Individual study: Impact of predator control on breeding success of dark-rumped petrels Pterodroma phaeopygia nesting on Floreana Island, Galápagos, Ecuador
Cruz J.B. & Cruz F. (1996) Conservation of the dark-rumped petrel Pterodroma phaeopygia of the Galápagos Islands, 1982-1991. Bird Conservation International, 6, 23-32
The nominate race of the dark-rumped petrel Pterodroma phaeopygia phaeopygia is endemic to the Galápagos archipelago (Ecuador), where it breeds on the islands of San Cristóbal, Santa Cruz, Santiago, Floreana and Isabela (although the location of the colonies on the latter are unknown). Agricultural development has reduced the available nesting habitat on Santa Cruz, Floreana and San Cristóbal, and predation by introduced mammals, such as black rats Rattus rattus, cats Felis catus, pigs Sus scofra and dogs Canis familiaris, on all of the islands has eliminated the species from all but the most protected nest sites. As part of a more general review of the conservation actions taken for the species in the Galápagos, this study describes the predator control measures implemented during 1983-1986 at the dark-rumped petrel colony at Cerro Pajas (1°18' S, 90°28' W) on Floreana, and documents the impact on petrel breeding success.
Poisoning of rats: From 1983, attempts were made to control black rats using the anti-coagulant rodenticide coumatetralyl, distributed in feeders consisting of 50 cm (later 80 cm, to protect the bait from rain damage) sections of 4-inch (10 cm) diameter plastic tubing. Each feeder contained 100-250 g of commercially prepared coumatetralyl bait mixed with ground corn and rice (0.075% active ingredient). In 1983 and 1984, 200 feeders were distributed at approximately 50 m intervals in the crater of Cerro Pajas. In response to a decline in rat numbers, the number of active feeders was reduced to 75 in 1986, distributed in a cordon encircling the petrel colony. To assess the impact of poisoning on the rat population, snap-traps were set on three successive nights (c.300 trap-nights) prior to poison distribution, three to four weeks later, and at the end of the programme.
Control of cats: Within the colony, six cats were shot in 1983-1984. In response to an increase in cat predation in 1985, a combination of shooting, hunting with trained dogs and poisoning (with 1080) was employed to control cat numbers. Poison baits were distributed at 100 m intervals in a cordon around Cerro Pajas, and in other areas where evidence (e.g. scats) of cats was found. Poison was also injected into the corpses of petrels killed by cats when it was suspected that they would return to consume the remains. In 1986, traps were also set at 500 m intervals in a cordon around the exterior base of the mountain.
Control of other mammals: Within the colony, larger mammalian predators and pests (e.g. pigs, goats Capra hircus and donkeys Equus asinus) were controlled using a rifle and trained hunting dogs. During 1983-1984, two pigs, c.25 goats and several donkeys were killed. Over 500 goats and 89 cats were eliminated by hunting during 1985-1986.
Monitoring of petrel breeding success: Petrel nests were monitored from January to September in 1983 (104 nests) and 1984 (100 nests), with nests usually checked weekly during the incubation period. A sample of 100 nests was also monitored in 1985 and 1986.
Pre-poisoning trapping caught 16 and 19 rats in 1983 and 1984, and four rats in both 1985 and 1986. No rats were caught during trapping three to four weeks after poisoning began in any of the four years, suggesting that poisoning was effective at maintaining the site relatively free of rats during the petrel breeding season.
In 1981, prior to the start of predator control, 31% of petrel nests were lost to predation by rats and cats, and 31% of eggs laid resulted in fledged young. In 1983 and 1984, none of the nests monitored was lost to predation, with fledging success increasing to 46% and 72% respectively. Losses due to rat predation were negligible in 1985 and 1986, but 49% of nests were lost to cat predation in 1985, with fledging success dropping to 23%. Following the start of more intensive cat control measures in 1985, nest predation fell to 10% in 1986, with fledging success recovering to 70%.
No evidence of secondary poisoning (e.g. of short-eared owls Asio flammeus) or poisoning of other non-target species was recorded during 1983 or 1984, but finches and cockroaches were observed consuming bait in 1985. Although no ill-effects or deaths due to the poison were observed, possible effects on fecundity or other life-history traits in these species are not known.
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