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WHY MOCKINGBIRD SPECIATION IN THE GALAPAGOS ARCHIPELAGO IS LIMITED
TO FOUR SPECIES
ERIKA HARRELL
DARWIN, EVOLUTION, AND GALAPAGOS
DURHAM
OCTOBER 2008
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Introduction
The central question of this study is why mockingbird speciation in the Galapagos is limited to
only four species. The answer to this question is significant becauses it pertains to the broader
question of what characters of a species or environment make for less selective pressures and
more limited speciation and what characters of a species/environment selectively pressure for
greater speciation. It also addresses the question of how specific characters of a species and an
environment can interact to selectively pressure for broad or limited speciation. I chose to study
these questions by observing the mockingbirds of Galapagos because these birds not only exhibit
limited speciation, but can conveniently be compared to the finches of the Galapagos, which
have speciated significantly more than the mockingbirds. Advantageously, both sets of species
are confined to the Galapagos archipelago and experience much habitat overlap. Therefore, by
comparing them, it is easy to determine what characters of each and of their environment
pressured for or against extreme speciation. My primary hypothesis is that mockingbirds are
resourceful generalists and omnivores, so there was no selective pressure on these birds to split
into specialists with regard to food consumption. Finches, on the other hand, have a diet that is
restricted by the fact that they are chiefly herbivores. Such a restriction is a signicant selective
pressure that the mockingbirds simply did not have. To test my hypothesis, I considered
experimentation and data gathered by accomplished researchers, and observed the different
species of both mockingbirds and finches in the Galapagos wild.
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Background
There are four species of mockingbird on the Galapagos archipelago. All four species are
cooperative breeders, meaning young, non-breeding “helpers” aid in feeding and protecting
fledglings. (Curry & Grant 1989:441) Each species also remains in its natal territory and, thus,
exhibits territorial behavior. All species are enthusiastic omnivores with long, curved
“multipurpose” breaks, prefer “hopping” on land to flying, and inhabit the island dry zones.
(Curry & Grant 1989: 443)
Hood Mockingbird using multi-use beak to resourcefully dig through sand for food on the island
of Espanola. (Harrell 2008)
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Hood Mockingbird pecking at sleeping marine iguana. Probably in search of food. (Harrell
2008)
Typical Galapagos Arid/Dry Zone. (Harrell 2008)
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The most widespread, Nesomimus parvulus, is commonly known as the Galapagos Mockingbird.
N. parvulus is found on all islands in the northwestern portion of the archipelago except for
Pinzon, where it was most likely outcompeted by feral rats. It is not found on any island which
contains another mockingbird species (Floreana, Espanola, San Cristobal). There are eight
subspecies of N. parvulus, each of which is found on its own island or cluster of islands. (Curry
& Grant 1989: 445)
Galapagos Mockingbird on the island of Santa Cruz. (Harrell 2008)
Nesomimus macdonaldi, the Hood Mockingbird, is found only on Espanola. It is the largest and
most aggressive of the mockingbirds of Galapagos. N. macdonaldi is also the most opportunistic
feeder and the most reluctant to fly. (Nelson 1968: 31)
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Hood Mockingbird, Espanola. (Hedlin 2008)
Hood Mockingbird aggression. (Easton 2008)
Nesomimus trifasciatus, the Charles Mockingbird, was once found only on Floreana but has
since been confined to Floreana’s satellite islands Champion and Gardner due to the introduction
of feral predators. There are less than one hundred of these rare birds left in Galapagos. (Harris
1982: 121)
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Charles Mockingbird (Harrell 2008)
Lastly, Nesomimus melanotis, the Chatham mockingbird, can be found on San Cristobal. Its
plumage is intermediate between that of the Galapagos and Hood Mockingbirds and it is the
shyest species. N. melanotis exhibits less cooperative breeding than the other species and has its
nests highest due to predation. (Castro & Phillips 1996: 120)
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Chatham Mockingbird (Schwarz 2006)
There are thirteen species of finch on the Galapagos islands. The founding finch population is
thought to have been herbivorous, and according to Darwin’s proposed “principle of divergence”
competition for food pressured the finches to their current specialized diets of grubs, ticks, blood,
cactus pulp, nectar, leaves, buds, and a wide variety of seeds. (Kricher 2006: 136) The resulting
speciation/adapative radiation was much more extreme than that of the mockers. The finches
inhabit the same arid zones of each island that the mockingbirds do. (Kricher 2006: 138)
Hypotheses
The main hypothesis that I considered while examining my research question is that, due to their
omnivorous tendencies, there was no selective pressure on the mockingbirds of Galapagos to
specialize their diets and, therefore, a low degree of mockingbird speciation occurred. The
mockingbirds are a truly hearty set of species and, while researching in Galapagos, I received a
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firsthand account from a tour guide of Hood Mockingbirds commonly consuming sea lion
placenta. One alternate hypothesis is that not enough time has passed since the mockingbirds
first reached Galapagos for the birds to have speciated significantly. However, this hypothesis
can be dispelled by the fact that mainland mockingbirds demonstrate limited speciation in much
the same way that the Galapagos mockingbirds do. Because mockingbirds have narrow
speciation globally, time cannot be considered a speciation constraint for the birds.
Methods
The first measures taken were to research the ancestry of the mockingbirds. After general
ancestry was verified, the confluence of wind-driven currents at the archipelago was observed in
order to determine how mockingbird colonization of the Galapagos most likely occurred. Next,
research was conducted to ascertain the cause of greater mockingbird speciation in the southern
islands than in the northern islands. Throughout this series of research, the diets of the finches
and the mockingbirds were also verified and compared, as well as the vegetation found in the
arid zones of each island in Galapagos.
Findings
A team composed of B. Arbogast, S. Drovetski, R. Curry, P. Boag, G. Seutin, P. Grant, B. R.
Grant, and D. Anderson conducted DNA testing on the mockingbirds revealing that the
Galapagos Mockingbird is most closely related to a mainland species known as the Tropical
Mockingbird while the Hood, Charles, and Chatham Mockingbirds are most closely related to
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the Chilean Mockingbird. DNA testing also revealed that the Galapagos Mockingbird was not
genetically similar to the Chilean Mockingbird. The Tropical Mockingbird is found in Central
America and the Caribeean while the Chilean Mockingbird inhabits the west coast of South
America. Further research indicated that the Humboldt Current, Southern Equatorial Current, and
the Panama Current intersect at the Galapagos, and that the islands to the south are upwind of the
northern islands. (Arbogast 2006: 4) The mockingbird diet was verified to include everything
from seeds, fruits, and cactus pulp to insects, spiders, baby birds, eggs, sea lion placentas,
unattended tourist food and whatever else the birds found available to them at any given time.
(Kricher 2006: 134) Each finch species had its own specialized diet and had adapted traits,
particularly with regard to beak size and shape, that are advantageous in the acquisition and
consumption of each specific food/foods in its diet. (Kricher 2006: 139) For example, the Large
Ground Finch has a very large beak relative to its body size in order to facilitate the crushing of
larger seeds.
Large Ground Finch on Espanola. (Harrell 2008)
According to a study conducted by Robert Bowman, each island of Galapagos has distinct
vegetation variation. Bowman’s studies determined that this variation even occurs from island to
island within the same ecological zones. (Kricher 2006: 144)
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Conclusions and Recommendations
The Galapagos Mockingbird is not genetically similar to the ancestor of the Charles, Chatham,
and Hood Mockingbirds, therefore, it probably descended from a different species than the latter
three, which were all genetically similar to their likely ancestor the Chilean Mockingbird.
(Arbogast 2006: 4)
Dotted line separates the two clusters of islands affected by each separate mockingbird
colonization event. (Arbogast 2006: 2)
In conclusion, there were probably two mockingbird colonization events. The winds that drive
the Humboldt Current most likely distributed the Chilean Mockingbird across the southernmost
islands of the archipelago and the winds driving the Panama Current probably distributed the
Tropical Mockingbird across the northwestern islands of the archipelago. Because the islands to
the south are upwind of the islands to the north, it follows that the Tropical Mockingbird, now
the Galapagos Mockingbird, would keep to the north rather than flying upwind and colonizing
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the islands to the south. The southernmost islands are also considerably isolated from the
northern islands and from eachother. Therefore, because mockers remain in their natal territories
and spend more time on land than in flight, and because the southern islands are somewhat cut
off from the rest of the archipelago and from eachother, the Chilean mockingbird did not attempt
to colonize the northern islands. Nor was there contact between the populations of mockingbird
on each southern island with each of the other southern populations. Therefore, the individual
populations of mockingbird on Floreana, Espanola, and San Cristobal speciated into three
separate species, one on each of the islands, while the northern Galapagos Mockingbird became
widespread in a tighter cluster of islands. The founder’s effect, the rapid divergence that results
from a limited gene pool, no doubt allowed for rapid speciation on Floreana, Espanola, and San
Cristobal, so that if any of these island’s species did stray to another of the islands, it surely
would have been outcompeted by birds already there that had adapted to that island’s unique
environment. This was most likely also true of mockers that strayed north: the subspecies of
Galapagos Mockingbird on each northern island is so well-suited to the northern islands that
other mockingbirds would have been outcompeted. Not to mention the fact that any interspecies
mating would have resulted in inviable offspring, so there was virtually no way for other
mockingbird species to survive on islands that they did not already inhabit.
Based on Bowman’s conclusions on the variation of vegetation in Galapagos. It is evident that
this variation in itself would have been enough of a selective pressure for finch speciation
without the added pressure of competition for resources. Because the finches and mockingbirds
both inhabit the island dry zones, they shared the pressure created by plant variation. However,
the additional pressure of competition over food, which the mockingbirds did not have because
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they are resourceful omnivores, is therefore the most likely cause of intense adaptive radiation in
the finches of Galapagos. Another factor worth considering when looking at the diets of the
finches and the mockingbirds is that herbivorous finches were also subject to greater seasonality
in their food supply than the mockers, who were able to tailor their diets to the seasons due to
their aforementioned resourcefulness.
Although my conclusion is, in fact, speculative, the data collected not only supports my
hypothesis but offers an even more elaborate answer to the question of why mockingbird
speciation in the Galapagos is so limited that also addresses the distribution of mockingbird
species in the archipelago. Further research that would be worthwhile would include an analysis
and comparison of the specific plant life and environmental conditions of each of the islands of
Galapagos and the role that these environmental factors play in the lives of each individual
species of mockingbird. It would also be interesting to study the mockingbird and finch
interactions and observe whether or not character divergence played a role in their speciation as a
result of overlapping habitats.
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Works Cited
Arbogast, Brian S., Sergei V. Drovetski, Robert L. Curry, Peter T. Boag, Gilles Seutin, Peter R. Grant, Rosemary Grant, and David J. Anderson. "The Origin and Diversification of Galapagos Mockingbirds." Evolution 60 (2006): 370-82. Castro, Isabel, and Antonia Phillips. A Guide to the Birds of the Galapagos Islands. Princeton, NJ: Princeton UP, 1996. Curry, Robert L., and Peter R. Grant. "Demography of the Cooperatively Breeding Galapagos Mockingbird, Nesomimums parvulus in a Climactically Variable Environment." Journal of Animal Ecology 58 (1989): 441-63. Harris, Michael. The Collins Field Guide to the Birds of Galapagos. New York, NY: The Stephen Greene P, 1982. Heinzel, Hermann, and Barnaby Hall. Galapagos Diary: A Complete Guide to the Archipelago's Birdlife. Berkeley, CA: University of California P, 2000. Kricher, John C. Galapagos : A Natural History. New York: Princeton UP, 2006. Nelson, Bryan. Galapagos, Island of Birds. New York, NY: William Morrow & Company Inc., 1968. Stewart, Paul D., and Richard Dawkins. Galapagos : The Islands That Changed the World. New York: Yale UP, 2007. And a special thanks to students from the Stanford Travel/Study Galapagos Field Seminar 2008 for their photographs.