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Margot Bellon

Dr. Bill Durham

HumBio 17C

September 19th, 2018

The Mysterious Diet of Sharp-Beaked Ground Finches

Introduction

Geospiza difficilis (G. difficilis), or the sharp-beaked ground finch, is one of the greatest

examples of adaptive radiation in the Galápagos Islands. This genus is widely dispersed across

this dynamic archipelago, each individual species containing a unique set of adaptations relevant

to their location. In this paper, I explore the

background and evolutionary adaptations of sharp-

beaked ground finches, and attempt to explain their

current beak morphologies and feeding behaviors

through the presentation of two distinct hypotheses. I

discuss how geographical barriers have triggered

speciation, and outline the various genetic explanations for the appearance of new adaptations on

different islands. I cite research that has been conducted on these finches in the Galápagos

islands, and detail the conclusions I have derived from existing observations and data. Finally, I

describe how climate change may influence the prevalence of such unique feeding behaviors,

and how humans can mitigate the warming-induced ecological stresses affecting these finches

and their respective ecosystems.

Background

G. difficilis is especially interesting to study because it has many congeneric species, and

represents allopatric differentiation in the speciation process.i Within G. difficilis, species differ

more in morphology than they do in any other genus (Grant, 1986). The sharp-beaked ground

finch is commonly found on several Galápagos islands, including Pinta, Fernandina, and

Geospiza difficilis. https://www.worldbirdphotos.com/photo/finch-sharp-

beaked-ground-geospiza-difficilis-male-galapagos-2/

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Santiago. These are larger, less arid, higher elevation islands that typically receive more rain

during the cool-dry season (June-December of each year). G. septentrionalis (the vampire finch),

however, is only found on smaller, lower Galápagos islands in the arid zone. The only islands on

which it has been observed are Wolf and Darwin islands (830 ft. and 550 ft. respectively), which

are the northern-most islands of the archipelago. An important point is that in recent years, G.

difficilis has been phylogenetically split between the northern, low island clades (Genovesa,

Wolf, and Darwin), and the higher island clades. This distinction is based on genetic lineages

that reflect divergent ecology, morphology, and song.

The map above illustrates the omnipresence of G. difficilis in the Galápagos,

demonstrating that it is particularly concentrated in the northern and western islands of the

Galápagos, mostly above the equator. It is distributed across three low elevation islands that are

furnished with Croton scouleri (a shrub), Opuntia cacti, grasses, and several species of low

herbs. Genovesa has similar vegetation, plus a ground covering of drought-deciduous trees, such

Map of G. difficilis presence in the Galápagos. Phylogenetic tree of G. Difficilis. https://www.nature.com/scitable/knowledge/library/molecular-genetic-techniques-and-markers-for-ecological-15785936

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as Bursera graveolens. All of the islands have a wide areas of forest cover, except Wolf and

Darwin, which simply have low growing patches of vegetation. Seeds are available on all islands

at most times as well, except arthropods are more common on the higher islands where there is

typically more access to rainfall.

Furthermore, the diagram to the right of the map displays the evolutionary progression of

G. difficilis; it is clear that the genus spread to occupy

different niches, which, as a result, created adaptive

differences within the genus. The diagram portrays that

G. difficilis_D, G. difficilis_W, and G. difficilis_G

evolved the latest, which makes sense because since

these are the northern-most and northeastern-most

islands, it would have taken the sharp-beaked ground finches longer to colonize these ecosystems

and develop the adaptations that make these arid islands more habitable.

Hypotheses

I. Diet influenced the development of sharp-beak morphology in the sharp-beaked ground

finch (G. difficilis).

II. Blood-drinking behavior evolved because of food shortages at low elevations in the cool-

dry season.

Methods

The methods that I employed in this research process included reading a variety of

research articles on G. difficilis and G. septentrionalis in order to better understand feeding

patterns and the geographic distribution of these sharp-beaked ground finches. Much of the

literature that I read were primary accounts by the Grants and other researchers of behaviors they

Bursera graveolens. .alamy.com/stock-photo-twisted-branches-of-

the-dwarf-palo-santo-tree-bursera-graveolens-malacophylla-139671088.html

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directly witnessed on the islands. Other literature included more quantitative analyses of the

distribution of Geospiza difficilis populations on the different islands of the archipelago, as well

as quantified evidence of the diets of Geospiza difficilis, Geospiza septentrionalis, and Geospiza

fuliginosa. I also consulted climate data to better understand heating trends in the Galápagos, and

to connect sharp-beaked ground finches’ adaptive features to changing climate trends and

exacerbated El Niños. I cited literature on endangered species in the Galápagos to connect

conservation efforts to the long-term viability of Geospiza difficilis, as well as Nazca boobies, in

the Galápagos archipelago. Finally, I took primary photographs of the various species discussed

in this paper throughout my visit to the Galápagos, even though we did not directly witness

vampire finch parasitism on Wolf and Darwin islands.

Findings

The primary feature that makes vampire finches so uniquely distinct from any other

sharp-beaked ground finch is the fact that it draws blood from prey for nourishment. To survive

the long dry spells in the cool-dry season on small, arid islands, the finches peck the skin of

Masked boobies and red-footed boobies until they

draw blood. This behavior primarily occurs because

of a lack of fresh water on the islands, though the B-

vitamins and cofactors in the blood also provide a

source of food supplementation for the finches.ii

Traditionally, sharp-beaked ground finches (and most

other finches) feed on leaves, seeds, flowers, cactus

pulp, and insects. These various food sources are available in great abundance during the warm-

wet season (January- June) because of the influx in rainfall, which causes vegetative productivity

Opuntia Cactus. Photo by Margot Bellon

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and incurs a greater availability of seeds and insects for finch consumption. The vampire finch

parasitic behavior is quite cooperative, which is fascinating considering how competitive and

aggressive parasitism is within an ecosystem exhibiting food-scarcity. Furthermore, the vampire

finch only feeds on the blood of Nazca boobies or red-footed boobies. Several finches will line

up behind the boobies, and as soon as one leaves because it has finished its feeding frenzy,

another finch will promptly take its place. These finches also steal booby eggs from unguarded

nests using the “bill-bracing technique”

until the egg shell is broken. The bill-

bracing technique involves the finches’

using their bills as a lever to lift the eggs

out of the nest, or to dig their bills into the

ground and use their feet as a mechanism

to steal the eggs.iii What is most

interesting about the vampire finch’s

feeding behavior is that it did not begin as a parasitic pattern. In fact, the blood-sucking behavior

evolved because vampire finches would remove parasites from the white feathers of the booby

by digging their beaks deep into the elbow of the booby wings. The finches would be nourished

by the arthropods, and the boobies would be freed of parasites. When the finches accidentally

drew blood while serving this mutualistic act, they adapted to obtaining the nutritious dietary

supplement of blood, and suddenly, and quite deceptively, became a parasitic threat to the

innocent birds.

In contrast, the Nazca and red-footed boobies did not evolve fast enough to combat this

new-found parasitism, and have been recorded to have very passive reactions to the blood-

Sora.unm.edu. (2018). [online]

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sucking behavior. According to Koster and Friedemann,

“Courting pairs seem virtually unaware of the ‘vampires’ riding

on their backs, pecking and drawing blood from the feather

quills. With single boobies, however, a reaction of discomfort

can frequently be observed.”iv The boobies will walk around the

cliff, now and then shaking their wings to throw off blood-

sucking persecutors, but are inevitably followed by up to five or six finches patiently awaiting

their turns to sip the blood. Furthermore, there is no evidence that the boobies are severely

injured by the parasitic behavior. Based on visible inspection (because no deeper probe was

available for Koster and Friedemann), there is no direct harm of the finch bill probe on the young

growing feather, although the blood-filled quills are punctured by the finches and partially

drained of their blood. Even though feather growth is not stunted, though, the boobies are still at

risk for disease transmission through these parasitic feeding patterns, especially if multiple

vampire finches feed on multiple Nazca boobies, since some infections can easily be transmitted

through the blood.v

Schluter and Grant performed a research study in 1982 to determine what ground finches

were eating on the different Galápagos islands. Food supply was assessed during the cool-dry

season (June-December). It is during this season that crop and food production reach their

lowest values. They made approximately 300 observations per bird, and they identified the

food each bird was consuming based on food present in the beak or food that was missing from

the parent plant. The following chart demonstrates what food is eaten by G. difficilis on which

islands. The most notable point is that blood is only observed on Darwin and Wolf islands, and

concealed seeds are observed in all of the islands, emphasizing that seeds are a universal food

Vampire Finch.

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staple for the entire genus.

Schluter, D. and Grant, P. (1984). Ecological Correlates of Morphological Evolution in a Darwin's Finch, Geospiza

difficilis. Evolution, 38(4), p.856.

According to the Grants, on the low island

of Genovesa, G. difficilis has become

smaller in size over time, and this is

correlated with a diet of small seeds and

flower nectar. These data provide partial

support for the idea that G. difficilis on

Darwin and Wolf combine the feeding

niches of the absent G. fuliginosa and G.

scandens by scavenging, eating

invertebrates, and consuming Opuntia cacti, since these other finch species are not present in

these northern-most islands to feed on the cacti and invertebrates. Additionally, G. fuliginosa

(small ground finch) and G. difficilis differ marginally in beak depth and ranges of seeds

consumed, but differ significantly in beak shape (due to G. difficilis’ extensive arthropod diet).vi

The image above illustrates the beak morphological diversity of G. fortis across islands.

Although this is a different genus of finch (the medium ground finch), it effectively displays how

https://press.princeton.edu/titles/10282.html

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many diverse expressions of a single trait can exist across islands. The diagram just beneath the

beak chart illustrates what is meant by beak

depth in discussing finch bills: it is the

vertical measurement of the finch bill. On

the graph to the left produced by the

Grants, it is clear that beak depth increases

with seed hardness, meaning that on islands

with softer seeds (perhaps due to more

rain), the average beak depths should be of

smaller values.

Additionally, it has been found that

the Genovesa population of G. difficilis is

more closely related to many other species of Darwin's finches than it is to more centrally

located populations of G. difficilis. This can be explained through a phenomenon called

introgression, which is the introduction of new species to an island and their hybridization with

native species, which causes species to quickly evolve away from their original form.vii

Peripheral populations, such as the finches found on Darwin and Wolf, may have been

differentially affected by past introgression, causing their beak morphologies to be inconsistent

with other G. difficilis species on the islands. However, considering how isolated Darwin and

Wolf are, it is perhaps more plausible that these vampire finches evolved the bills that they did

because of isolation, genetic drift, and random gene flow, more than simply hybridization.viii

Grant, P. (2017). Ecology and evolution of Darwin's

finches. Princeton University Press.

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ix

Moreover, in discussing hypothesis 2, I will

be referring to the table above. This study aimed to

explain seed density as a function of drought on

the islands. 1973 was the wet year, and 1977 was

the drought year in the study above. Predictions of

finch numbers and foraging activity in 1977 were

conditional upon food supply. Small and soft seeds were absolutely and relatively rarer in the

drought year than in the wet year, and food supply and finch numbers were severely depressed

during years of little or no rainfall, partly as a consequence of interspecific competition for food.

As one can see from the chart, G. scandens finches (cactus finches) devoted more time to flower

buds in 1977 than in 1973. This is associated with a lower abundance of flowers in 1977 (during

the drought). Therefore, it is clear that years of contrasting rainfall may not necessarily produce

Grant, P. and Grant, B. (1980). Grant, P. and Grant, B. (1980). Grant, P. and Grant, B. (1980).

Cactus Finch. Photo by Bill Durham

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directly contrasting food conditions, since food was still available during the drought year, but

food supply and finch numbers are definitely smaller in drought years, which incurs higher rates

of interspecific competition for food.x

Conclusions

My speculation regarding why the vampire finches evolved to be so cooperative with one

another with regards to feeding on boobies is that due to the limited supply of water on the

islands, the finches had to share the boobies that were available for parasitism with each other in

order to preserve the viability of the entire species. Furthermore, since the parasitism never

actively kills the Nazca and red-footed boobies, there will almost always be a reliable supply of

blood, so the need for harsh, dangerous competition between vampire finches appears

unnecessary. However, when the finches steal eggs from unguarded nests, they have been

recorded to viciously and aggressively fight over the embryo, dragging it out of its shell and

ripping it apart on the spot.xi Because a single egg is a very limited supply of food, it makes

sense that the finches would be more aggressive in this particular context.

Additionally, I speculate that the reason for which white Nazca boobies are more

common sources of food than red-footed boobies is because of their white feathers. Since

multiple finches will line up behind a booby once one finch has clearly penetrated the feathers,

the contrast of the red blood on the white feathers will clearly signal to the other finches that prey

has been discovered. However, since the red-footed boobies are darker in pigmentation, the

feeding frenzy is less visibly obvious, and, thus, it is reportedly less common for finches to feed

on brown red-footed boobies.xii

Moreover, with respect to hypothesis 1, which predicts that the sharp-beaked ground

finch’s sharp-beak morphology evolved as a function of diet, I have concluded that in order to

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have absolutely conclusive results, research must be conducted at the end of the dry season when

there is the greatest food scarcity and stress. According to Schluter and Grant, a correlation

between morphology and diet is expected only under food limitation, when natural selection may

most strongly influence morphology, and so performing this study under high food-stress

conditions would help reinforce that diet strongly correlates with morphology, as opposed to

finding a causal relationship between beak structure and a variety of other factors, such as

introgression or random genetic drift. Unless researchers observe natural selective response to

food-stress over a very prolonged period of time, it will be difficult to ever showcase causation

between a certain diet and beak structure.

Furthermore, based on the evidence presented in the literature, the long beak of G.

difficilis is either due to the adaptation to feed on and probe Opuntia flowers, or to probe through

the feathers and skin of seabirds or eggs. However, given the fact that shorter-billed birds on

Genovesa also feed on Opuntia flowers, long bills are thought to be more associated with

bleeding, and with consuming hard seeds (as demonstrated in the beak depth graph in the

“Findings” section). These results are ambiguous, however, since the same bill can be used

moderately efficiently for different tasks (nectar-feeding and seed crushing), so it is difficult to

distinguish one diet as the primary influencer of beak morphology. As the Grants found in 1984-

1986, which were years with unusually rainy weather, there was a higher abundance of small,

soft seeds on the islands, and a reduced number of tough, larger ones that finches tend to

avoid.xiii It makes sense that G. septentrionalis, which is found on the drier islands in the

archipelago, would have evolved bills with larger beak depths in order to feed on the tough seeds

available, as the soft, small ones associated with moist conditions are rapidly ravaged by all of

the birds. Therefore, it is, again, difficult to isolate one food source as the primary cause of a

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certain beak morphology, since long bills and deep bills are adaptive for blood-sucking, Opuntia

cactus feeding, and tough seed cracking. Therefore, my results for hypothesis one are

inconclusive.

Additionally, with respect to my second hypothesis, I am able to justify that G.

septentrionalis developed its parasitic feeding behavior because of ecological stress on the arid

islands. According to the Grant paper, seed number and biomass (including insects) are lower in

drought years, and lower on low elevation islands where there is less rainfall. These dry

conditions cause a lower abundance of

small, soft seeds, which could have

created a stress for the development of a

sharper beak in the vampire finches.

Further, the aggressive nature of vampire

finch parasitism could have evolved from

the interspecific competition for the

scarce soft seeds. Once species with

harder beaks proved their success in

sucking blood and in eating harder seeds, natural selection eagerly drove the G. difficilis beak

morphology in one direction.

Opuntia Cactus. Photo by Margot Bellon

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Future Research Recommendations

My primary question moving forward is

whether other finch species will develop parasitic

feeding behaviors as climate change makes other

islands more arid and amplifies food scarcity.

Evidence suggests that the Galápagos are only

getting wetter through time, so it is unlikely that

other finch species on other islands will adopt

blood-drinking behaviors because the abundance

of fresh water across the archipelago will eventually amplify food abundance. xiv The graph

above demonstrates that precipitation trends increased dramatically across the islands in the 30

years between 1950 and 1980. Furthermore, there is evidence that the Galápagos are wetter now

than they were in ice-age times, droughts were more severe in ice-age times, and aerial expanse

of deserts were much increased due to the lowering of sea level in ice-age timesxv. On low

islands, finches now experience droughts every 1 in 3 years, whereas droughts occur far less

frequently on higher islands.

COLINVAUX, P. (1972). Climate and the Galapagos Islands. Nature,

240(5375), pp.17-20.

Grant, P. (1985). Climatic Fluctuations on the Galapagos Islands and Their Influence on Darwin's Finches. Ornithological Monographs, (36), pp.471-483.

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However, although more rainfall (which is associated with climate change) might benefit

seed productivity and finch survival in

the short term, it could eventually lessen

food supply for boobies. This is because

rising ocean temperatures associated

with El Niño and increasing climate

change is causing the sardines

(Clupeidae), which Nazca boobies

primarily feed on, to migrate south to

find cooler-temperature, more suitable waters.xvi Consequently, the Nazca boobies would be left

without their principal food supply in future years with worsened El Niños and wetter climates,

and they could potentially starve. As a result, the vampire finches would be left without a blood

source, and the mutualistic/parasitic relationship between Nazca boobies and vampire finches

would be adversely affected.

This prompted me to speculate whether a surge in sharp-beaked ground finch populations

on the islands due to food abundance and generally favorable mating conditions would cause

more parasitism on the arid islands to occur, due to higher competition for food with an

increased number of finches. It would be interesting to conduct further research on how

increased food abundance might affect sharp-beaked finch populations, and whether a surge in

population would make them more aggressive with respect to feeding, as opposed to maintaining

their generally cooperative behavioral patterns.

Nazca booby. Photo by Margot Bellon

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Additionally, it would be interesting to study how sharp-beaked ground finches will

eventually adapt to exacerbated El Niños

and wetter climates. If climate change

were making the islands drier and more

arid with increasing temperatures, it

would make sense that other finches

would adopt parasitic feeding behaviors,

too, in order to hydrate in the face of a

lack of water. The graph to the left

demonstrates the warming trends

affecting La Niña and El Niño over a

period of 40 years; these warming trends are not necessarily correlated with drier climates. Since

the islands are getting warmer, there are other micro-adaptations that the finches have evolved to

acclimate to changing atmospheric conditions. For example, Geospiza fuliginosa breed at

moderately high elevations on the south side of Santa Cruz only in dry years, because in higher

altitude there is greater access to water. However, in wetter years, Geospiza fuliginosa breeds at

lower elevation. Ultimately, finch populations on small islands are not morphologically static

because they change under temporary selective pressures in short time periods, illustrating their

adaptability as a species.

Connection to Conservation

In considering conservation, it is important to recognize that the vampire finch has a very

restricted range and small population size. Therefore, it can easily be threatened by the

introduction of invasive predators or disease, and could be driven to a state of Critically

Cpc.ncep.noaa.gov. (2018). Climate Prediction Center - Warm Episodes. [online]

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Endangered.xvii The sharp-beaked ground finch is an example of a small peripheral population

that is easily subject to introgression or genetic drift, or even full eradication if the conditions

prove unfavorable.

To further discuss the human impacts in the Galápagos, over half of the industrial era is

responsible for increases in ocean warming occurred in the past 20 years, with over a third of that

heat being accumulated below 700 meters.xviii As a result, there is abnormal cloudiness and

rainfall in the Galápagos, especially in the boreal winter and spring seasons. Consequently, seed

type produced with more rainfall could be incompatible with long, sharp-beaked finch bills that

G. difficilis has so keenly evolved to have. Although more rainfall through El Niño could benefit

the sharp-beaked finches, by providing greater food supply of seeds and arthropods, it could also

negatively impact the vegetation and other species that have adapted to low elevation, arid

climates, such as the red-footed and Nazca boobies on Genovesa. Therefore, as a society, we

should be mindful of the warming trends we are incurring on our planet, especially around the

equator where El Niño and La Niña are exhibited the most seriously. Too much rain and

moisture could change the natural course of the ecosystem in unforeseeable ways, and when we

are dealing with fragile, small populations such as G. difficilis and G. septentrionalis, preserving

the ecosystem in its existent, natural state is of critical importance.

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References

i Farrington, H., Lawson, L., Clark, C. and Petren, K. (2014). THE EVOLUTIONARY

HISTORY OF DARWIN'S FINCHES: SPECIATION, GENE FLOW, AND INTROGRESSION

IN A FRAGMENTED LANDSCAPE. Evolution, 68(10), pp.2932-2944. ii Husnik, F. (2018). Host–symbiont–pathogen interactions in blood-feeding parasites: nutrition,

immune cross-talk and gene exchange. Parasitology, 145(10), pp.1294-1303. iii Koster, H. and Friedemann (1983). TWELVE DAYS AMONG THE "V AMPIRE-FINCHES"

OF WOLF ISLAND. Noticias de Galapagos, [online] (38), pp.4, 5. Available at:

http://aquaticcommons.org/9976/1/NG_38_1983_Koster_Twelve_days.pdf [Accessed 1 Sep.

2018]. iv Koster, H. and Friedemann (1983). TWELVE DAYS AMONG THE "V AMPIRE-FINCHES"

OF WOLF ISLAND. Noticias de Galapagos, [online] (38), pp.4, 5. Available at:

http://aquaticcommons.org/9976/1/NG_38_1983_Koster_Twelve_days.pdf [Accessed 1 Sep.

2018]. v Husnik, F. (2018). Host–symbiont–pathogen interactions in blood-feeding parasites: nutrition,

immune cross-talk and gene exchange. Parasitology, 145(10), pp.1294-1303.

vi Grant, P. (2017). Ecology and evolution of Darwin's finches. Princeton University Press.

vii Grant, P. (2017). Ecology and evolution of Darwin's finches. Princeton University Press. viii Tebbich, S., Sterelny, K. and Teschke, I. (2010). The tale of the finch: adaptive radiation and

behavioural flexibility. [online] Philosophical Transactions of the Royal Society B. Available at:

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on Isla Daphne Major, Galapagos. Oecologia, 46(1), pp.55-62. x Hamilton, T. and Rubinoff, I. (1967). On Predicting Insular Variation in Endemism and

Sympatry for the Darwin Finches in the Galapagos Archipelago. The American Naturalist,

101(918), pp.161-171. xi Koster, H. and Friedemann (1983). TWELVE DAYS AMONG THE "V AMPIRE-FINCHES"

OF WOLF ISLAND. Noticias de Galapagos, [online] (38), pp.4, 5. Available at:

http://aquaticcommons.org/9976/1/NG_38_1983_Koster_Twelve_days.pdf [Accessed 1 Sep.

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Neotropica, [online] 10(4). Available at: http://www.scielo.br/scielo.php?pid=S1676-

06032010000400025&script=sci_arttext [Accessed 1 Sep. 2018]. xiii Pbs.org. (2018). Evolution: Library: Finch Beak Data Sheet. [online] Available at:

https://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html [Accessed 15 Oct. 2018]. xiv COLINVAUX, P. (1972). Climate and the Galapagos Islands. Nature, 240(5375), pp.17-20.

Grant, P. (1985). Climatic Fluctuations on the Galapagos Islands and Their Influence on

Darwin's Finches. Ornithological Monographs, (36), pp.471-483. xv COLINVAUX, P. (1972). Climate and the Galapagos Islands. Nature, 240(5375), pp.17-20.

Grant, P. (1985). Climatic Fluctuations on the Galapagos Islands and Their Influence on

Darwin's Finches. Ornithological Monographs, (36), pp.471-483. xvi Pbs.org. (2018). Evolution: Library: Finch Beak Data Sheet. [online] Available at:

https://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html [Accessed 15 Oct. 2018].

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xvii Encyclopedia of Life. (2018). Vampire Finch - Geospiza difficilis septentrionalis - Details -

Encyclopedia of Life. [online] Available at: http://eol.org/pages/1244235/details [Accessed 1

Sep. 2018].

xviii Cho, R. (2018). El Niño and Global Warming—What’s the Connection?. [online] State of the

Planet.