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First published online July 14, 2008
Journal of Experimental Biology 211, 2519-2523 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017327
Experimental reduction of ultraviolet wavelengths reflected from parasitic eggs affects rejection behaviour in the blackcap Sylvia atricapilla
iková1,2
1 Institute of Vertebrate Biology, v. v. i., Academy of Sciences of the Czech
Republic
2 Institute of Botany and Zoology, Faculty of Science, Masaryk University, Brno,
Czech Republic
* Author for correspondence (e-mail: honza{at}brno.cas.cz)
Accepted 19 May 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: brood parasitism, cuckoo, blackcap, UV spectrum part, rejection behaviour, parasitic egg
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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), and consists of laying eggs in nests of unrelated species,
the hosts, which incubate the eggs and feed the parasitic young
(Lack, 1968). It has been
documented that a successful parasitic act has a generally detrimental effect
on the host's fitness because of dramatic reduction of their reproductive
fitness (Davies, 2000). Many
studies have shown that interaction between avian brood parasites and their
hosts represent one of the best examples of coevolution (for a review, see
Rothstein, 1990), which has
led to a variety of adaptations and counter-adaptations on both sides, and has
been described as an evolutionary arms race
(Dawkins and Krebs, 1979;
Davies and Brooke, 1989). For
example, in the case of a host-parasite system in which the female parasite
specialises on a single host species, parasitic egg mimicry is one of the
outcomes of this struggle (Brooke and
Davies, 1988). However, this counter-adaptation of the brood
parasite must be very stressful for hosts, since egg discrimination has
primarily evolved as a consequence of other selection pressures associated
with nesting in dense colonies (Underwood
and Sealy, 2002).
A significant effort has been devoted to studying naturally parasitized
host populations to explore a role of parasitic egg mimicry
(Moksnes et al., 1993;
Edvardsen et al., 2001;
Moskát and Honza,
2002). Similarly, many conclusions have been drawn by an
experimental approach testing host responses towards both conspecific eggs
(Procházka and Honza,
2003; Procházka and
Honza, 2004; Lovaszi and
Moskát, 2004) and an artificial model egg
(Moskát and Fuisz,
1999; Honza et al.,
2004). Most of these studies have tried to explain why so many
different host populations accept non-mimetic eggs. This scenario could be
primarily influenced by the fact that humans and birds significantly differ in
their colour perception. Indeed, Honza et al.
(Honza et al., 2007) in a
study of song thrushes Turdus philomelos revealed how questionable
the use of egg categories such as mimetic versus non-mimetic might
be, since some colours of the parasitic eggs classified by humans as
non-mimetic were accepted by the hosts. This discrepancy could be explained by
the anatomical differences between bird and human eyes
(Chen and Goldsmith, 1986;
Harth et al., 1998), because
birds are able to detect UV wavelengths in the range of 300–400 nm,
invisible to humans (Huth and Burkhardt,
1972; Wright,
1972). Since the UV visual sensitivity plays an important role in
bird recognition ability (Wright,
1972; Cherry and Bennett,
2001), interest in the role of UV visual signals has been
increasing. Moreover, there is evidence about other functional significances
of the UV wavelengths in mate choice, foraging and navigation
(Bennett and Cuthill,
1994).
The evolution of the cues influencing rejection behaviour of hosts has
attracted enormous attention from many evolutionary biologists
(Moskát and Fuisz,
1999; Underwood and Sealy,
2006), however, most of these studies did not take into
consideration the significance of the UV wavelengths. The role of the UV
wavelengths in avian brood parasite–host systems was first investigated
by Cherry and Bennett (Cherry and Bennett,
2001), who identified the importance of this part of spectrum in
the evolution of egg mimicry in the red-chested cuckoo, Cuculus
solitarius. Moreover, there are other studies demonstrating that the
combination of the UV and the part of the spectrum visible to humans plays an
important role in avian brood parasites–host coevolutionary system
(Avilés and Møller,
2004; Avilés et al.,
2006a;
Polaiková et al.,
2007; Underwood and Sealy,
2008).
In our study, we investigated rejection behaviour in blackcaps, Sylvia
atricapilla Linnaeus 1758. Although, this species is rarely parasitized
at present, there is evidence that blackcaps have been used as hosts by the
common cuckoo Cuculus canorus in the past
(Honza et al., 2001). Since
the rejection behaviour of this species still persists, the blackcaps are
considered to be current winners in the coevolutionary struggle with the
cuckoo (Honza et al.,
2004).
The blackcap host is a passerine species, with the ability to perceive
short wavelengths including UV (Cuthill et
al., 2000). Moreover, our previous study on this species
(Polaiková et al.,
2007) revealed that the probability of egg rejection significantly
increased with decreasing brightness of host eggs at the blunt pole. In the
light of this finding, we investigated an influence of reduction of UV
wavelengths reflected from parasitic eggs on recognition behaviour in
blackcaps. We predicted that experimental manipulation of the UV wavelengths
reflected from parasitic eggshell should influence recognition behaviour of
this species. We expected that the model eggs with a reduced part of the UV
spectrum would be rejected from experimental nests at a different rate than
those with unmanipulated UV wavelengths. In our analyses, we investigated the
eggshells of both whole parasitic eggs and their blunt poles.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). All
eggs were marked with small numbers on the middle part of the eggshell using
waterproof ink to allow their individual identification. Experimental
parasitism was carried out only when at least the fourth host egg was laid.
Blackcap clutches were randomly parasitized by replacing an egg with one
randomly chosen egg from an original clutch. Even though the modal clutch size
in the blackcaps nest varies from four to five eggs, our previous study
(Polaiková et al.,
2007) on this host species documented that adding or switching
host eggs in experimental nests has no effect on host response. The
experimental nests were monitored daily for 5 days after manipulation. If the
parasitic egg disappeared within this period but the host eggs were left
unharmed in the nest, it was considered to have been rejected. Nest desertion
was classified as a mode of rejection. If the parasitic egg remained unharmed
in the nest, it was considered to have been accepted. Each experimental nest
was parasitized only once and each experimental egg was used only in one
experimental trial. Prior to each experimental procedure, we measured the
reflectance spectra of both experimental and host eggs in each clutch. Two
types of model eggs were used: (1) conspecific eggs coated in a UV blocker
(UV–), and (2) conspecific eggs coated in VaselineTM
petroleum jelly (UV0). The UV blocker consisted of mixture of an
octylmethoxysinnamate and benzophynone-3 which reduces a significant part of
the UV wavelengths. The spectral reflectance curves of the experimental eggs
for both treatments are shown in Fig.
1. The effect of blocker persisted for at least 48 h after coating
the experimental eggs, and it had no effect on curvature of the visible part
of the spectrum. All experimental eggs were recoated in the test substances
daily during examination of the nests. The third experimental group,
consisting of conspecific eggs without any coating, was used as a control.
This last group was not included in our analyses.
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Measurement of the reflectance spectra
We measured the spectral reflectance of all eggs from the parasitized
clutches in the range of 300–700 nm using a USB 2000 spectrophotometer
(Ocean Optics, Dunedin, FL, USA). We divided each egg into three regions
across its longitudinal axis: the blunt end, the middle and the sharp end.
Each region was a third of the length of the egg. Three randomly located
measurements on each part of the egg were taken (each covering 1
mm2). A deuterium and halogen light source was used (DT-Mini-GS,
Ocean Optics) that was shone on the eggs through a quartz optic fibre
(QR400-7-UV/VIS-BX, Ocean Optics), and was reflected from the eggs at an angle
of 45° to the surface. Data from the spectrophotometer were loaded into
OOIBase 32 software (Ocean Optics). The measurements were relative and
referred to a standard white reference (WS-2) and to darkness. Reference and
calibration were made before each measurement of each clutch. Total
reflectance was obtained for the UV (325–400 nm) interval, which is the
invisible part of colour spectrum for humans, and a visible part of a spectrum
(400–700nm). We calculated objective variables of the colour, namely
brightness (reflectance intensity), chroma (colour purity) and hue [peak
wavelengths; (Endler, 1990)].
Brightness was calculated as the sum of the total reflectance values within
the 325–700 nm range of wavelengths
(R325–700). Chroma for each part of the spectrum was
calculated using the following reflectance ratios:
R325–400/R325–700 (UV chroma),
R400–475/R325–700 (blue
chroma), R475–550/R325–700
(green chroma),
R550–625/R325–700 (yellow
chroma) and R625–700/R325–700 (red
chroma). Hue was estimated by the wavelength of the maximum reflectance
(Rmax).
Since our previous study
(Polaiková et al.,
2007) revealed that mainly characteristics of the blunt egg pole
are important for recognition behaviour in blackcaps, we calculated all colour
variables reflected from both the whole egg surface and the blunt egg pole
separately.
Statistical analyses
The effect of both colour variables characterizing the parasitic egg
appearance and contrast between host and parasitic eggs (both calculated for
the whole egg surface and blunt egg pole separately, before and after
experimental manipulation) on host's recognition ability was examined by a
binomial logistic regression with backward stepwise elimination, which
included experimental treatments (UV– and UV0) as
categorical variables and host response as a dependent variable (1=rejection,
2=acceptance). We started analysis by including all variables in the model,
dropping non-significant steps until the last step (step 28), which was
considered to be the most significant model.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Responses towards parasitic eggs
Fig. 2 shows rejection rate
of the experimental eggs in all three experimental groups. There were no
differences in rejection rate between UV0 and the control
unmanipulated group (
2-test=0.78, P=0.37) showing no
effect of the grease surface on the bird's response. However, significantly
more of the experimental eggs coated in the UV blocker (UV–)
were rejected than either the model eggs coated in the Vaseline
(UV0: 2=4.04, P=0.02). Similarly, a
logistic regression revealed that the only two variables has a significant
effect on the host response (rejection=1; model significance
2=15.7, P<0.001), the brightness of whole
conspecific eggs (Wald 2=7.06, d.f.=1, P=0.008) and
the UV brightness at blunt egg poles of experimental eggs (Wald
2=6.13, d.f.=1, P=0.013). The likelihood of alien egg
rejection increased with increasing values of the brightness of the whole
surface of experimental eggs [B=0.00±0.00 (± s.e.m.)],
but with decreasing UV brightness at the blunt poles of experimental eggs
[B=–0.01±0.0 (± s.e.m.)].
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). However,
other particular cues facilitating recognition of a foreign egg play an
important role in the host recognition processes. For example, Rothstein
(Rothstein, 1982b) in a study
of the American robin (Turdus migratorius) found that smaller
parasitic eggs of brown-headed cowbirds, Molothrus ater, elicited a
quicker host rejections, probably because of a combination of visual and
tactile stimuli, since undersized parasitic eggs are possibly easier to
reject. However, Underwood and Sealy
(Underwood and Sealy, 2006)
argued that warbling vireo (Vireo gilvus) hosts usually reject
brown-headed cowbird's eggs only based on eggshell spot pattern, which
indicates that heavy spottiness of the parasitic eggshells is primarily
responsible for egg recognition and this host species does not need a stimulus
summation (Underwood and Sealy,
2006). In the cuckoo-host system, there are only a few studies
documenting which parameters of the eggshell appearance affect the recognition
of the parasitic eggs (Stokke et al.,
1999; Procházka and
Honza, 2003; Procházka
and Honza, 2004). Moreover, it is not just the colour
characteristics of the whole eggshell that are important in host recognition
behaviour, but also particular eggshell parts
(Polaiková et al.,
2007).
Since Cherry and Bennett (Cherry and
Bennett, 2001) revealed the importance of the part of the spectrum
that is invisible to humans in host recognition of parasitic eggs, many
researchers have been investigating the significance of this trait
(Soler et al., 2003;
Avilés and Møller,
2004; Cherry et al.,
2007) (see also Moskat et al.,
2008). However, to our knowledge, there is only one study,
conducted by Avilés et al.
(Avilés et al., 2006a),
which experimentally manipulated UV reflectance to reveal the importance of
this part of the spectrum in host recognition. This study investigated
recognition behaviour of magpies (Pica pica) towards great spotted
cuckoo (Clamator glandarius) eggs, however, their research did not
reveal an effect of artificial reduction of the UV reflectance on host
perception. In contrast to this result, our study revealed a positive effect
of lowering UV reflectance at the blunt pole of experimental eggs on egg
rejection in blackcaps. To our knowledge, this is the first experimental
evidence showing that a reduction of UV wavelengths reflected from the
parasitic eggshells may significantly influence host recognition
behaviour.
There are several possible explanations of why our results differ from
those of the Avilés et al.
(Avilés et al., 2006a)
study. First, corvids and small passerines may vary in their UV signal
perception (Avilés et al.,
2006a). Second, the logistic regression in our study revealed that
it is not just UV perception that is important in recognition behaviour of
blackcaps but also other parameters including host eggs characteristics.
Another possibility for the differences could be the chemicals used. Our
UV-blocking chemical covered a larger range of UV wavelengths than that used
by Avilés et al. (Avilés et
al., 2006a) and the experimental eggs coated in this UV block were
darker (for comparison see Fig.
1 in both studies). Moreover, Avilés et al.
(Avilés et al., 2006b)
discovered that nest light properties may influence host discrimination
towards parasitic eggs as the UV wavelengths reflected from the egg surface
could be chosen to provide detectable cues for hosts in poorly lit
environments. However, this does not apply in our case, as blackcaps usually
breed in open nests. Finally, another reason for the different results could
be different recognition abilities of the two tested hosts (magpies and
blackcaps). Since blackcaps exhibit good recognition abilities resulting in a
high rejection rate (Honza et al.,
2004; Polaiková
et al., 2007), their egg recognition should favour a greater
variety of different shell cues in comparison with species that exhibit lower
recognition abilities. Variation in egg recognition by different hosts has
been explained by different duration of co-evolution between the brood
parasite and its host (Brooke and Davies,
1988; Soler and Møller,
1990). Although magpies live in sympatry with cuckoo, this species
belongs to a naive host group with lower recognition abilities, that are not
as sophisticated as those of blackcaps.
Our results show that not only UV wavelengths but also total brightness of
experimental eggs within the 325–700 nm wavelength range significantly
influenced host rejection behaviour. Our previous results on blackcaps showed
that birds reject every egg that is even a little bit lighter than the host
eggs and accept all eggs that are as dark as or much darker than their own
host clutch. This may be connected with the spot concentration of the
blackcap's eggshell (Makatsch,
1976), which might theoretically serve as a fingerprint of
individual females (Kilner,
2006). Alternatively, if synthesis and allocation of pigments into
the eggshell is costly (Moreno and Osorno,
2003), the darkness of the blunt end of the egg may be linked to
the quality and age of the bird (Moreno et
al., 2006; Siefferman et al.,
2006), which can be further reflected in the host rejection
response (Lotem et al.,
1995).
Our findings supported the significance of ultraviolet reflectance in the egg recognition processes of hosts. However, further studies are needed to disentangle the role of functional mechanism of the UV wavelengths in egg recognition for better understanding of this co-evolutionary brood-parasitism system.
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Acknowledgments |
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  K. Phillips BLACKCAPS EVICT IMPOSTORS WITH WRONG UV APPEARANCE J. Exp. Biol., August 1, 2008; 211(15): i - ii. [Full Text] [PDF]
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