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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.