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Darwin’s finches combat introduced nest parasites with fumigated cotton
Sarah A. Knutie1*, Sabrina M. McNew1, Andrew W. Bartlow1,
Daniela A. Vargas2, and Dale H. Clayton1

Affiliations:1

Department of Biology, Univ. of Utah, 257 South 1400 East, Salt Lake City, UT 84112, USA.

2

Facultad de Biosciencias, Univ. Autonoma de Barcelona, Cerdanyola del Valles, 08193
Barcelona, Spain.

*E-mail: saknutie@gmail.com

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Main text
Introduced parasites are a threat to biodiversity when naïve hosts lack effective defenses

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against such parasites [1]. Several parasites have recently colonized the Galápagos Islands,
threatening native bird populations [2]. For example, the introduced parasitic nest fly Philornis
downsi (Diptera: Muscidae) has been implicated in the decline of endangered species ofDarwin’s finches, such as the mangrove finch (Camarhynchus heliobates) [3]. Here, we showthat Darwin’s finches can be encouraged to “self-fumigate” nests with cotton fibers that havebeen treated with permethrin. Nests with permethrin-treated cotton had significantly fewer P.
downsi than control nests, and nests containing at least one gram of cotton were virtuallyparasite-free. Nests directly fumigated with permethrin had fewer parasites and fledged more
offspring than nests treated with water.
Adult P. downsi flies, which are not parasitic, lay their eggs in the nests of Darwin's
finches and other land birds in the Galápagos. Once the eggs hatch, the fly larvae feed on the
blood of nestlings and adult females when they sit on the nest. Several previous studies haveshown that P. downsi reduces the reproductive success of Darwin's finches [reviewed in 4]. In
some years, 100% of nests at a given location can fail due to P. downsi [4,5,6]. It is therefore

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critical that control measures be developed to help reduce the effect of P. downsi on endangered
Darwin's finches and other birds [3,7].
Our study was conducted January-April, 2013 at the El Garrapatero field site on Santa
Cruz island [4,5]. The study was prompted by observations of several species of Darwin’s
finches incorporating cotton fibers from laundry lines into their nests (Figure 1A). To determine
whether finches can be encouraged to self-fumigate their nests, we placed 30 cotton dispensers(Figure 1B) at 40-meter intervals along two transects through our study site (Figure S1).

	  

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Preliminary trials showed that finches transport cotton up to 20 meters (Supplemental
information).

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We used two types of (interspersed) dispensers: 1) experimental dispensers, whichcontained cotton treated with a 1% permethrin solution, and 2) control dispensers, whichcontained cotton treated with water. Processed and unprocessed cotton were used to distinguishbetween the treatments. The two types of cotton were similar in appearance, but could be
distinguished upon close inspection. A coin toss determined which treatment was assigned towhich cotton type: processed cotton was used for the experimental treatment and unprocessed
cotton for the control treatment. A preliminary experiment showed that finches do notdiscriminate on the basis of cotton type or fumigant (Figure 1C; Supplemental information).
Over the course of the study, we searched once a week for active nests within 20 metersof each dispenser. When a nest was found, it was checked with a camera on a long pole toconfirm breeding activity. After the birds finished breeding, the nests were collected anddissected to quantify the number of P. downsi in each nest. Cotton and natural nest materials
were separated and weighed.
We located 26 active Darwin’s finch nests, 22 (85%) of which contained cotton (Figure

UU IR Author Manuscript1D). None of the nests contained more than one type of cotton. Thirteen nests had experimental(permethrin) cotton and nine nests had control (water) cotton. Nests were constructed by four
species of Darwin’s finches: Geospiza fortis, G. fuliginosa, Camarhynchus parvulus, andPlatyspiza crassirostris. Nests with experimental cotton had a mean (± SE) of 14.69 ± 9.54
parasites; control nests had a mean of 29.89 ± 7.69 parasites (Mann-Whitney test: U = 31.00, P =0.03). The effect of the experimental cotton was dose-dependent. Of the eight nests that
contained at least one gram of experimental cotton, seven had no parasites and the eighth had	  University of Utah Institutional Repository
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only four parasites (Figure 1E). There was no relationship between cotton and parasite load
among control nests (Figure 1E).

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Monitoring reproductive success requires climbing to nests and banding nestlings, which
could interfere with self-fumigation behavior. We therefore quantified the effect of fumigation
on host reproductive success using another 37 Darwin’s finch nests adjacent to the selffumigation transects. We sprayed experimental nests with a 1% permethrin solution and control
nests with water. Nestlings were banded with color bands, enabling us to confirm fledging
success by identifying individual birds after they left the nest [4,5]. Once all of the nestlings in anest had fledged or died, the nest was collected and dissected to quantify the number of parasites.
The twenty experimental nests sprayed with permethrin had no parasites, while the 17
control nests sprayed with water had a mean of 17.00 ± 3.89 parasites (Mann-Whitney test, U =20.00, P < 0.0001). Nineteen of the twenty experimental nests (95%) fledged at least one
offspring, while only 11 of the 17 control nests (65%) fledged any offspring (Fisher’s Exact, P =0.03). Overall, 50 of 60 nestlings (83%) fledged from experimental nests, compared to just 29 of
54 nestlings (54%) from control nests (Figure 1F).
Our study shows that Darwin's finches can control P. downsi with permethrin-treated

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cotton, and that fumigation increases fledging success. There are currently no other effective
methods for controlling P. downsi. Self-fumigation may thus be a viable approach forcombatting P. downsi in the nests of Darwin's finches. The mangrove finch is the most criticallyendangered species of Darwin’s finch, with a population of less than 100 individuals restricted toa home range of less than 1km2 on Isabela Island [3]. Sixty cotton dispensers could treat thisentire population. Self-fumigation may be a particularly efficient approach because mangrove
finches often build their nests high in mangrove trees, where they are relatively inaccessible [3].

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Our study is the first to demonstrate the effectiveness of self-fumigation against parasites.
This approach has been tried previously where mice were encouraged to incorporate fumigatedUU IR Author Manuscript

cotton into their nests to kill ticks that vector Lyme Disease; however, the effectiveness of themethod is not clear [8]. Self-fumigation might also be useful for controlling the fleas that vector
plague, which can contribute to the local extinction of black-tailed prairie dogs (Cynomys
ludovicianus) [9]. Because prairie dogs incorporate plant fibers into their burrows, it might bepossible to encourage them to use fumigated materials. Self-fumigation also has potential for thecontrol of parasites in other threatened and endangered bird species. For example, it might beuseful for combating explosive increases in lice that appear to have contributed to the decline of
the Hawaiian endemic akepa honeycreeper (Loxops cocineus cocctneus) [10].

Supplemental Information
Supplemental Information including experimental procedures and one figure can be found withthis article online at http://dx.doi.org/10.1016/j.cub.2014.03.058.

Acknowledgments

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We thank the Galápagos National Park, Charles Darwin Research Station, Fred Adler, Elena
Arriero, Emily DiBlasi, Jordan Herman, Michael Thompson and Sarah Windes for assistance.
We thank Charles Brown, Sarah Bush, Franz Goller, Jennifer Koop, Cagan Sekercioglu, and two
anonymous reviewers for comments on the manuscript. The work was supported by NSF grant
DEB-0816877 to D.H.C. and a University of Utah GCSC grant and RocketHub Crowdfunding to
S.A.K. We are grateful to U.S. Cotton™ for donating supplies.

	
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References
1.

Daszak, P., Cunningham, A. A., and Hyatt, A. D. (2000). Emerging infectious diseases of

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wildlife – threats to biodiversity and human health. Science 287, 443–449.
2.

Wikelski, M., Foufopoulos, J., Vargas, H., and Snell, H. (2004). Galápagos birds anddiseases: invasive pathogens as threats for island species. Ecol. Soc. 9, online:
http://www.ecologyandsociety.org/vol9/iss1.

3.

Fessl, B., Young, G. H., Young, R. P., Rodríguez-Matamoros, J., Dvorak, M., Tebbich, S.,
and Fa, J. E. (2010). How to save the rarest Darwin’s finch from extinction: the mangrove
finch on Isabela Island. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 365, 1019–1030.

4.

Koop, J. A. H., Huber, S. K., Laverty, S. M., and Clayton, D. H. (2011). Experimental
demonstration of the fitness consequences of an introduced parasite of Darwin’s finches.
PLoS One 6, e19706.

5.

Koop, J. A. H., Owen, J. P., Knutie, S. A., Aguilar, M. A., and Clayton, D. H. (2013).
Experimental demonstration of a parasite-induced immune response in wild birds:
Darwin’s finches and introduced nest flies. Ecol. Evol. 3, 2514–2523.

6.

O’Connor, J. A., Robertson, J., and Kleindorfer, S. (2013). Darwin’s finch begging

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intensity does not honestly signal need in parasitised nests. Ethology 119, 1–10.
7.

O’Connor, J. A., Sulloway, F. J., Robertson, J., and Kleindorfer, S. (2010). Philornis
downsi parasitism is the primary cause of nestling mortality in the critically endangered
Darwin’s medium tree finch (Camarhynchus pauper). Biodivers. Conserv. 19, 853–866.

8.

Ostfeld, R. S., Price, A., Hornbostel, V. L., Benjamin, M. A., and Keesing, F. (2006).
Controlling ticks and tick-borne zoonoses with biological and chemical agents. Bioscience56, 383–394.

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9.

Stapp, P., Antolin, M. F., and Ball, M. (2004). Patterns of extinction in prairie dog
metapopulations: plague outbreak follow El Niño events. Front. Ecol. Envionment 2, 235–

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240.
10.

Freed, L. A., Cann, R. L., and Bodner, G. R. (2008). Incipient extinction of a major
population of the Hawaii akepa owing to introduced species. Evol. Ecol. Res. 10, 931–
965.

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Figure Legend

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Figure 1. Incorporation of permethrin-treated cotton into nests by Darwin's finches.
(A) Female medium ground finch (Geospiza fortis) removing fibers from a cotton laundry line at
the Charles Darwin Research Station, Galápagos. (B) Cotton dispenser at the field site; cotton
has been removed from the lower half by finches. (C) Small ground finch (G. fuliginosa)
removing cotton from a dispenser in a preliminary experiment. (D) Finch nest containing about
one gram of cotton. (E) Parasite abundance was negatively correlated with the mass ofexperimental cotton (Spearman rank correlation: rs = -0.62, P = 0.03), but not with the mass ofcontrol cotton (rs = 0.22, P = 0.58). (F) Experimental nests treated with permethrin fledged
more offspring than control nests treated with water (Fisher’s Exact test: P = 0.001). Orange
bars are the total number of nestlings monitored; green bars are the total number of nestlings thatfledged.

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Figure 1

B
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Supplemental Information
Supplemental Figure

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Figure S1. A partial representation of the field site with cotton dispensers. Light gray area
corresponds to the area searched for nests. The experiment had a total of 30 dispensers, with 15
along each side of the road in the pattern shown here.

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Supplemental Experimental Procedures
Study system and field site

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Our field site, El Garrapatero, is a 5km x 1.5km area in the southeastern arid coastal zoneof Santa Cruz Island, Galápagos. Several species of Darwin’s finches are abundant at this site[S1], including the medium ground finch (Geospiza fortis), small ground finch (G. fuliginosa),
small tree finch (Camarhynchus parvulus), and vegetarian finch (Platyspiza crassirostris). Finchnests are dome-shaped and constructed mainly of plant fibers. Finches build their nests one to
five meters above the ground at this site in endemic tree cacti (Opuntia echios gigantea) or
Acacia trees. All procedures in our study were approved by the University of Utah IACUC(protocol #10-07003).

Cotton dispensers
Dispensers were made from 19-gauge hardware cloth, which held cotton in place (Figure
1B). A piece of hardware cloth was folded in half with each side bound together by cotton string
along the edges. Two wooden perches were placed approximately 4cm from the bottom of thedispenser. A black plastic roof was attached to the top of each dispenser to slow the degradation

UU IR Author Manuscriptof permethrin from exposure to sunlight and rain. Processed and unprocessed cotton were used
to distinguish between the experimental and control treatments. Both types of cotton wereobtained from U.S. Cotton™. The only difference between the cotton types is that processed
cotton is combed to align the fibers.

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Discrimination test
Prior to our main study, we tested whether finches discriminate against cotton type and/or

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permethrin. We placed four dispensers at 100m intervals at the Charles Darwin Research Station.
Each dispenser was loaded with 3g of each type of cotton and treatment: permethrin-treatedprocessed and unprocessed cotton and water-treated processed and unprocessed cotton (Figure1C). After 14 days, the cotton was weighed to the nearest 0.001g to determine how much ofeach type was removed from the dispensers. Over the course of two weeks, there was no
significant difference in the type of cotton birds removed from the dispensers: finches removed amean (± SE) of 0.83 ± 0.46g processed permethrin cotton, 1.10 ± 0.64g unprocessed permethrin
cotton, 0.90 ± 0.70g processed water cotton, and 0.95 ± 0.60g unprocessed water cotton (Kruskal
Wallis, H = 1.027, P = 0.80).

Distance traveled for cotton
We also tested how far finches will transport cotton to their nests. We placed a dispenser
with cotton in the field at a location away from our main study site. About four weeks later, wecollected nests within 200 meters of the dispenser after birds were finished using the nests

UU IR Author Manuscript(Darwin's finches do not re-use the same nests [S2]). We dissected each nest to determine
whether it contained any cotton. Two nests closest to the dispenser (7 and 17 meters) had cotton,
but ten more distant nests (all >25 meters away) contained no cotton. Thus, we concluded thatDarwin’s finches at this site will transport cotton up to about 20 meters.

	
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Self-fumigation experiment
Based on the preference test, 30 cotton dispensers were hung from trees 40 meters apart

UU IR Author Manuscript(approximately 2 meters above the ground) along two transects through our field site (FigureS1). Experimental dispensers contained processed cotton treated with a 1% permethrin solution;
control dispersers contained unprocessed cotton treated with water. Thirty-five grams ofexperimental or control cotton were placed evenly over the bottom three-quarters of eachdispenser. The cotton was re-sprayed with permethrin or water every 8-10 days.
We searched for active Darwin’s finch nests once a week for approximately 2 months
after the dispensers were placed in the field. Once a nest was found, breeding activity (eggs ornestlings) was confirmed by checking the nest with a fiber optic camera (31mm in diameter, 36
mm in length; Sony®, Tokyo, Japan) attached to a 4m collapsible pole. During this visit, we
also identified the species of Darwin’s finch associated with each nest by briefly observing nest
activity with binoculars from at least 5m away. Six of the experimental nests were built by G.
fortis, five by G. fuliginosa, one by C. parvulus, and one by an unidentified finch species. Twoof the control nests were built by G. fortis, one by G. fuliginosa, two by C. parvulus, one by P.
crassirostris, and three by unidentified finch species.

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Once nestlings had died or fledged, each nest was collected and sealed in a plastic bag.
The nest was dissected within eight hours and any P. downsi larvae, pupae, and eclosed pupal
cases were counted. First instar larvae can burrow subcutaneously into nestlings, making them
impossible to quantify reliably [S3]. Therefore, as in previous studies [S3], total parasite
abundance was the sum of all second and third instar larvae, pupae, and eclosed pupal cases in
the nest material. Larvae and pupae removed from nests were reared to confirm theiridentification as P. downsi [S4].

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All cotton was removed from nests and weighed to the nearest 0.001g. Non-cotton nest
material was also weighed to the nearest 0.001g. The amount of cotton used in nests did not

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differ significantly by treatment. Thirteen experimental nests had a mean (± SE) of 2.12 ± 0.62g
cotton; nine control nests had a mean of 1.04 ± 0.47g cotton (Mann-Whitney test: U = 40.00, P =0.23). The percent of nest material comprised of cotton did not differ significantly by treatment(experimental nests were 5.61 ± 1.87% cotton; control nests were 2.50 ± 1.23% cotton; MannWhitney test: U = 39.00, P = 0.20). Four nests did not contain any cotton; these nests had amean of 48.25 ± 16.68 parasites.

Effect of fumigation on fledging
Active nests were visited every other day between 0600 and 1100h to record the numberof eggs and nestlings present. Nests were randomly assigned to the experimental or control
group. Experimental nests were sprayed with a 1% permethrin solution; control nests were
sprayed with water. Nests were treated when the first nestling hatched, and again 4 days later.
Nestlings, eggs and a thin layer of material from the bottom of the nest were removed before thenests were treated. Parents were quick to return to their nests following treatment, and there

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were no cases of nest abandonment due to treatment. Nestlings were marked shortly after
hatching by coloring one toenail with a permanent marker. At ~8 days of age, nestlings werebanded with a numbered monel metal band and three color bands. Banded nestlings were thenre-sighted within seven days of leaving the nest to confirm fledging success. After the nest
failed or all nestlings had fledged, the nest was collected and sealed in a plastic bag to quantify
P. downsi, as described above. Three nests in the control treatment were overrun by fire ants
(Solenopsis geminata) and therefore excluded from the analyses.

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Supplemental References
S1.

Foster, D. J., Podos, J., and Hendry, A. (2008). A geometric morphometric appraisal ofUU IR Author Manuscript

beak shape in Darwin’s finches. J. Evol. Biol. 21, 263–75.
S2.

Grant, P. (1999). Ecology and Evolution of Darwin’s Finches (Princeton, New Jersey:
Princeton University Press).

S3.

Koop, J. A. H., Huber, S. K., Laverty, S. M., and Clayton, D. H. (2011). Experimental
demonstration of the fitness consequences of an introduced parasite of Darwin’s finches.
PLoS One 6, e19706.

S4.

Dodge, H. R., and Aitken, T. H. G. (1968). Philornis Flies from Trinidad (Diptera:
Muscidae). J. Kansas Entomol. Soc. 41, 134–154.

	
  
	
  

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