Action: Control mammalian predators on islands for songbirds
- Two before-and-after trials in the Seychelles and Cook Islands describe population increases in magpie robins and monarch flycatchers following cat and rat control. A before-and-after study from New Zealand found that the population of South Island robins Petroica australis australis was almost identical before and after rat control.
- Two studies found higher reproductive success in monarch flycatchers and shrikes in areas with rodent control, compared to areas without control. However, this was climate dependent in shrikes.
- A before-and-after study from Hawaii found lower predation on artificial nests after intensive rodent control.
The effectiveness assessment of the control of mammalian predators on islands was carried out across all bird species groups.
Supporting evidence from individual studies
A before-and-after study on Frégate (210 ha), Seychelles, between 1981 and 1984 (Watson et al. 1992), found that the population of Seychelles magpie robins Copsychus sechellarum (a species confined solely to Frégate at the time) increased from 18 to 25 birds following the removal of at least 56 feral cats Felis cattus in 1981-2 and the probable eradication of cats from the island. The authors argue that a reduction in foraging habitat due to agricultural abandonment hindered further population growth.
A before-and-after study in 1987-9 on Breaksea Island (170 ha), South Island, New Zealand (Taylor & Thomas 1993) found an almost identical number of South Island robins Petroica australis australis after a rat eradication programme as before. This study is discussed in detail in ‘Do birds take bait designed for pest control?’.
A paired before-and-after study in Totokoitu Valley, Rarotonga (6,700 ha), Cook Islands (Robertson et al. 1994), found that the nesting success of kakerori (Rarotonga flycatchers) Pomarea dimidiate was significantly higher in areas with an intensive black rat Rattus rattus poisoning programme (62% success for 71 nests in 1988-93), compared to the same areas in the 1987-8 breeding season (no nesting attempted) and to other areas without a poisoning programme (26% success for 47 nests in 1987-93). In areas with rat control adult mortality also fell significantly, from 24% in 1989-90 to 6% in 1989-93. The population of kakerori increased from 29 birds in 1989 to 60 in 1993. This study also used nest guards, discussed in ‘Physically protect nests with individual exclosures/barriers’.
A before-and-after study on O’ahu, Hawaii, USA, in 1998 (VanderWerf 2001), found that predation rates on artificial O’ahu ‘elepaio Chasiempis sandwichensis ibidis nests were significantly lower following intensive rodent control compared to before control began. There was a 55% reduction in predation rate for 40 nests placed on the ground and a 45% reduction for 40 nests placed in trees. Egg survival rates over 15 days were approximately 80% following rodent control and 20-40% before control. Survival of artificial nests in trees was not significantly different from genuine ‘elapaio nests. Control was through trapping and poisoning (with diphacinone).
A controlled, replicated study on San Clemente Island, California, USA, between 2000 and 2006 (Heath et al. 2008) found that 172 pairs of San Clemente loggerhead shrikes Lanius ludovicianus mearnsi produced 1.1 more fledglings when rodents were controlled in their territories during April-July, compared to control pairs. In drier-than-average years, experimental pairs also raised 1.1 extra fledglings to independence (40 days old). However, there was no effect of rodent control on fledgling success in wetter-than-average years. Management in December-March did not increase either measure of productivity. Rodents were controlled with cholecalciferol rodenticide. This study also investigated the impact of supplementary feeding in addition to rodent control, which is discussed in ‘Provide supplementary food to increase reproductive success’ and the success of captive-bred individuals, in ‘Release captive-bred individuals’.
- Watson J., Warman C., Todd D. & Laboudallon V. (1992) The Seychelles magpie robin Copsychus sechellarum: ecology and conservation of an endangered species. Biological Conservation, 61, 93-106
- Taylor R.H. & Thomas B.W. (1993) Rats eradicated from rugged Breaksea Island (170 ha), Fiordland, New Zealand. Biological Conservation, 65, 191-198
- Robertson H.A., Hay J.R. & Saul E.K. (1994) Recovery of the Kakerori: An Endangered Forest Bird of the Cook Islands. Conservation Biology, 8, 1078-1086
- VanderWerf E.A. (2001) Rodent Control Decreases Predation on Artificial Nests in O'ahu 'Elepaio habitat. Journal of Field Ornithology, 72, 448-457
- Heath S.R., Kershner E.L., Cooper D.M., Lynn S., Turner J.M., Warnock N., Farabaugh S., Brock K. & Gracelon D.K. (2008) Rodent control and food supplementation increase productivity of endangered San Clemente Loggerhead Shrikes (Lanius ludovicianus mearnsi). Biological Conservation, 141, 2506-2515