Individual study: Predator control of the black rat Rattus rattus and its implications for pink pigeon Columba mayeri nesting success, Mauritius
Jones C. G., Swinnerton K. J., Taylor C. J. & Mungroo Y. (1992) The release of captive-bred pink pigeons Columba mayeri in native forest on Mauritius. A progress report July 1987-June 1992. Dodo, 28, 92-125
Oceanic islands harbour some of the world's most threatened species. Anthropogenic causes such as habitat destruction and fragmentation are factors which have resulted in population declines of island fauna and flora. Introduced species also pose a major threat to the long-term survival of native island communities and have been responsible for many extinctions.
The pink pigeon Columba mayeri, endemic to Mauritius, has suffered severe population declines due to habitat loss and predation. Introduced predators include mongooses Herpestes auropunctatus, feral cats Felis catus, crab-eating macaques Macaca fascicularis, and two species of rat: the rarer brown rat Rattus norvegicus and the widespread black rat Rattus rattus. These species may predate adult birds, but more notably reduce reproductive success by predation of eggs and nestlings. Macaques and rats are considered the most serious pink pigeon nest predators.
Nest predation by rats is distinguishable from that of macaques by nest and egg remains. Rats leave the nest unharmed and eggs have a distinct 'boat-shaped' hole, or are often in two halves, whereas with macaque predation, the nest is almost completely destroyed and only tiny egg fragments remain.
The wild population of pink pigeons had a 59% recorded nest failure (13 of 22 nests) due to predation in the years 1989 to 1991. Rats accounted for at least 31% (7) of these failures. In the released (captive-bred) population 29% (2 of 7) of nests with eggs were predated by rats.
In order to try and reduce nest predation of wild pink pigeons, a predator control programme was established.
Initial studies were carried out by trapping and poisoning black rats Rattus rattus and brown rats R.norvegicus around feeding stations and breeding sites in 1991. Due to the high abundance of rats (up to 20 rats were trapped per day) a more permanent control system, using poison bait stations, was established in the wild pink pigeon breeding area.
In December 1991, a 50 m spaced grid system of poison bait stations was set up in the main area of occupancy of the wild pink pigeon population at Pigeon Wood. Bait stations comprised open-ended (to allow rat access) plastic tubes (30 cm long × 10 cm diameter) with the bait placed centrally inside. The poison used was the anti-coagulant poison Brodifacoum (tradename 'Klerat', supplied in commercial wax blocks). Poison-bait was replaced every seven to 10 days.
Following the permanent rat control by poisoning (which started in December 1991), pink pigeon nest failure due to egg predation noticeably decreased. By June 1992, only 12% (1/8) of nests had been predated by rats at Pigeon Wood, compared to 31% lost to rats over the prior two breeding seasons of 1989-1991.
Conclusions: Although the 1992 breeding season was still ongoing, and thus the final level of rat predation following control was not known, results suggested that continuous control of rat predation greatly increased pink pigeon breeding success. This form of rat control was continued in the recovery programme of the pink pigeon. Due to the success of the rat control programme at Pigeon Wood, a similar permanent bait-station grid was established in the core area where releases of captive-bred pigeons have taken place at Plaine Lièvre in the Black River Gorges.
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