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J. Orn. 134, 1993: S. 425--434
Cuckoo Cuculus canorus Parasitism on AcrocephMus Warblers
in Southern Moravia in The Czech Republic
Arne Moksnes, Eivin Roskaft, Vit~zslav Bi~ik, Marcel Honza and Ingar J. Oien
The Reed Warbler, Acrocephalus scirpaceus, in the Lednice area, Southern Moravia in
the Czech Republic, was parasitized by the Cuckoo, Cuculus canorus, at a rate of at
least 18.0 %. The Cuckoo eggs showed poor mimesis with the Reed Warbler eggs, but
showed a greater resemblance to the eggs of other species breeding in the area, including
the Great Reed Warbler, A. arundinaceus. This latter species was also parasitized, but
we did not find enough nests to obtain a reliable estimate for the rate of parasitism.
The parasitized Reed Warblers rejected the Cuckoo eggs at a high rate (42.1%) and
therefore both the hatching success and the breeding success of the Cuckoo was considerably lower than shown by comparable results from Britain. On the background
of these results (poor mimesis of the Cuckoo eggs and a high rejection rate by the
hosts) the question of the degree of specialization versus generalism in the Cuckoo's
host preference is discussed.
Introduction
During the course of coevolution between the Cuckoo, Cuculus canorus, and its
host species different Cuckoo strains, termed gentes (singular: gens), have evolved
(JouP.DAIN 1925, CHANCE 1940, BAKEr. 1942, LACK 1968, WYLLI~ 1981, BROOKE&
DAVIES 1991, DAVIES& BROOKE1991). According to this theory, the females of each
gens lay distinctive eggs that mimic the eggs of the particular host species on which
they specialize. Alternatively, each gens may parasitize a group of host species, all of
which have similar eggs and nest sites (WYLLIE1981, MOKSN~S& ROSr~FT 1987), or
they may search for nests at random (BRooKE & DAVIES1991). At least fifteen such
distinctive gentes are distinguished in Europe (WYLLIE1981).
The Cuckoos that parasitize the Reed Warbler, Acrocephalus scirpaceus, and the Great
Reed Warbler, A. arundinaceus, belong to two widely-distributed genres. According to
WYLLIE(1981), and judging from an analysis of the geographical distribution of about
2500 Reed Warbler clutches, each containing at least one Cuckoo egg, held in European
museum collections (MOKsNZS& ROSKAFTunpubl.), the Reed Warbler is parasitized
frequently in Western Europe and as far north as Southern Scandinavia, and also
regularly in Central Europe_ According to SCHULZE-HAGEN(1992), who has summarized 34 published studies on the parasitism rates for Reed Warblers, Cuckoo
parasitism seems to be greater in the northwestern than in the eastern and southern
parts of Central and Western Europe- The Cuckoo gens that parasitizes the Reed
426
A. MOKSNESU.a.
I J" 134Orn"
Warbler has evolved eggs which well mimic those of the hosts (BROOK~ & DaviEs
1988, DAviEs & BROOKE 1988, 1991).
Furthermore, the Great Reed Warbler is frequently parasitized in Eastern Europe,
especially in Hungary, Romania, the Ukraine and in the Vojvodina in Serbia
(MOLNAR 1944, MALCHEVSKY1960, WYLUE 1981, STEVANOVICet al. 1989). The eggs
of the Cuckoo gens that parasitizes the Great Reed Warbler frequently mimic the eggs
of the host very well and are sometimes almost impossible to distinguish from those
of the host (SouTHErN 1954, WYLLIE1981).
A certain proportion of individual birds of both the Reed Warbler and the Great
Reed Warbler reject the Cuckoo's egg when they are parasitized (DAvIEs & BROOKE
1988, BROWNet al. 1990, LOTEMet al. 1992). The rejection rate normally increases as
the degree of mimesis decreases (DAVIES & BROOI~ 1988, 1989, BROWNet al. 1990).
According to experimental evidence such discrimination by the host seems to have
been a strong selective force for the evolution of egg mimesis among Cuckoos
(BROO~ & DAVIES 1988, DAVIES& BROOM 1988, 1989, HIGUCHI 1989, MOKSNESet
al. 1991).
In this study we report data for the parasitism rates of both the Reed Warbler and
the Great Reed Warbler from Southern Moravia in the Czech Republic. For the Reed
Warbler we also present data on the rejection rates of genuine Cuckoo eggs in relation
to the degree of host egg mimesis. Finally, we present data for the breeding success of
the Cuckoo in that region. Against this background, we discuss the question of the
degree of specialization versus generalism in the host preferences of the Cuckoo.
Study area and methods
The study area is situated in the vicinity of the village of Lednice,48 o 47' N, 16 o 49' E, about
40 km south of Brno in Southern Moravia in the Czech Republic. The nests of the
Acrocephalus warblers were found in the littoral vegetation surrounding three fish ponds:
Ml~nsk~ pond (107 ha in area), Prost~edni pond (49 ha) and Hloboveck~ pond (104 ha). The
vegetation of all the ponds is dominated by reed beds, Phragmites australis, accompanied in
places, by the reedmaces, Typhaangustifolia. All the Great Reed Warbler nests and most of the
Reed Warbler nests were found in the reed beds, but some nests of the latter species were also
found in stands of Typha vegetation. The ponds are surrounded by and separated from arable
land by a belt of old parkland, up to 200 m in breadth, consisting of deciduous trees. A more
detailed description of the area is given by HUr)EC(1975).
The field work was carried out during the period 10 June to 16 July, 1992. Nests were
systematically searched for in the littoral vegetation, mostly in the reed beds. Each nest was
assigned a number and the position marked with a small coloured plastic tag placed at the edge
of the open water zone as soon as it was found.
When the nests were first visited, the number of eggs was recorded. The eggs were floated
(HaYs-& L~cl~oY1971) to determine whether they were freshly laid or had already been incubated for some time In addition to direct observation of laying or hatching dates, this
method enabled us to estimate the laying dates for all the nests.
All the nests were checked regularly (i. ~ daily during the laying period) for signs of
parasitism. The degree of mimesis between any Cuckoo's eggs found and the host's eggs was
Heft 4 ]
1993 ]
Cuculus canorus
427
scored according to a scale from one to five: 1) Perfect mimesis; When inspecting the clutch,
the Cuckoo's egg was hard to pick out, except for the fact that it was normally larger than
the host's eggs. The colour and patterning of the Cuckoo's egg was similar to those of the host,
and it was only possible to identify the Cuckoo egg after a closer inspection of the clutch.
2) Good mimesis; The Cuckoo's egg mimicked the host's eggs quite well, but small deviations
in both colour and pattern made the Cuckoo's egg relatively easy to distinguish after a closer
inspection of the clutch. 3) Moderate mimesis; The Cuckoo's egg was easily distinguished
when inspecting the clutch, but either the colour or the pattern was similar of those of the
host's eggs. 4) Poor mimesis; The Cuckoo's egg differed from the host's eggs in both colour
and pattern. 5) Non-mimetic; The Cuckoo's egg did not resemble the host's eggs in any respect
(see also HAt~t<isoN 1968).
All the clutches containing Cuckoo eggs were photographed according to a standard procedure. The length and breadth of the Cuckoo's egg(s) and of one of the host's eggs were
measured.
We recorded whether the Cuckoo eggs were subsequently rejected, either by desertion or by
ejection. The fate of the Cuckoo egg was recorded for all parasitized nests. Parasitism of the
nest was considered to have been successful if the young Cuckoo became fledged, or if it was
still alive at an age of 14 days. Breeding success is defined as the number of successful young
reared in relation to the number of eggs laid. Breeding success may have been overestimated
by this method, because there is a possibility that we overlooked some Cuckoo eggs that had
been quickly ejected by the hosts after laying, and because some Cuckoo chicks may have died
between reaching an age of 14 days and fledging. Hatching success is defined as the number
of eggs hatched in relation to the number of eggs laid. Fledging success is defined as the
number of young fledged in relation to the number of eggs hatched.
All the statistical analyses of the data are two-tailed, except where stated otherwise. Statistical
tests were calculated by Statgraphics version 3.0 (STCS 1991).
We are indebted to M. K~3c~ and P. OLSENfor their assistance in the field and to J. CHYTILwho, on
behalf of the Czech authorities, gave us the permission to carry out field work in the protected areas at
Lednic~ Thanks also to J. LWJELDat the Zoological Museum in Oslo and O. HOGSTADat the Museum of
the University of Trondheim for allowing us to photograph eggs in their collections. Finally thanks to
P. TALLANZW.~for improving the English. This study was supported by a grant from the Nansen Foundation.
Results
Rates of parasitism
O u t of the total of 128 Reed Warbler nests investigated 23 (18.0 %) were eventually
parasitized. However, because a proportion of these nests were first found after the egglaying period had finished, and because some Reed Warblers normally eject Cuckoo
eggs (see e, g. DAVIES & BROOKE 1988) this rate is probably an underestimate. That this
was so is further suggested by the fact that in seven nests recorded as unparasitized,
one host egg disappeared during the egglaying period or early on in the incubation
period. Because the Cuckoo normally removes one or more of the host's eggs during
the act of parasitism (see e, g. G~TIxmR 1981, WYLLIE 1981, MOKSNES & R~S~FT
1987), these nests m a y have been parasitized and the Cuckoo egg thereafter ejected by
the host. When these nests are included, an estimated parasitism rate of 23.4 % (30/128)
is obtained, which is probably closer to the true rate of parasitism. However, this may
428
A. MOKSNESu.a.
[ J" 134Orn"
still represent a minimum value` On the other hand, Cuckoos may remove host eggs
from nests which they do not afterwards parasitize (Gkr,~,~ER 1981).
Of the 23 Reed Warbler nests recorded as parasitized, four were found with a Cuckoo
chick and 19 with Cuckoo eggs. Of these latter nests, four (21.1%) contained two
Cuckoo eggs and 15 a single Cuckoo egg, i. e` altogether 23 Cuckoo eggs. The cases
in which two parasitic eggs were found in the same nest always showed a clear colour
difference, indicating that the eggs had been laid by different female Cuckoos.
We found 19 Great Reed Warbler nests which could be checked for parasitism. Only
one nest was recorded as parasitized, it contained a Cuckoo chick. This yields a rate
of parasitism of 5.3 %.
Vertical rows from left to right: 1 and 2: ReedWarbler eggsand Cuckoo eggs,respectively,from deserted
parasitizednests of the ReedWarblerin the Lednicearea,3: GardenWarbler,4: Red-backedShrike,5: Great
Reed Warbler.
C u c k o o egg m o r p h s and m i m e s i s
Most of the Cuckoo eggs found in the Reed Warbler nests showed a poor degree of
mimesis when compared to the host's eggs (Table 1, Fig.). As many as 20 of the 23
Cuckoo eggs found had a whitish ground colour and a spotting which made them
look more like Cuckoo eggs mimicking the eggs of the Great Reed Warbler, the
Garden Warbler, Sylvia borin, or the Red-backed Shrike, Lanius col&rio (Fig.). These
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1993 J
429
Cuculus canorus
are also common Cuckoo hosts, as shown by examination of museum collections of
eggs from Central Europe (MOKSNES& ROSKAFTunpubl.). Only three Cuckoo eggs
had an appearance which, on a subjective basis, would have allowed them to be
classified as a "Reed Warbler type" (or gens) (see BROOKE & DAVIES 1988).
Nonetheless, even these eggs were easily distinguishable from those of the host
(Table 1).
Tab. 1. Acceptance versus rejection of Cuckoo eggs by Reed Warblers in relation to the degree
of colour mimesis between the Cuckoo's egg(s) and the host's eggs. The scale 1--5 denotes
a gradient from perfect mimesis (1) to maximum contrast (5) (see text for further explanation).
1
2
Degree of mimesis
3
4
Accepted
Rejected
6
2
Total
8
51)
51)
10
5
Total
1
11
8
1
19
1) Includes 2 clutches with 2 Cuckoo eggs.
H o s t b e h a v i o u r t o w a r d s C u c k o o eggs
According to Table 1, the Reed Warblers rejected the Cuckoo's egg(s) in 8 of the 19
parasitized nests (42.1%). Seven of these nests were deserted, but in one nest the
Cuckoo egg was ejected. However, ejection rate may have been higher, because we probably failed to discover some of the ejected eggs. The previously-mentioned disappearance of one of the host's eggs from some of the nests may be an indication that
ejection behaviour is more frequent. On the other hand, there is no reason to believe
that deserted nests would have been overlooked, because the probability that we would
have discovered these nests is the same as for non-deserted nests.
No significant relationship was found between the rate of rejection and the degree
of mimesis between the Cuckoo's and the host's eggs (Table 1; median test, Fisher exact, p -- 0.17). However, there was a slight tendency for poorer mimesis to exist in
the rejected clutches (Median: 4, N = 8) than in the clutches in which the Cuckoo's
egg was accepted (Median: 3, N = 11). In the four nests which contained two Cuckoo
eggs, the degree of mimesis between all the eight Cuckoo eggs and the host eggs was
recorded as 4 on the scale (Table 1). Two of these nests were later deserted, and in the
two other nests the Cuckoo's eggs were accepted, whereafter the first of the Cuckoo
chicks to hatch ejected all the host's eggs and the second Cuckoo's egg as well.
In six of the deserted nests the hosts seemed to have abandoned their nests more or
less immediately after the appearance of the Cuckoo's egg because these eggs were cold
already by the time of our first visit after parasitism had taken place, and the parents
were never again seen at the nest. One nest was deserted between the seventh and the
ninth day after the Cuckoo had laid her egg. The single case of ejection observed, occurred between the second and the fifth day after parasitism had taken place.
430
A. Mot(sNEs u.a.
[ J' Orn.
[ 134
B r e e d i n g success of the C u c k o o
Seven of the 23 Cuckoo eggs laid in Reed Warbler nests hatched (Table 2), i. e. an
estimated hatching success of 30.4 %. Two of these young Cuckoos were predated during the nestling stage, and two others fell into the water by accident and drowned. The
remaining three young were observed during the first two weeks of their nestling
period. Assuming that they survived to fledging, this would yield an estimated fledging
success (number of fledglings leaving the nest in proportion to the number of eggs hatched) of 42.9 %. If so, the breeding success was 13.0 % (3 of 23 Cuckoo eggs resulted
in fledged Cuckoos).
Tab. 2. Fate of the Cuckoo eggs laid in Reed Warbler nests.
Cuckoo eggs
Number of eggs
% eggs
Rejected
7
4
2
10
30.4
17.4
8.7
43.5
Total
23
100.0
Accepted
Hatched
Unhatched
Predated
Discussion
Rates of p a r a s i t i s m
The density and breeding of the birds in the Southern Moravian ponds which we
studied to observe Cuckoo parasitism in 1992, had also been investigated by HVDEc
(1975) during the period 1959-1968. As we did in 1992, he observed parasitism by
the Cuckoo only in nests of the Reed Warbler and the Great Reed Warbler, but at different rates; 5.6 % (N = 187) for the Reed Warbler and 15.7 % (N -- 156) for the Great
Reed Warbler. This contrasts with the relationship between these two species as
Cuckoo hosts that we found, namely a minimum rate of parasitism of 18.0 % for the
Reed Warbler and 5.3 % for the Great Reed Warbler. However, we found relatively few
nests of the Great Reed Warbler (19) that we could subsequently check for parasitism,
compared to nests of the Reed Warbler (128). HUDEC (1975) checked about the same
number of nests of both species. The breeding season of the Great Reed Warbler in
this area lasts from early May until late June or the beginning of July, whereas the Reed
Warbler breeds later on, mostly in June and the first half of July (HuDzC 1975). Since
we did not start our field work before the second half of June, it could be argued that
we may have overlooked many Great Reed Warbler nests, including some parasitized
ones. However, if this had been the situation, then we should have expected to have
found many empty Great Reed Warbler nests from which the young had fledged, but
we found only a few such nests. The marked difference of our data from those of
HUDEC (1975) would seem to indicate that a relatively heavy decrease in the Great
Reed Warbler population in the area has occurred during the past 25 years (this is also
confirmed by K. HUDEC, pers. comm.) or, at least, that its population density in 1992
was low,
Heft 4 ]
1993 /
Cuculus canorus
431
The estimated minimum rate of parasitism by the Cuckoo of the Reed Warbler in
the Lednice area (18.0 %), is still relatively high. The median rate of parasitism compiled by SCHULZE-HAGEN(1992), from 34 different studies, was 9.0 %. The highest frequencies so far recorded locally in Central Europe are 55 % (ScHIERMANN 1926) and
63 % (SCH~UI~S 1963).
We did not obtain a reliable estimate for the parasitism rate of the Great Reed
Warbler in this study because we found too few nests. However, this species, like the
Reed Warbler, is often parasitized at high rates, of about 20 % in Japan for example
(LOT~M et al. 1992).
C u c k o o egg m o r p h s , m i m e s i s and b r e e d i n g success
The eggs of the Reed Warbler Cuckoo gens usually show good mimesis with the eggs
of its host (WYLLIE1981, BROOKE & DAVIES 1987, 1988, DAVIES & BROOKE 1988,
1991). In museum egg collections, 1225 European Cuckoo eggs were classified by
MOKSNES & ROSKAFT (unpubl.) as belonging to the ~Reed Warbler" egg-morph and
their degree of mimesis with the Reed Warbler host eggs was scored. Perfect mimesis
(1) was recorded for 1.3 % (N = 16), good mimesis (2) for 33.2 % (N = 407), moderate
mimesis (3) for 53.3 % (N = 653), poor mimesis (4) for 11.8 % (N = 144) and no
mimesis (5) for 0.4 % (N = 5). These results represent a statistically significantly better
mimesis than that we found for the Cuckoo eggs in the Reed Warbler nests in the
Lednice area, where no better degree of mimesis than 3 was scored at all and the median value was 4 (Table 1; Kendall's TauB = 0.064, p = 0.0006). This poor degree of
mimesis of the majority of the Cuckoo eggs from the Lednice area therefore indicates
that the Cuckoos active in this area do not belong to a Reed Warbler gens. This is also
indicated by the high rate of rejection recorded (42.1%; 8 of 19 nests), a rate which
is considerably higher than that observed for Reed Warblers in Britain (19.0 %; 8 of
42 nests, X2 = 3.59, df = 1, p = 0.058) (DAVIES& BROOKE 1988). The rejection rate
in the Lednice area (8/19), in fact, did not differ statistically from the rejection rate
recorded experimentally in Britain when non-mimetic model Cuckoo eggs were introduced into Reed Warbler nests (61.8 %; 34/55, Z2 = 2.24, df = 1, NS; DAVIES&
BP,OOKt~ 1989).
Because of this high rate of rejection, the Cuckoos parasitizing the Reed Warblers
in the Lednice area had a relatively low rate of hatching success (30.4 %), which is
statistically significantly lower than the corresponding value from Britain (56.2 %, N
= 385, Xa = 5.77, df = 1, p <0.05; BROOKE& DAVIES 1987). This is the reason why
the breeding success recorded for Lednice (13.0 %) was almost significantly lower than
that for Britain (31.9 %, N = 442, Xa = 3.64, df = 1, p = 0.057).
There are at least two explanations, which are not mutually exclusive, that may be
put forward to explain the existence of this poor mimesis. (1) If there (as indicated)
had been a decline in the Great Reed Warbler population in the Lednice area in recent
years, or in the particular study year, then the Cuckoos that normally parasitized Great
Reed Warblers might have been forced to parasitize Reed Warblers. The Cuckoo eggs
432
A. MOKSNESU.a.
[ J' Orn.
k 134
found in the Reed Warbler nests bore a greater resemblance to Great Reed Warbler eggs
than to Reed Warbler eggs. (2) The Cuckoos in the Lednice area are generalists, that
parasitize several host species including the Acrocephalus warblers. The resemblance
observed between the Cuckoo eggs and the eggs of, for example, the Garden Warbler
and the Red-backed Shrike also suggests this possibility.
At present we have no data that would enable us to decide which of these two explanations is the most probable. However there are two arguments which can be put
forward to support the second explanation:
(1) BROOKE& DAVIES(1991) found no support for the existence of host imprinting
in the Cuckoo. This is a necessary mechanism for maintaining host specialization (the
"host preference" hypothesis; B R o o ~ & DAVIES 1991).
(2) An alternative to host specialization is that female Cuckoos show no particular
host preference, but return to breed in their natal habitat (the "natal philopatry"
hypothesis; BROOKE& DAVIES 1991). There is some evidence for this migration pattern from ringing recoveries (SE~L 1977). The question as to whether this hypothesis
could explain the existence of Cuckoo egg mimesis has been discussed by BROOKE&
DAVIES (1991). If the Cuckoos search for nests at random, species that either occur at
high densities or have easily found nests, will have the greatest chance of being
parasitized. In uniform habitats, where one or only a few species predominate, random
searching could result in most of the Cuckoo's eggs being laid in the nests of one and
the same host species, a necessary condition for the maintenance of egg mimesis. Examples of this situation may be seen in the excellent mimesis of the Cuckoos that
parasitize the Meadow Pipit, Anthus pratensis, nesting on the bogs and heaths of Northern Europe and the Great Reed Warbler nesting on the Hungarian marshes
(SOUTHERN 1954). In fragmented habitats with a greater diversity of host species, on
the other hand, one would expect that random searching would result in the Cuckoos
laying their eggs in the nests of many different species and, in consequence, the eggs
will show a poorer degree of mimesis (see also HAI~RISON 1968). This is what we
found in the Lednice area; poor mimesis in combination with a fragmented habitat
(reed beds, fishpond shores, patches of forest, fields etc.).
Summary
The Reed Warbler, Acrocephalus scirpaceus, in the Lednice area, Southern Moravia in the
Czech Republic, was parasitized by the Cuckoo, Cuculus canorus, at a rate of at least 18.0 %.
The Cuckoo eggs showed poor mimesis with the Reed Warbler eggs, but showed a greater
resemblance to the eggs of other species breeding in the area, including the Great Reed Warbler,
A. arundinaceus. This latter species was also parasitized, but we did not find enough nests to
obtain a reliable estimate for the rate of parasitism. The parasitized Reed Warbters rejected the
Cuckoo eggs at a high rate (42.1%) and therefore both the hatching success and the breeding
success of the Cuckoo was considerably lower than shown by comparable results from Britain.
On the background of these results (poor mimesis of the Cuckoo eggs and a high rejection
rate by the hosts) the question of the degree of specialization versus generalism in the Cuckoo's
host preference is discussed.
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1993 J
Cucu&s canorus
433
Zusammenfassung
Teichrohrs~inger im Gebiet von Lednice (Siidm~ihren, Tschechei) waren mindestens zu 18 %
vom Kuckuck parasitiert. Die Kuckuckseier glichen jenen der Teichrohrs~inger nur wenig,
waren jedoch den Eiern anderer im Gebiet b~tender Arten einschlief~lich des
Drosselrohrs~ingers ~ihnlicher. Drosselrohrs~inger waren auch parasitiert; wir fanden jedoch
nicht genug Nester, um eine Parasitierungsrate ermitteln zu k6nnen. 42,1% der Kuckuckseier
wurden vom Teichrohrs~inger nicht angenommen; daher waren Schlhpf- und Ausfliegerate des
Kuckucks merklich geringer als in vergleichbaren Ergebnissen aus Grof~britannien. Vor dem
Hintergrund unserer Ergebnisse -- geringe Angleichung der Kuckuckseier und hohe
Ablehnungsrate durch die Wirtsart -- wird die Frage yon Spezialisierung und Generalismus
in der Wirtswahl des Kuckucks diskutiert.
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Authors' addresses: (A. M., E. R., I. J. 0.) Department of Zoology, University of Trondheim,
N-7055 Dragvoll, Norway. (V. B.) Department of Zoology, Palack~ University, T~. Svobody
26, 771 46 Olomouc, The Czech Republic. (M. H.) Institute of Systematic and Ecological
Biology, CAS Kv&n~i 8, 603 65 Brno, The Czech Republic.