Darwin’s finches beat back bloodsuckers with cotton balls

Bloodsucking parasites hitching rides on boats have colonized Ecuador’s Galapagos Islands over the last couple decades, crushing the iconic native populations. For Darwin’s finches, the solution could be as simple as cotton balls laced with pesticides.

The introduced nest fly Philornis downsi lays its eggs in the nests of land birds. Adult flies aren’t parasitic, but once their eggs hatch, the larvae feed on the blood of newborn nestlings and their mothers.

A team led by Sarah Knutie from the University of Utah wondered if birds could be encouraged to self-fumigate their nests. Several species of Darwin’s finches have been seen taking cotton from frayed laundry lines, toilet paper, and towels to weave into their domed-roof nests of plant fibers. To find out, they dispersed cotton treated with a 1 percent permethrin solution, the mild pesticide found in head-lice shampoo. They placed 30 wire-mesh dispensers at 40-meter intervals along transects through the El Garrapatero field site on Santa Cruz Island.

nest with cottonWhen the birds finished breeding, the team collected nests found near dispensers. Four territorial species — Geospiza fortis (pictured below), G. fuliginosa (above), Camarhynchus parvulus, and Platyspiza crassirostris — wove the cotton into their nests. Of the 26 nests the researchers dissected, 22 contained cotton. Nests with treated cotton had much fewer P. downsi than control nests containing cotton treated with water: 15 parasites on average, compared with 30 for the controls. Nests containing at least one gram (pictured right) of the treated cotton were virtually parasite-free.

In another experiment, the researchers manually sprayed 20 nearby nests with the permethrin solution and banded the nestlings to track them. They dissected the nests after all the nestlings fledged (or died). None of the nests had parasites, and all but one fledged at least one finch (95%). Meanwhile, the control nests sprayed with water had more than a dozen parasites each, and only 65% fledged any offspring.

Overall, more nestlings fledged from experimental nests than from control nests, suggesting that self-fumigation may be a simple, effective way to combat P. downsi. These maggots have already been implicated in the decline of the mangrove finch (Camarhynchus heliobates), the most critically endangered of Darwin’s finches. Because many nests are difficult for us to reach, helping finches help themselves may be the best stopgap solution. — Janet Fang | 8 May 2014

Source: Knutie, S.A. et al. Darwin’s finches combat introduced nest parasites with fumigated cotton, Current Biology (2014). dx.doi.org/10.1016/j.cub.2014.03.058

Images: Sarah Knutie, University of Utah

finch on clothes line