OK plan to save Farallones’ birds
There are over 10,000 species of birds in the world, many of which owe their existence to islands, which serve as safe nesting areas free from predators where seabirds can raise their young. Many rare and beautiful land birds are found only on islands, having evolved over geological time in isolation from their continental relatives. These islands can be nurseries for entire new species as well as a haven for young birds.
Unfortunately, during the past 500 years, islands have also become hot spots of extinction. Over 150 bird species have become extinct during this time, with more than 90% of these extinctions occurring on islands. More often than not, this is because birds that evolved on islands never developed ways to resist mammalian predators, making them easy marks for mice, rats and cats introduced by seafaring humans, either accidentally or purposefully.
One close-to-home example of a human-caused catastrophe is the Farallon Islands, 30 miles west of the Golden Gate and visible on a clear day from San Francisco. In the early 1800s, the fur trade extirpated seals from the islands. In the 1850s, millions of seabird eggs were collected annually for sale in Gold Rush-era San Francisco. In the process, meanwhile, house mice were introduced to the island and, in the absence of predators, multiplied prodigiously.
Today, the islands are a National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service with support from Point Blue Conservation Science. Thirteen species of seabirds nest on the islands, including half of the world’s ashy storm-petrels, a 1.3-ounce bird whose Farallon population has declined by 42% during a recent 20-year period. Other nesting birds include thousands of common murres, Brandt’s cormorants and dramatically plumed tufted puffins.
However, the introduced house mice now number over 60,000 and threaten the ability of these bird species to reproduce.
In addition to devouring bird eggs, the mice feed on a cricket that’s endemic to the islands. Stocked with ample food supply, they have thus attracted an additional threat to the seabirds in the form of burrowing owls. These small mainland-based owls are drawn to the Farallones in the early fall, when the mouse population reaches its annual peak. Then in January and February, when the mouse population dwindles with seasonal vegetation, the owls move on to eating storm-petrels before returning to the mainland.
The Fish and Wildlife Service has developed a plan to remove the mice, using the rodent poison brodifacoum. The amount of rodenticide to be used is under 2 grams dispersed in about 3 tons of cereal grains, which would be air dropped on the islands. This method has been successfully used on 70 islands around the world (including Anacapa Island off the Southern California coast), leading to total rodent removal, enhanced bird populations and richer adjacent seabed habitat from seabird guano. According to the Fish and Wildlife Service’s final environmental impact statement, it’s the only method that’s demonstrated consistent success for eliminating the house mouse. From 2005-15, 30 mouse eradications on islands were attempted, all using brodifacoum — 93% were successful.
New Zealand has the most experience, recently clearing sub-antarctic Antipodes Island of an estimated 200,000 mice. The British cleared huge South Georgia Island in the southern Atlantic of both rats and mice in 2018, also using brodifacoum.
In 2019, the Fish and Wildlife Service plan was presented to the California Coastal Commission, which expressed concern that it did not sufficiently address prevention of by-kill and possible accidental spillage. On Thursday, Fish and Wildlife will present the commission with an updated plan, which is supported by Golden Gate Audubon Society, Marin Audubon and many other conservation organizations in the Bay Area.
A few wildlife organizations oppose the plan arguing that it’s not acceptable to use brodifacoum because of potential harm to other species. In the case of this island, and of most isolated islands, however, there are no other mammals to harm.
Predatory birds like owls and gulls are a concern since they may eat some poisoned mice. The plan, however, calls for trapping owls and any other predators and hazing (scaring away) gulls to minimize this problem. The endemic cricket is not expected to be impacted, and a population of endemic salamanders will be protected in terrariums during the bait application.
Some opponents suggest avoiding rodenticide by using a gene editing or contraceptive technique, but such approaches don’t exist today and would be a dangerous untried experiment. They would also require a continued human presence on the island, constantly disturbing sensitive wildlife, rather than a one-time rodenticide treatment.
On the Farallones, tens of thousands of avian lives are at stake. Even more significant, the ashy storm-petrel’s survival as a species lies in the balance.
It’s time for the Coastal Commission to approve this judicious and scientifically vetted plan. To do nothing is to continue our human string of irresponsible environmental actions and abandon the islands that have been hailed as California’s Galapagos.