The Ecology Group at the Institute of Agriculture and Environment, Massey University, Palmerston North 4474, NEW ZEALAND.
1. The braided rivers of the South Island, New Zealand, provide feeding and breeding habitat for a range of endemic bird species. Many of these species are entirely reliant on braided river habitat for breeding, but because of a reduction in habitat quantity and quality due to weed infestation, hydroelectric power development and water abstraction, and high predation rates by introduced mammals, the survival of many of these bird species is under threat.
2. Predation is the primary cause of mortality for most braided river bird species, but most studies have only measured the effects of predation at the egg stage. In this study. I monitored breeding success of banded dotterels (Charadrius bicinctus) and black-fronted terns (Sterna albostriata) on the Ohau River, South Island, from 1998-2000, and examined the effects of predation on both species. The black-fronted tern is an endangered species about which very little is known, so I also investigated aspects of its breeding biology.
3. I used time-lapse video cameras at 39 nests of banded dotterels and determined that nest monitoring did not affect nest survival rates, nor do predators use human scent trails to locate nests. I attached radio transmitters to 49 banded dotterel chicks to assess rates and causes of mortality, but although a minimum of 18% of chicks were killed by predators, the transmitters did not provide information on the relative importance of the different predator species. Hatching success at 338 banded dotterel nests was 68%, and 48% of hatched nests fledged at least one chick.
4. Hatching success for 1022 black-fronted tern nests was 50.2%, and a minimum of 27.6% of 897 chicks survived through to fledging. Cause of mortality was assessed for 148 chick, juvenile and adult terns, and I found that predators were responsible for 47% of deaths. Feral cats (Felis catus) and Norway rats (Rattus norvegicus) were probably responsible for 19% and 51% of predator related deaths, respectively. Mortality rates remained high immediately after fledging because of predation. Annual adult mortality was estimated at 88-92% but the associated confidence intervals were wide (57-99%). I document aspects of black-fronted tern ecology such as incubation period, fledging period, egg and chick weights, and develop a preliminary method for sexing adult black-fronted terns based on body measurements. I measured black-fronted tern chick growth and survival and found that hatching asynchrony results in lower growth and survival in second hatched chicks. I collated all existing data black-fronted tern populations and from these data estimate that the population size is less than 10 000 but conclude that better data on population size and trends are required.
5. Using population viability analysis models, I compare the survival of banded dotterels and black-fronted terns on the braided rivers. Higher productivity and shorter generation times, rather than any behavioural differences, are probably the key factors that result in stronger population growth for banded dotterels, despite both species being subject to similar levels of predation. Simulations indicate that predator control could be the most effective way to increase black-fronted tern survival.
6. This study provides the first comprehensive record of black-fronted tern breeding biology. I provide evidence that the black-fronted tern population is almost certainly in decline and the species urgently needs further research. In addition, the interrelationships between rabbit (Oryctolagus cunniculus) abundance, predator abundance and nest predation rates are poorly understood and urgently need attention in order to better manage braided river communities and ensure the survival of black-fronted terns and other vulnerable bird species.