Land Birds


 

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Darwin's Finches - PAGE 2

It takes some effort and interest to learn to identify the finches. What makes Darwin's finches so difficult to identify is the variability of their beaks (which can sometimes be exacerbated by interbreeding) and the fact that the beak of one species may overlap into the range of another. The best way to begin is to look at finches on islands that have relatively few, distinct species (see the table below). The four finches below are a good example of the difficulty. Finch #1 looks like a large ground finch and may well be, but according to the bird guide, a finch that looks like a large ground finch but is in a mixed flock is probably a medium ground finch that is at the very large end of the medium finch range. Large ground finches are more solitary than medium ground finches. I photographed finch #1 at the tortoise pool at the Charles Darwin Research Station on Santa Cruz and there were lots of medium finches around that day. Finch #2 is definitely a large ground finch because I photographed it on Genovesa.. I always thought that Finch #3 was a small ground finch. I photographed it while anchored off Daphne Major. The bands indicate that it is part of the study by Peter and Rosemary Grant. The finches on Daphne Major are predominantly medium ground finches, but they tend to be small. Finch #4 is definitely a small ground finch because I photographed it on Espanola. The table below shows the distribution on islands that have visitor sites

Modified from "A Field Guide to the Birds of Galapagos"

The difficulty in identifying the finches is rooted in precisely what makes them so interesting and important - the evolutionary process. If we believe that two species share a common ancestor, then as one traces the species back in time, they should become closer and closer in form. At the branch point, the species should become ambiguous. That is precisely the point at which we find the Darwin's finches. They are in the process of separating, but they haven't completely done so at this point in time. The definition of the term "species" includes the presence of a fertility barrier between individuals of different species. In the case of Darwin's finches, those barriers are not completely formed yet, and there is a certain amount of documented hybridization between species. This also contributes to the ambiguity of the birds.

Comprehensive studies over the past 20 years by the Grants (reviewed in a more palatable, layman form in "The Beak of the Finch") has revealed many interesting lessons about the evolutionary process. Our current understanding of evolution is that new species are born when the population of the ancestor species is split. Once the gene pool is separated, the two populations may be subject to different natural selection pressures, and hence, evolve in separate ways. The splitting of a population followed by subsequent evolution is known as allopatric speciation. At some point, the populations may come back together again, that is, they may become sympatric. A variety of possibilities arise when two populations, born in allopatry become sympatric;

1). If the two populations have not diverged too greatly, then they can simply merge back into a single population
2). The two populations may compete, one eventually becoming extinct.
3). The two populations may avoid competition by specializing. In this case, they would continue to diverge in sympatry.

Finch evolution seems to be driven by a combination of allopatric and sympatric events. For example, a glance at the table of distribution shows that similar finches, such as the cactus finch and the large cactus finch do not coexist on the same islands. The large cactus finch shows what can happen in the presence and absence of a competetor species.

There are well-known populations of the large cactus finch (G. conirostris) on Genovesa and Espanola, but their beaks are different. On Genovesa, the large ground finch coexists with the large cactus finch. During the wet season, when there is lots of food to go around, the two species can feed in each others niches with no competition. However, the dry season, and its scarcity of food forces the two to specialize. Assuming that the two originally evolved in allopatry, their association in sympatry has continued to make them diverge. Despite the fact that finches show a broad variation within each beak type, the diversity of the large ground and large cactus finches on Genovesa is minimal. Any large cactus finch whose beak varies towards the large ground finch will be unable to compete with members of its own, or opposite species. The same applies to the large ground finch. The picture is quite different, however, on Espanola. On Espanola, the large ground finch either never arrived, or it became extinct. Whichever is true, Espanola only has the large cactus finch. But with no competition, the beak of the large cactus finch can exhibit more of its variability and, in fact, its beak is somewhat intermediate between the two finches on Genovesa, and it can feed equally well in both niches all year round. This phenomenon is known as character displacement.

 

Darwin's finches have many other evolutionary tales to tell. Darwin himself used the finches in the The Voyage of the Beagle to quietly announce the theory of evolution:

"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."

As such, it is entirely appropriate that these small birds carry the name of the scientist who gave the theory of evolutioan to the world, and who put their island home on the intellectual map.



    for more info, contact Dr. Robert Rothman: rhrsbi@rit.edu