Our research, published today in Proceedings of the Royal Society B, reveals how the parasite deforms the beak. This has the effect of weakening the male bird’s mating call, and making it no longer clearly distinguishable from that of other closely related species.
A changed song can have an important effect on the male finch’s ability to find a mate.
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It’s a simple tune consisting of one syllable repeated 3 to 15 times, depending on what species of finch he belongs to. Larger-bodied finch species produce a slower song with few syllable repeats, and smaller-bodied finch species produce faster song with many syllable repeats.
In the case of the Medium Tree Finch (Camarhynchus pauper), a critically endangered species that only occurs on Floreana Island of the Galapagos Islands, its species-typical song has a bright resonance that rings across the forest canopy.
An accomplished male singer that can hit the high notes is quickly swooped up by a female looking to pair with a proficient singer.
The parasitic Philornis larvae in a finch nest.
Sonia Kleindorfer, Author provided
Since then, the devastating impacts of its larval feeding habits on nestling birds have been coming to light. The adult fly is vegetarian, but the females lay their eggs into bird nests and their larvae feed on nestling bird beaks from the inside out.
Many Darwin’s finch species now have beaks with massively enlarged nostrils because of damage the feeding fly larvae have caused during the nestling stage. We discovered that a changed beak apparatus measurably affects the song of Darwin’s tree finches with consequences for pairing success.
A Medium Tree Finch male with extremely enlarged nostrils is unable to hit the high notes.
Male finches that produce song with a narrower frequency bandwidth, because their song has a lower maximum frequency, have poor quality song. These males are less likely to be chosen by females, a pattern we documented in both the Medium Tree Finch and the Small Tree Finch.
Previously, we discovered evidence of hybridisation in Darwin finches. This is where two separate species of finch breed which could potentially produce a new species, phase out one of the species, or cause the collapse of the two existing species into one.
This female choice seems to be paying dividends, because hybrid pairs with greater genetic diversity also sustained fewer of the parasitic larvae in the nest. And that could lead to fewer birds with infected beaks.
There are concerted efforts underway to develop control and eradication methods for P. downsi on the Galapagos Islands, building on a collaborative relationship between the Charles Darwin Foundation and the Galapagos National Parks. The Philornis downsi Action Group is an international consortium of concerned scientists working to develop biological control methods.