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Mice Plague Islands: Seabirds in Peril
16 Aug
Summary
- Invasive mice have reached densities of 1,270 per hectare on Farallon Islands.
- Mice impact burrowing owls, which then prey on vulnerable Ashy storm-petrels.
- An eradication project aims to remove mice using helicopter-dropped rodenticide.
Invasive house mice, introduced to the Farallon Islands in the 19th century, have created severe ecological disruptions. Population densities have surged to as high as 1,270 mice per hectare, impacting the largest seabird colony in the contiguous United States. These rodents alter the food web, notably affecting the endemic Farallon camel cricket and influencing populations of migrating burrowing owls.
As mouse numbers peak in autumn, burrowing owls feed heavily on them. When mouse populations decline in winter, these owls turn to Ashy storm-petrels, a globally vulnerable seabird. The Farallon Islands host half the world's breeding population of Ashy storm-petrels, a species slow to reproduce, making it susceptible to even small increases in adult mortality.
To combat this threat, the U.S. Fish and Wildlife Service, with conservation partners, is implementing an eradication project. This involves using rodenticide bait dispersed by helicopters during late autumn, timed to minimize impact on seabirds. Safety measures include temporarily relocating resident burrowing owls and deterring Western Gulls during bait application.
This initiative, approved after years of environmental review, aims to restore the native ecosystem. Researchers anticipate that removing the mice will improve native seed dispersal, aid the recovery of native invertebrates and reptiles, and end the cycle of owls preying on storm-petrels.
