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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/
2
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
3
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
4
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
5
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]
6
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.
7
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
8
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.
9
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
10
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
11
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
12
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
13
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.
14
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
15
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]
16
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.
17
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:
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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.