Consistency in egg rejection behaviour: responses to repeated brood parasitism in the
blackcap (Sylvia atricapilla)
Konzistence v odmítání vajec: reakce na opakovaný hnízdní parazitismus u pěnice černohlavé
(Sylvia atricapilla)
Marcel Honza, Milica Požgayová, Petr Procházka & Emil Tkadlec
To evaluate host responses towards repeated brood parasitism we experimentally parasitized and
continuously videotaped blackcap nests in two consecutive trials. The ejection of a foreign egg was the
most common response (94.5%) in both trials. The general method of ejection was puncturing. In 9.8%
of identified ejections, already punctured eggs stuck to the abdominal feathers of the incubating bird and
were carried out of the nest. Females were responsible for the majority of ejections in both trials and their
response time was significantly shorter than that of males. Blackcaps exhibited consistency in the sex
responsible for egg ejection over the two trials; but in five (20.8%) experiments individuals changed their
behaviour. Repeatability for host responses within the nest was very high. In ejections accomplished by
the same bird, the response was significantly quicker in the second trial, indicating the presence of certain
learning abilities. Our results suggest that cuckoo hosts are quite consistent in their responses towards
parasitic eggs when parasitized repeatedly within one breeding attempt.
K vyhodnocení reakcí hostitelů na opakovaný hnízdní parazitismus jsme experimentálně parazitovali a
filmovali hnízda pěnic černohlavých ve dvou následujících experimentech. Nejčastější reakcí bylo vyhození
cizího vejce (94,5 %). Hlavní metodou vyhození bylo naklování a vyhození. V 9,8 % identifikovaných
vyhození se ale naklovaná vejce přilepila na břišní pera inkubujícího ptáka a byla vynesena z hnízda.
Samice byly zodpovědné za většinu vyhození a jejich doba vyhození byla významně kratší než u samců.
V rámci páru byli při vyhazovaní ptáci mezi oběma pokusy konzistentní, pouze v 5 případech (20,8 %) se
změnil pták, který vejce vyhodilo. Opakovatelnost hostitelských reakcí v rámci jednoho hnízda byla velmi
vysoká. U vyhození provedených stejných ptákem byla reakce v druhém pokusu mnohem rychlejší, což
naznačuje jisté schopnosti učení. Naše výsledky naznačují, že hostitelé kukačky jsou poměrně konzistentní
ve svých odpovědích na opakovaný hnízdní parazitismus.
Keywords: brood parasitism, cuckoo, blackcap,
Granty:
GAAV (A600930605), GAČR (524/05/H536) and Institutional Research Plan (AV0Z60930519).
Ethology
Consistency in Egg Rejection Behaviour: Responses to Repeated
Brood Parasitism in the Blackcap (Sylvia atricapilla)
Marcel Honza*, Milica Požgayová* , Petr Procházka* & Emil Tkadlec*à
* Institute of Vertebrate Biology, Academy of Sciences of the Czech Republic, Brno, Czech Republic
Institute of Botany and Zoology, Masaryk University, Brno, Czech Republic
à Department of Ecology & Environmental Sciences, Palacký University, Olomouc, Czech Republic
Correspondence
Marcel Honza, Institute of Vertebrate Biology,
Academy of Sciences of the Czech Republic,
Květná 8, CZ-603 65 Brno, Czech Republic.
E-mail: [email protected]
Received: June 12, 2006
Initial acceptance: July 13, 2006
Final acceptance: October 22, 2006
(K. Riebel)
doi: 10.1111/j.1439-0310.2007.01340.x
Abstract
To evaluate host responses towards repeated brood parasitism we experimentally parasitized and continuously videotaped blackcap nests in two
consecutive trials. The ejection of a foreign egg was the most common
response (94.5%) in both trials, but desertion (4.1%) and acceptance
(1.4%) also occurred. The general method of ejection was puncturing.
In 9.8% of identified ejections, already punctured eggs stuck to the
abdominal feathers of the incubating bird and were carried out of the
nest. Females were responsible for the majority of ejections in both trials
and their response time was significantly shorter than that of males.
Blackcaps exhibited consistency in the sex responsible for egg ejection
over the two trials; but in five (20.8%) experiments individuals changed
their behaviour. Repeatability for host responses within the nest was
very high. In ejections accomplished by the same bird, the response was
significantly quicker in the second trial, indicating the presence of certain learning abilities. Our results suggest that cuckoo hosts are quite
consistent in their responses towards parasitic eggs when parasitized
repeatedly within one breeding attempt.
Introduction
Brood parasites can dramatically reduce host fitness
(Payne 1977b; Øien et al. 1998b). Such strong
selective pressure will favour the evolution of host
defence mechanisms such as the recognition of the
parasite’s egg and its rejection. These, in turn, will
select for counteradaptations in the parasite in terms
of evolution of egg mimicry, setting in motion a coevolutionary arms race between the host and the
brood parasite (Davies & Brooke 1988; Rothstein
1990).
Most studies on host defence mechanisms to date
particularly aimed to establish basic types of host
responses towards foreign eggs (Davies 2000), but
few of them focused on behaviour of hosts immediately after they were parasitized (Moksnes et al.
1994; Sealy & Neudorf 1995; Soler et al. 2002).
However, a number of unresolved questions of host–
344
parasite interactions ask for detailed observations of
the hosts’ behaviour after being parasitized. For
example, the issue of which sex is responsible for
egg recognition is crucial for models explaining the
arms race between brood parasites and their hosts
(Kelly 1987; Takasu 1998). Several authors have
assumed that only host females eject parasitic eggs,
because in most host species only the females incubate the clutch (Rothstein 1975b; Davies & Brooke
1988; Lotem et al. 1992). If both sexes are responsible for egg rejection, the rejection alleles will
spread faster in the host population than when only
one sex discriminates against alien eggs (Rothstein
1975b; Sealy & Neudorf 1995).
Another crucial factor in the evolution of egg
rejection behaviour concerns rejection costs (Rothstein & Robinson 1998). Detailed observations on
egg ejection can reveal whether the costs arise from
occasional accidental ejection of host’s own eggs in
Ethology 113 (2007) 344–351 ª 2007 The Authors
Journal compilation ª 2007 Blackwell Verlag, Berlin
M. Honza et al.
the absence of parasitism (i.e. recognition errors –
Davies & Brooke 1988) or from accidentally damaging or removing their own eggs when trying to
remove the parasite’s egg (Rothstein 1976; Rohwer
et al. 1989; Moksnes et al. 1991).
Multiple brood parasitism, i.e. when at least two
foreign eggs are present in one host nest at a given
moment (Honza & Moskát 2005), is relatively common in many host-parasite systems (Friedmann
1963; Payne 1977a; Martı́nez et al. 1998). In common cuckoo hosts, this phenomenon is relatively
rare, although it can occur in local populations with
a high rate of parasitism (Molnár 1944; Moskát &
Honza 2002). Repeated brood parasitism is a situation when two or more parasitic eggs appear in one
host nest subsequently (this study). This situation
allows studying repeatability of individuals’ behaviour as it provides a measure of consistency of an
individual’s response to a particular stimulus (Boake
1989; Roff 1997). Nonetheless, this approach has
often been discouraged because of concerns over
pseudoreplication (Boake 1989), but to estimate how
consistent behaviour is, repeated observations on the
same individual are needed.
Here we address the issue of blackcap behaviour
after being parasitized using an experimental
approach. Simulating repeated brood parasitism
within the same host pair during one breeding
attempt, we tested consistency in egg rejection behaviour. We hypothesized (1) the sex which spends
more time incubating to be more likely to reject the
parasitic egg and (2) that the time necessary for egg
learning along with the previous rejection experience to be possible factors influencing the timing of
host reaction.
Methods
The study was carried out in a forest near Dolnı́ Bojanovice, Czech Republic (4851¢N 1702¢E) in 2003–
2004. Blackcaps occur in sympatry with the cuckoo
here, but no cases of parasitism were recorded. The
blackcap is a rare cuckoo host (Moksnes & Røskaft
1995), which rejects alien eggs at a high rate (Honza
et al. 2004). Sexually dichromatic plumage makes
the species suitable for a study of sex roles in egg
ejection.
An industrial DCR-1000 camera placed on a tripod
5–10 m from the nest was connected with a videocamcorder SONY GV-D800E via a cable c. 50 m from
the nest. The equipment allowed us to monitor the
birds’ behaviour continuously on the videocamcorder
monitor, as well as to exchange videocassettes withEthology 113 (2007) 344–351 ª 2007 The Authors
Journal compilation ª 2007 Blackwell Verlag, Berlin
Consistency in Egg-Rejection Behaviour
out disturbing the birds. The camera was concealed
by a cover and the nests were videorecorded continuously during daylight usually between 0500–
2100 h Central European summer time.
The experiment consisted of two consecutive trials.
After we allowed the birds to habituate to the set-up
over a period of 90-min videotaping, we parasitized
the nest by adding the first experimental egg (first
trial). The nests were parasitized during the egg-laying stage after the fourth egg had been laid or soon
after the clutch completion (incubation usually commences with the penultimate egg and typical clutch
size is five). The nest content was observed on the
camcorder screen and when the parasitic egg was
ejected from the nest, we stopped recording and
checked the host clutch for possible damage of hosts’
own eggs. The day after the ejection we parasitized
the same nest by the same egg type again and videotaped the nest until ejection (second trial). Each nest
was used for one experiment only and we did not
test two nests within the same territory within the
same year either. However, nest owners were not
ringed, thus we cannot exclude that some individuals were tested in two subsequent years. There were
no differences (t-test ¼ 0.21, p ¼ 0.65) between the
study years in the date of laying of the first egg
(2003: mean SD ¼ 36.35 8.23, n ¼ 34; 2004:
33.88 8.12, n ¼ 36; 1 ¼ 1 April), the data were
thus pooled.
We used mainly unfertilized real eggs of Bourke’s
parrot, Neophema bourkii. The eggs of other species
(blackcap; yellowhammer, Emberiza citrinella; house
sparrow, Passer domesticus; and chaffinch Fringilla
coelebs) were collected from abandoned nests and
used only in 14 cases of 73 trials. Egg size had no
effect on duration of host pecking behaviour (rs ¼
0.197, p ¼ 0.224, n ¼ 59). We standardized the colour of the experimental eggs by painting them light
blue to resemble the eggs of the redstart, Phoenicurus phoenicurus, cuckoo gens found in the Czech
Republic.
For each sex we extracted from the videotapes the
following data: time spent on the nest, start of pecking
the egg, type of ejection, and the sex responsible for
ejection. Pecks were vigorous strokes by the bill,
which clearly differed from movements used for egg
turning. We distinguished two types of ejection:
(1) puncturing – the host made a hole into the eggshell and ejected the egg grasping it by its mandibles
in the punctured area; and (2) sticking – already
punctured egg was glued to host’s abdominal feathers
and most probably accidentally carried out from the
nest. These cases were omitted from subsequent
345
Consistency in Egg-Rejection Behaviour
calculations of consistency in egg rejection behaviour,
because the sticking occurred also in birds that only
incubated and did not peck the egg and the exact time
of ejection was accidental. When both parents pecked
the egg, we considered this as co-operation of the two
mates. We also scored costs of rejection, i.e. when the
host’s eggs were damaged or the clutch was deserted.
Not all variables could always be scored from the
videotapes, resulting in sample sizes smaller than the
total number of trials for some of the variables.
One variable characterizing the behavioural
response of parents towards a parasitic egg is the time
from inclusion until ejection. The distribution of this
variable was quite strongly skewed to the right, we
thus log-transformed it to meet the assumptions of
linear models. Response times might differ between
the first and second trial due to learning processes.
If so, then the second response time could be more
reduced in nests with the same ejector than in nests
with the different-sex ejectors. In addition, the
response time may equally well be sex-dependent.
To explore all these possibilities, and considering the
fact that the data often came from the same bird violating the assumption of independence, we fitted a
linear mixed model incorporating 2 factors and its
interaction: (1) experiment with 2 levels (trial 1 and
trial 2) and (2) ejector sex with 3 levels (F – ejectors
in both trials were females, M – both ejectors were
males, and FM – ejectors were of different sex). In
addition, the model included a nest identifier as random effect. We used F-tests with the KenwardRoger-type denominator degrees of freedom to assess
the significance of fixed effects. Then we explored
whether either sex was equally likely to become an
ejector of the egg. Therefore, we fitted a generalized
linear mixed model to estimate a grand mean (intercept) for all data, i.e. with no fixed effects but including the nest identifier as a random effect. This
model assumed binary error distribution and used
the logit link function to relate the response variable
to the predictor. We used the 95% confidence interval of the intercept to assess whether the probability
of becoming a female ejector deviated from 0.5. This
model was also used to assess the consistency of binary responses between the two trials within the nest
by estimating repeatability as the intraclass correlation r ¼ VCb/(VCb + VCo + VCw) where VCb denotes
the between-nest variance component, VCo the component due to overdispersion and VCw the withinnest residual component estimated as p2/3 (e.g. see
Guo & Zhao 2000). As another measure of consistency between the sex of ejectors in trials 1 and 2 we
used McNemar’s test for paired observations. Then
346
M. Honza et al.
we examined whether the proportion of time spent
incubating was equally distributed between the sexes
using the same model structure and significance test,
now assuming binomial error distribution and nest
identifier as a random effect. Finally, we explored the
predictive effects of the proportional time allocated
by females to incubation on their probability of
becoming the ejector. Again, we used the same binary mixed model as that for the analysis of sex probabilities. All statistical tests were performed using
procedure GLIMMIX implemented in SAS (version
9.1.3; SAS Institute Inc. 2005).
Results
Responses towards Parasitic Eggs
We parasitized and videotaped 41 and 32 blackcap
pairs in the first and second trial, respectively. The
most frequent responses were ejections (69/73,
94.5%), whereas desertions occurred only in three
cases (4.1%) and acceptance was the rarest response
(1/73, 1.4%; for detailed data for each trial see
Table 1). Before ejection, blackcaps punctured all
parasitic eggs. In the majority of cases, the birds then
grasped the eggshell in the punctured area and carried the egg away from the nest (55/61 of recorded
cases, 90.2%). However, in 9.8% (6/61) nests the
parasitic egg was rejected by sticking. The proportion
of these two ejection techniques differed between
sexes (trials pooled; puncturing vs. sticking – males
10 : 4, females 45 : 2; Fisher’s exact test: p ¼ 0.021)
and no individual used the latter technique in both
trials. Blackcaps suffered costs in four ejections
(0.068 of own eggs damaged per rejection, n ¼ 73)
and three desertions (0.192 of own eggs deserted per
rejection, n ¼ 73). For 24 pairs we obtained information about both ejection events and these are
hereafter considered in the analyses.
Timing of Ejection
The overall response time in the experiment was
much shorter in females than males (3.4 vs. 10.4 h;
Table 1: Host responses towards experimental eggs
Ejection
Trial
Acceptance
Desertion
Male
Female
Sex not recorded
1
2
Total
1
0
1
2
1
3
8
6
14
23
24
47
7
1
8
Ethology 113 (2007) 344–351 ª 2007 The Authors
Journal compilation ª 2007 Blackwell Verlag, Berlin
Consistency in Egg-Rejection Behaviour
M. Honza et al.
F1,46 ¼ 4.30, p ¼ 0.044). Also, the response time in
the second trial was about 10 h shorter in nests
where the same individual ejected the eggs than in
nests where the other mate ejected in the second
trial (contrast F and M vs. FM, same-sex ¼ 4.05 h,
different-sex ¼ 14.4 h, F1,45 ¼ 4.79, p ¼ 0.034).
There was a remarkable shortening of the response
in trial 2 (trial 1 ¼ 9.3 and trial 2 ¼ 1.9 h, F1,23 ¼
34.4, p < 0.001), indicating the presence of learning
abilities. However, we found no evidence for these
abilities to be sex-specific (interaction experiment · ejector sex, F2,21 ¼ 1.50, p ¼ 0.25).
Sex Responsible for Ejection in Relation to
Incubation Effort
In both trials, females were significantly more likely
to eject eggs than males. The estimated probability
was 0.83 with 95% c.i. 0.63–0.93. Females also invested proportionally more time into incubation than
males, the estimated proportion being 0.84 (0.69–
0.92). The proportion of time the female spent incubating increased her probability of becoming the ejector; however, this effect just missed to be significant
(F1,46 ¼ 3.80, p ¼ 0.057; Fig. 1). The probabilities for
a female to become an ejector in trial 1 and 2 were
estimated to be 0.81 and 0.84, respectively, the difference being insignificant (F1,46 ¼ 0.08, p ¼ 0.77).
female ejected the egg in 17 nests, the same male in
2 nests. In five cases (20.8%) the sex responsible for
egg ejection changed in the second trial (McNemar’s
change test v2 ¼ 0, p ¼ 1.0). Repeatability for binary
responses (see Methods) within the nest was very
high attaining a value of 0.568.
Interestingly, there was only little co-operation
between the mates in egg rejection; in most cases
when one sex started pecking, the second rarely
helped with the egg ejection (Table 2). If we consider only situations where both birds were incubating
during both trials and thus had the opportunity to
respond, the same bird ejected the parasitic eggs in
66.7% (10/15) cases.
Discussion
Ejection Types and Rejection Costs
Videotaping revealed two types of ejection: (a) puncturing and then removing the egg by grasping the
shell (in the blackcap for the first time documented
by Moksnes et al. 1994 and observed in other
cuckoo hosts as well, e.g. Lotem et al. 1992, 1995;
Table 2: The relationship between sex role in egg pecking and in egg
ejection
1st experiment
2nd experiment
Sex pecked
Male
rejected
Female
rejected
Male
rejected
Female
rejected
Male
Female
Both
4
0
0
1
16
2
2
0
1
1
18
2
Consistency in Egg Ejection Behaviour
Fig. 1: Probability for female of becoming an
ejector in relation to her proportion of incubation time. Logistic model with parameters:
Probability (F) ¼ exp ()3.75 + 7.25)/[1 + exp
()3.75 + 7.25)]
Ethology 113 (2007) 344–351 ª 2007 The Authors
Journal compilation ª 2007 Blackwell Verlag, Berlin
probability for female of becoming ejector
In 19 of the 24 nests (79.2%), the same bird was
involved in both ejections, indicating that the
response over individuals was highly consistent
(McNemar’s change test v2 ¼ 0, p ¼ 1.0). The same
1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
proportion of incubation time spent by female
347
Consistency in Egg-Rejection Behaviour
Martı́n-Vivaldi et al. 2002; Soler et al. 2002) and (b)
sticking to abdominal feathers after the eggshell was
punctured. The occurrence of the latter might be
more common in the blackcap (10% in this study)
or other species as well, however, to our knowledge
there have been no quantitative data reported for
any hosts. However, this type of rejection behaviour
is most probably an accidental consequence of egg
puncturing.
Puncture ejection is supposed to be relatively
costly for many hosts (Rohwer et al. 1989; Martı́nVivaldi et al. 2002). The same blackcap population
parasitized by conspecific eggs incurred similar rejection costs (0.30 of own eggs lost per rejection, Stokke et al. 2002; compared to 0.26 of own eggs per
rejection in this study). However, true recognition
errors (i.e. own eggs ejected when host unparasitized) are more important in evolutionarily terms
(Rothstein 1990; Rothstein & Robinson 1998).
Stokke et al. (2002) recorded such errors in chaffinches (7.3%) but not in the blackcaps. While filming the nests before clutch manipulations, we
documented a possible case of recognition error – an
unparasitized host vigorously pecked its own eggs
and after egg insertion it ejected at least one own
egg along with the introduced one. Few recognition
errors (4.4%) were also found by Marchetti (1992,
2000) in the Hume’s leaf warbler Phylloscopus humei.
Yet, Røskaft et al. (2002) reported no such errors in
two Acrocephalus warblers. It is possible that recognition errors are underestimated and egg losses, usually explained by partial predation or jostling, in fact
represent recognition errors.
Even though the blackcap is not nowadays a frequently used cuckoo host (Moksnes & Røskaft 1995;
Honza et al. 2001, but see Berthold et al. 1995), in
accordance with previous findings (Moksnes et al.
1991; Moksnes & Røskaft 1992; Soler et al. 2002;
Honza et al. 2004) it rejected all but one of experimentally added eggs. This species is supposed to
have retained relic and relatively cost-free egg rejection behaviour (Honza et al. 2004).
Timing of Rejection
There is a high variability in timing of egg rejection
among various hosts. Northern orioles (Icterus galbula)
and warbling vireos (Vireo gilvus), for example, eject
alien eggs very quickly (Underwood 2003). On the
other hand, cedar waxwings (Bombycilla cedrorum)
often delay their responses towards parasitic eggs
(Rothstein 1976). However, there have been few
studies which were able to determine the exact rejec348
M. Honza et al.
tion time (Sealy & Neudorf 1995; Soler et al. 2002;
Underwood 2003). We found that females were
quicker than males in their responses against introduced eggs and that the response time was significantly shortened in the second trial. Moreover,
rejection in the second trial was faster if the ejector
was the same individual, suggesting that there might
be some learning component involved (Rothstein
1978; Lotem et al. 1992, 1995).
Sex Responsible for Recognition and Rejection of
Eggs
It has been commonly assumed that only host
females are responsible for egg ejection (Rothstein
1975b; Davies & Brooke 1988; Lotem et al. 1992;
Palomino et al. 1998). This is because in many species it is females that lay and incubate the clutch
and spend more time in the nest than males. Underwood (2003), however, recorded that male warbling
vireos ejected cowbird eggs in about 20% of nests.
Sealy & Neudorf (1995) predicted that species where
males incubate or are more involved in attending
the nest are more likely to have evolved egg recognition and ejection also in males. This agrees with
experimental results of Soler et al. (2002) who documented that in species where only females incubate,
only females recognize and eject alien eggs, whereas
in species where both sexes incubate, both sexes are
involved in ejections. Moreover, they found almost
the same sex ratio among rejecters in blackcaps. In
our study, females were responsible for the majority
of ejections (77.0%) suggesting that females might
also be more effective in recognizing and ejecting
foreign eggs, although both sexes incubate. The
results support the assumption that the role of sex in
egg ejection depends on the amount of incubation.
This should be considered especially in models of
coevolution between the host and its parasite as
ejections by males may have important theoretical
implications in spreading a rejecter trait (Rothstein
1975c; Kelly 1987; Sealy & Neudorf 1995). An alternative explanation could be that a male mated with
a different female each year, could have different
experiences with eggs than the female after her first
breeding attempt and thus be less likely to discriminate and eject the parasitic egg.
Sealy (1996) found that warbling vireo males were
responsible for two of three unsuccessful ejection
attempts, whereas females were successful in all
cases. Similarly, Sealy & Neudorf (1995) reported
that damage to the northern oriole eggs occurred
proportionately more often during male ejections,
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Consistency in Egg-Rejection Behaviour
M. Honza et al.
which suggests that males may be less experienced
egg ejectors. As males in our study population did
not exhibit rejection behaviour frequently, it may be
expected that their inexperience would affect the
efficacy of egg ejection. However, our results do not
support this assumption.
Consistency in Egg Ejection Behaviour
Variation among individuals in rejection responses
can be a consequence of genes and/or environmental cues (differential responses to different environmental stimuli). Antiparasitic adaptations among
host populations can have a genetic basis, depending on different selection pressures from brood parasites, costs and benefits of responses and the
amount of gene flow. However, unlike morphological traits, behaviour is supposed to be flexible.
Changes in host defences thus need not necessarily
reflect loss of or regaining an adaptation, but may
represent phenotypic plasticity related to the risk of
parasitism perceived by the hosts in the absence or
presence of brood parasitism (Brooke et al. 1998;
Lindholm 2000; Rothstein 2001). If differences in
rejection rates are considered to be a conditional
response to parasitism (Øien et al. 1998a), then it
is reasonable to assume that the timing of rejection
will also fluctuate according to present circumstances.
In our population, only 20.8% of individuals
changed their responses towards parasitic eggs during subsequent trials, while the majority showed the
same response across the two trials. This finding
might seem somewhat surprising, because previous
studies (Lotem et al. 1995; Soler et al. 2000) reported higher flexibility in host responses. Soler et al.
(2000) assumed that the change in behaviour resulted from the trade-off between costs and benefits
related to egg rejection. It must be stressed, however, that the authors cited above carried out their
experiments over two years, not within one breeding attempt, as we did.
The mates did not co-operate in egg rejection –
when one pair member started pecking, the other
did not engage in egg puncturing, although it had
previously incubated the eggs and had the opportunity to learn their appearance. This makes it unlikely
that the other bird did not reject because of no
opportunity to respond or was prevented from doing
so because its mate had done it earlier. As already
stated by Rothstein (1975a), nest cleaning behaviour
might be an important evolutionary stage towards
ejection of parasitic eggs. Breaking an egg in the nest
Ethology 113 (2007) 344–351 ª 2007 The Authors
Journal compilation ª 2007 Blackwell Verlag, Berlin
is dangerous to host’s own eggs as the spilt content
may cause attaching the eggs to each other or to the
nest lining. Moskát et al. (2003) reported that great
reed warblers rejected dummy cuckoo eggs less often
than non-egg shaped objects. Similarly, some hosts
of parasitic cowbirds ejected non egg-shaped objects
from their nests with significantly higher rate than
experimental eggs (Ortega & Cruz 1988; Underwood
& Sealy 2006). However in blackcaps, an egg already
punctured by one pair member does not seem to
represent a sufficient stimulus for rejection by the
other bird or, alternatively, the mate who pecks first
is more alert and quicker than the second one.
Whatever the answer is, we should not always classify individual birds as acceptors or rejecters (Grim
2005) in species where both pair members reject but
simply admit that one pair member may pre-empt
the other’s rejection response.
In conclusion, egg-rejection behaviour in blackcaps was quite consistent when hosts were parasitized repeatedly within one breeding attempt. In
ejections accomplished by the same bird, the
response was significantly quicker in the second
trial, indicating the presence of certain learning abilities.
Acknowledgements
Comments of K. Riebel, M. Reichard and two
anonymous referees greatly improved earlier versions of this manuscript. Experiments were conducted following the Academy of Sciences Animal Care
Protocol, licence number 0008/98-M103. The study
was supported by GAAV (A600930605), GAČR (524/
05/H536)
and
Institutional
Research
Plan
(AV0Z60930519).
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