On the Evolution of Darwin’s Finches

A close-up photograph of a female Medium Ground Finch from South Plaza Island.Adapt or Die

Finches in the Galápagos Islands are being threatened by a parasitic fly that attacks their young, placing the same species of birds that helped Charles Darwin refine his theory of evolution in danger of extinction.  But the authors of a new study say that human intervention could alleviate the risk.

The successful establishment of introduced pathogens and other parasites pose a threat to global biodiversity.  Such species can trigger epidemics that cause the decline of even seemingly abundant host populations.  Unfortunately, the threat can go unnoticed until a species is decimated.

The blood-sucking maggots of the parasitic nest fly Philornis downsi now infest all land birds there, including most of the 14 species of Darwin’s finches, two of which are endangered: fewer than 100 mangrove finches remain on Isabela Island, and only about 1,620 medium tree finches exist, all on Floreana Island.  Nest flies have been implicated in the population declines of Darwin’s finches, including the two endangered species.

A new mathematical model suggests that the birds may succumb to this pest within 50 years.  The model is based on data from five years of experimental field work documenting the effect of P. downsi on the reproductive success of medium ground finch Geospiza fortis populations on Santa Cruz Island.  Island populations of host can be particularly at risk due to limited habitat size and lack of genetic diversity.

 

A World map showing the exploratory round voyage of HMS Beagle from 1831 to 1836.
Second Voyage of HMS Beagle around the World from 1831 to 1836  Source: Wikimedia

The Voyage of the Beagle

In 1831, English naturalist Charles Darwin set sail for a five-year voyage of exploration aboard HMS Beagle that would change his life and revolutionise the scientific world.  On the Galápagos Islands, Darwin thought in terms of “centres of creation” and rejected ideas concerning the transmutation of species.

Initially unaware of the significance of the birds in the islands, Darwin was interested in the geographical distribution of species, particularly the links between the species present on oceanic islands and on nearby continents.

These birds were to play an important part in the inception of his theory of evolution by natural selection.  During his time on the Galápagos Islands, Charles Darwin noticed that finches on different islands in the chain were quite similar but had large variations in their beaks, depending on the local food source.

Darwins' original drawings of Galapagos Finches.
Darwin’s Finches: The beaks of the finches recorded by Darwin were a key part of the evidence for evolution.  Source: Wikimedia

Because the islands – which belong to Ecuador – are so far from the mainland, Darwin concluded that the birds had begun as one species and then started to evolve into separate varieties of finch.  The birds vary in size from 10 to 20 cm and weigh between 8 and 38 grams.  The smallest ones are the warbler-finches and the largest is the vegetarian finch.  The birds are all dull-coloured.

Finches breed more successfully in years when relatively high rainfall increases the food supply.  They tend to do badly during extremely wet or dry years.

The most important differences between species are in the size and shape of their beaks, and the beaks are highly adapted to different food sources.

In 1839, Darwin conceived of his now-famous theory of natural selection.  He added in his closing remarks:

“Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends”.

A chart showing the different types of Galapagos finches and their specific diets.
A few varieties of finches from the Galápagos Islands and their preferred diet. Source: bio1100.nicerweb.com

According to Charles Darwin’s theory, the birds rapidly adapted to the different food sources that were available in their new home, where they faced little competition from other birds.  Among these key adaptations were different beak shapes, i.e. stronger, blunter beaks for cracking tough nuts, seeds or insects.

About 1.5 million years ago, the species started branching out from a common ancestor, which colonised the relatively young Galapagos archipelago – thrust from the ocean by volcanic activity.  There are between 14 and 18 species of finches on the Galápagos – but the study looked at one of the most common, the medium ground finch.  Around 270,000 of these birds are found on Santa Cruz island.

 

A photograph showing a very poorly, fly-infested, finch nestling.
A fly-infested finch nestling. Image: Jennifer Koop

A Fly in the Ointment

The finches are threatened by a nest fly which lays parasitic larvae in their nests which attack the young, usually at night.  Some eggs are laid in the nostrils of the nestlings, and when the eggs hatch, the maggots begin feeding immediately in the nostrils, so in the worst cases you can see perforations through the bill.

The flies are believed to have arrived in the Galápagos in the 1960s.  Prof Dale Clayton and his colleagues developed a mathematical model using five years of data to project the impact of fly damage on the reproduction rates.

The model showed that the critical elements in their survival were the probability that a given nest was infested by flies and to a lesser extent the proportion of adult finches that survive from one year to the next. 

In two of the three scenarios modelled by the scientists, the finch populations were declining and at risk of extinction.  The worst case scenario demonstrates that the birds could be gone in 50 years.

 

A photograph showing a finch picking cotton from a dispenser, that will be used to construct (and fumigate) its nest.
A finch in Ecuador’s Galápagos Islands pulls a cotton ball from a dispenser set out by scientists. Source: phys.org

Stemming Extinction

The model also shows that if you reduce the probability of infestation, you can significantly alleviate the risk of extinction.  Sensitivity analyses reveal that even a modest reduction in the prevalence of the parasite could improve the stability of finch populations.  The researchers argue that the number of infected nests ought to be reduced by 40% to lift the risk of extinction.

A number of human interventions could achieve this goal, including the introduction of wasps that would lay their eggs in the fly larvae which would destroy them.  Another idea is to use pesticides.  Cotton balls could be treated with chemicals and left for the birds to incorporate into their nests.

The pesticide is safe for the birds.  Permethrin is the same stuff that is used in head-lice shampoo.

A gram of fumigated cotton would be enough to kill 100% of the flies in their nests.  Knutie and Clayton are confident that their method might help the endangered mangrove finches, with only 60 cotton dispensers needed to cover the less than half a square mile inhabited by the birds.  The question is to see whether the flies will evolve resistance.

The scientists say that the stress resulting from the flies infestation might provoke an evolutionary response in the finches.  They point to a related species where the nestlings learned to become more vocal when attacked by the pest, as a result the parents fed them more and their survival rates improved.  Researchers are concerned that these threatened finches may simply not have enough time to adapt.