BRANTA — Nola Parsons
Quantifying abundance, breeding and behaviour of the African Black Oystercatcher
Institution: University of Cape Town, South Africa
Details: PhD 2006 (Completed)
Address: Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa. (Sep 2006) Email
Species Keywords: African Black Oystercatcher Haematopus moquini
Abstract
The African Black Oystercatcher Haematopus moquini was studied at the Koeberg Nuclear Power Station and Nature Reserve from November 2002 until June 2005. This protected mainland site has low levels of human disturbance both within the harbour area and on the adjacent beaches where a variety of terrestrial and avian predators occur. The oystercatchers breed on artificial rocky shore within the harbour and the sandy beaches. The oystercatcher is long-lived, strongly territorial and mate-faithful with a low reproductive rate. It has been studied extensively on offshore islands, Western Cape rocky shores and Eastern Cape sandy shores but not on Western Cape sandy shores. Breeding productivity and abundance data were collected by conducting weekly counts (adults, pairs, nests, chicks and ledglings) and each breeding pair was followed throughout the breeding season. The dates of hatching, fledging and departure from the parental territory were recorded for each chick. Total time commitment to breeding for each pair was calculated. Breeding productivity data of oystercatchers from other sites were compared. Feeding behaviour was observed and quantified. Prey was identified and correlated to sampling done at the site. Behaviour activity data were collected byinstantaneous scans of birds on the rocky and sandy shores; tide, weather, daylength and time of day were used as explanatory variables in generalized linear models. Each behaviour was modelled. Behaviour between the rocky and sandy shores was compared. Management practices and other seabirds and shorebirds breeding at the site were observed. A model to evaluate reproductive effort in relation to time investment and egg production is introduced; the large variation in these explanatory variables will enable their relative importance to be detected. A graphical method to display time commitment to breeding and to provide an overview of breeding events is introduced and used to facilitate visual comparisons between different seasons and sites. Oystercatcher density increased over 30 years; in the harbour there were 46 birds/km and 13 birds/km on the beach. The oystercatcher pairs in the artificial shore of the harbour have a similar productivity to an adjacent area of shoreline five times as long as the original shore. Over the three-year study period, the breeding productivity of oystercatchers at Koeberg was below that estimated to maintain a stable population (0.35 fledglings per pair per year). The poor productivity at Koeberg is mainly attributed to egg and chick loss as a result of predation; in addition there were high levels of human disturbance in the 2004/2005 breeding season. Data from 18 breeding sites around the South African coastline, on different shore types, were grouped into protected island sites, protected mainland sites and unprotected mainland sites. Egg loss resulting from predation caused poor breeding success at protected mainland sites; human disturbance caused chick loss at unprotected mainland sites. Site-specific conditions played the largest role in determining breeding success and reasons for variation in breeding success often remained unclear. However, it is clear that the best place for an oystercatcher to breed is on an island with no predators. African Black Oystercatchers foraging diurnally on sandy shores of the Western Cape, feed mainly on polychaetes (Scololepsis, Nephtys and Glycera), also on small crustaceans (especially associated with the drift-line kelp) and Donax serra. This sandy shore offered a diverse array of possible prey items throughout the tidal cycle and although specialization was present (particularly for an individual bout of feeding) the adaptable oystercatchers used a variety of techniques and opportunistic behaviours to utilise the available prey. The effects of several variables on the different behavioural activities of the oystercatcher on rocky and sandy shores on a Western Cape mainland site were quantified. The main difference seen is the dependence of the rocky shore birds on the tide height, especially feeding, sleeping and lying and the relative importance of all the variables (tide, weather conditions, daylength and time of day) on all behaviour activities on the sandy shore birds. Feeding on the sandy shore occurs throughout the tidal cycle but was more dependent on weather variables. The Koeberg Nuclear Power Station and Nature Reserve meets the criteria for an Important Bird Area and a Ramsar Wetland of International Importance. The nuclear power station is a National Key Point site having strict security for entry and therefore low human disturbance within the harbour area. Practical conservation recommendations are given.