystercatcher, Haematopus moquini, on Robben Island, South Africa

Abstract Breeding success of African Black Oystercatchers (Haematopus moquini) was monitored over three austral summers on Robben Island, South Africa from 2001 to 2004. Robben Island is a busy tourist destination which has a resident population that live and work on the island in addition to tourists that visit the island daily. Potential predators of birds’ eggs and chicks include feral cats (Felis catus), Kelp Gulls (Larus dominicanus), Mole Snakes (Pseudaspis cana) and House Rats (Rattus rattus). The mean number of fledglings per pair declined from 0.74 in the first breeding season to 0.41 and 0.35 in the subsequent breeding seasons. This fledgling success slightly exceeded the threshold estimated to maintain a stable population, and is similar to fledging success observed in other oystercatcher species. The lower fledging success in the second and third breeding seasons compared to the first breeding season were increased depredation of eggs and chicks and high-tide events on 17 February 2003 and 10 February 2004. Predation can be aggravated by human disturbance, and it was found that the area with greatest resident and tourist activity had the greatest egg and chick losses in all three breeding seasons. The area adjacent to the disturbed area had reduced breeding success in the second breeding season, possibly a result of increased predation associated with the settlement. Breeding success near the Kelp Gull breeding colony decreased in the third breeding season; the period during which the greatest number of Kelp Gulls were breeding. The potential success of a breeding attempt decreased if incubation of the clutch started later in the breeding season.

Growth and its relationship to fledging success of African black oystercatcher Haematopus moquini chicks

We investigated the growth of African black oystercatcher Haematopus moquini chicks on Robben Island, South Africa, over three austral summers, 2001-2004. Using a robust regression analysis to determine the growth parameters of chicks of known and unknown age we found that oystercatchers from our study population had a Gompertz growth rate coefficient that was 2% less than predicted for body mass based on the equation for waders. Leg growth lagged initially, then increased and slowed again as the chicks became older, whereas wing growth was slow initially but increased with age. Chicks with small growth rate coefficients for body mass exhibited retarded growth of all body measures except wing length. This enabled these chicks to fledge in a shorter period of time than their slow growth would otherwise allow. The growth rate of body mass was observed to vary greatly between chicks. Fast-growing African black oystercatchers had a shorter pre-fledging period; were larger at fledging and were more likely to fledge successfully. African black oystercatchers display sibling rivalry, and once a dominance relationship is established, the larger chick remains so during the pre-fledging period. Larger siblings fledged earlier and at a heavier mass than the smaller siblings and this may improve their chances of survival. Neither hatching date nor brood size influenced the growth rate coefficients.

Pre-fledging energy requirements of the nocturnally fed semi-precocial chicks of the Spotted Thick-knee (Burhinus capensis)

Abstract Studies of pre-fledging growth and energetics in precocial (self-feeding) shorebird species have shown that chicks expend large amounts of energy on thermoregulation and activity. We investigated the pre-fledging growth and energetics of Spotted Thick-knee (Burhinus capensis) chicks, which are semi-precocial (fed by their parents), near Cape Town, South Africa, to determine whether semi-precocial development would reduce the energy expenditure of chicks and enable them to grow faster. In addition, because Spotted Thick-knees are nocturnally feeding shorebirds, we hypothesised that their more limited foraging time (compared with diurnal feeders) would negatively influence growth and energy expenditure of chicks during their spring-summer breeding season. The growth-rate coefficient, resting metabolic rate, peak daily metabolisable energy and total metabolisable energy of Spotted Thick-knee chicks were similar to the values predicted for birds of their size. Therefore, the potential negative impact of nocturnal feeding on growth of chicks may be being countered by reduced chick energy expenditure on thermoregulation and activity owing to parental feeding, and adults potentially extending their foraging time into the day as their chicks become larger. The relative energy requirements of growing Thick-knee chicks were greater than those of precocial shorebird chicks in the same climate of the Western Cape, but lower than those of shorebirds breeding in temperate or arctic zones. The semi-precocial mode of development of Spotted Thick-knees did not result in reduced energy expenditure or faster growth than anticipated for shorebirds in general, but their relative growth was greater than that of precocial species at the same latitude. Therefore, semi-precocial development benefits the growth of Spotted Thick-knees.