Response from CDF to article on parasites in Darwin's finches

Recently, an article by Birgit Fessl and Sabine Tebbich (Philornis downsi - a recently discovered parasite on the Galápagos archipelago - a threat for Darwin's finches? 2002, Ibis 144: 445-451) has attracted much interest to the plight of the endemic Galápagos avifauna in relation to introduced species. The work by Fessl and Tebbich describes parasitism by fly larvae, especially of the genus Philornis, the warble-flies, on chicks of several species of Darwin's finches (genera Geospiza, Camarhynchus and Certhidea), a group found only in the Galápagos Archipelago. A large majority of nests they studied exhibited the presence of the fly parasites, and 27% of nests exhibited brood loss due to the parasites, although only 19% exhibited complete brood loss (no chicks surviving to fledging) due to the parasites. Nonetheless, an additional source of mortality, even at a level of 20%, is a cause for concern.

The first records of Philornis flies occurring in the Galápagos Islands are from the 1980s, and the understanding of their taxonomy is still confused. There is still debate as to whether the species is native or introduced. It has been described as a new, endemic species (Philornis isla Couri 2000), but most likely is represented by Philornis downsi, known previously from Trinidad in the Caribbean. It is therefore most likely an introduced species, as also suggested by Fessl et al. (Fessl, B., Couri, M.S. & Tebbich, S. Philornis downsi Dodge & Aitken, new to the Galápagos Islands (Diptera, Muscidae). 2001, Stud. Dipterol. 8: 317-322).

The distribution of the parasitic flies within the Galápagos Islands is not known, although it has been recorded on the four large, inhabited islands (Floreana, Santa Cruz, San Cristóbal, and Isabela). It is not known if the flies are distributed throughout the large islands such as Isabela. All study to the present has been carried out on Santa Cruz Island. It has been reported from both arid and humid habitats.

The level of threat to Darwin's finches and other native birds is difficult to assess, because the distribution, abundance, and effects are not understood. Extremely rare species such as Mangrove Finch (Camarhynchus heliobates) cannot tolerate much additional loss of reproductive effort. However, it is not clear that Philornis now occurs within the range of Mangrove Finch. All introduced species are potential threats to the avifauna of the Galápagos, but the level of threat may vary significantly. At present it is too early to assess the level of threat posed by Philornis to the Mangrove Finch, but we expect to have more information concerning this threat by mid-2003.

The Charles Darwin Foundation has an active research program underway to monitor the distribution and potential effects of the parasitic flies on the avifauna of the islands. Further information on their distribution should be forthcoming.

David A. Wiedenfeld, Ph.D.
Ornithologist
Charles Darwin Foundation
Puerto Ayora, Galapagos, Ecuador

Those wishing to support such research within the Galápagos can donate online or contact gct@gct.org

For further information on these and other articles, visit
Charles Darwin Research Station

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