Darwin’s finches combat introduced nest parasites with fumigated cotton

Introduced parasites are a threat to biodiversity when naïve hosts lack effective defenses against such parasites [1]. Several parasites have recently colonized the Galápagos Islands, threatening native bird populations [2]. For example, the introduced parasitic nest fly Philornis downsi (Diptera: Muscidae) has been implicated in the decline of endangered species of Darwin’s finches, such as the mangrove finch (Camarhynchus heliobates) [3]. Here, we show that Darwin’s finches can be encouraged to ‘self-fumigate’ nests with cotton fibers that have been treated with permethrin. Nests with permethrin-treated cotton had significantly fewer P. downsi than control nests, and nests containing at least one gram of cotton were virtually parasite-free. Nests directly fumigated with permethrin had fewer parasites and fledged more offspring than nests treated with water. Knutie et al. show that Darwin’s finches can incorporate insecticide-treated cotton into their nests to suppress a threatening nest parasite.