J Ornithol (2006) 147:649–652 DOI 10.1007/s10336-006-0097-x
SHORT NOTE
Great reed warbler Acrocephalus arundinaceus and reed warbler Acrocephalus scirpaceus respond differently to cuckoo dummy at the nest Andrzej Dyrcz Æ Lucyna Hałupka
Received: 14 February 2006 / Revised: 26 July 2006 / Accepted: 26 July 2006 / Published online: 23 August 2006 Ó Dt. Ornithologen-Gesellschaft e.V. 2006
Abstract A cuckoo Cuculus canorus dummy was exposed at 24 nests of great reed warbler Acrocephalus arundinaceus (GRW) and 34 nests of reed warbler Acrocephalus scirpaceus (RW) during the egg-laying stage. The eight GRW pairs attacked the cuckoo directly, striking the dummy, but such a behaviour was not recorded in RWs. Also, other behavioural measures (closest distance from the model, duration of distress calls and number of excitement calls) indicated a lower level of defence by RWs compared to GRWs. In the study area, the parasitism rate was much lower in GRWs (1.7% of nests) than in RWs (11.3%). We suggest that one of the reasons for the lower level of cuckoo parasitism on GRWs is its stronger nest defence and hence higher risk of injury or even death for the cuckoo during egg dumping.
cuckoo Cuculus canorus. However, some populations are not parasitised by the cuckoo. Stokke et al. (submitted) analysed 16 different European sites where the cuckoo and RW occur sympatrically, and found that, in nine of them, RWs did not host the cuckoo at all. The parasitism rate is high in some places (e.g. Hungary; Moskat and Honza 2002) but low in others (e.g. Czech Republic; Moksnes et al. 1993). In our study area, the level of parasitism was low in GRW (1.7% of nests) compared to RW (11.3%) (Dyrcz and Halupka, in press). The aim of this study was to test the hypothesis that the lower rate of cuckoo parasitism in GRW compared to RW may result from a higher level of nest defence by this species against the nest parasite.
Keywords Acrocephalus arundinaceus Æ Acrocephalus scirpaceus Æ Dummy Cuculus canorus
Methods
Introduction The ranges of great reed warbler (GRW) and reed warbler (RW) overlap in Europe (Cramp 1992). Both species are among the favourite hosts of the common Communicated by F. Bairlein A. Dyrcz (&) Æ L. Hałupka Department of Avian Ecology, University of Wrocław, Sienkiewicza 21, 50-335 Wrocław, Poland e-mail:
[email protected]
L. Haupka e-mail:
[email protected]
The study was carried out in 2004 and 2005 at three groups (1,583, 750 and 390 ha) of Milicz fish-ponds, Wrocław Province, SW Poland. Some of these ponds support extensive reed-beds, with patches of cattail (Typha sp.), and in many places the dikes surrounding the ponds are vegetated with old deciduous trees and luxuriant bushes. Ponds in the study region were 10– 180 ha in area. Some ponds are several hundred years old. The water depth is mostly 50–150 cm. Part of the area is protected as a nature reserve with extensive carp production. The cuckoo dummy used in the experiments was a wooden sculpture made by an artist and painted realistically (an earlier single experiment with a stuffed cuckoo ended with the total destruction of the model by an attacking GRW pair). The dummy was placed
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facing the nest on a small platform at the height of the nest cup, ca. 1 m from the nest. A tape recorder was placed next to the dummy. The experiment started at the moment of placing the cuckoo model near the nest, when the observer had been already hidden in a blind. Each experiment lasted 10 min. During the trial, a recorded voice of the cuckoo (female bubble cry) was played three times (after 1, 5 and 8 min from the start of an experiment). Observations were carried out from the mobile blind placed ca. 10 m from the nest. A second person started the tape recorder, accompanied the observer to the blind, and went away. The observer used the stopwatch and counting device. During the experiment, the following behavioural measures of defence were taken: (1) the number of direct attacks on the dummy (associated with physical contact); (2) the closest distance between a bird and the dummy; (3) the total time (in s) of uttering distress-calls or alarm rattle (Cramp 1992), which is frequently associated with an aggressive posture; (4) the number of excitement-calls (Cramp 1992); and (5) the number of flights between reed stems by both pair members. Only one experiment was conducted at each nest. All experiments were performed during egg-laying (nests contained at least two eggs). Tests with GRWs were done between 10 and 29 May 2004 and with RWs between 25 May and 24 July 2005. The hour of observation and nest stage did not differ in experiments with GRWs and RWs (Mann–Whitney U test: hour, U = 337, P = 0.226; nest stage, U = 303, P = 0.226). A total of 24 experiments were carried out at GRW nests, and 34 at the nests of RW. Birds were not individually marked, but the probability of testing the same pair twice (pseudoreplication) was low owing to the large distances between the experimental nests. Differences in level of defence between the two species were analysed with Mann–Whitney U test. All tests were two-tailed.
Results Great reed warblers parents arrived at the nest after 1– 360 s from the start of an experiment, most often (12 of 23 cases) after 60 s (that is after the first bubble cry of a female cuckoo; median = 60.0, Q1 = 60, Q3 = 120). In RW birds came after 1–540 s, most often after 60 s (14 of 28 trials) (median=60.0, Q1=60, Q3=120). The difference between the two species was not statistically significant (Mann–Whitney test: U = 337.5, P = 0.752). In 8 of 24 (33%, exact 95% confidence limits: 16–55%) tests with the GRW, the cuckoo was directly attacked. Such a behaviour was never observed in 34
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experiments with RWs (see Table 1). The probability that the RW does attack the cuckoo but that we did not observe such a behaviour due to sampling error is negligibly small. For example, if we assume that in the general population 16% of RWs (the lower 95% confidence limit calculated for the GRW) attack the cuckoo, the probability that we did not observe any instance in the sample of 34 experiments is less than 0.003. Thus we can conclude that in the RW direct attacks on cuckoos approaching the nest are very rare, if occurring at all. The number of attacks during a single experiment ranged from 4 to 212; in four nests it was clearly higher (97–212) than in others. In most cases, both mates attacked the cuckoo, although in some experiments females were more aggressive but we do not have enough data to test these differences. Birds frequently struck the occipital area of the skull, probably the most sensitive part. Great reed warblers approached the dummy significantly closer than RWs (Table 1), the minimum value of closest distance for both species was 0 and 0.25 m, respectively. The total duration of distress calls by GRWs was significantly longer (0–520 s) compared with RWs (0– 420 s; Table 1). The number of excitement calls by GRW ranged from 0 to 300, and from 0 to 7 in RW, and the level of calling was significantly higher in GRW (Table 1). The number of flights did not differ significantly between the two species.
Discussion The studied RWs never attacked the cuckoo mount (such a behaviour was observed occasionally in a Table 1 Differences between great reed warblers Acrocephalus arundinaceus (GRW) and reed warblers Acrocephalus scirpaceus (RW) in their responses (measured by five behavioural variables: duration of distress calls, number of excitement calls, flights, closest distance from the mount and attacks) to the cuckoo Cuculus canorus dummy at the nest Variables
Distress calls (s) Excitement calls/min Flights/min Closest distance (m) Attacks/min
Median (Q1–Q3) GRW
RW
20 (0–185) 2 (0–15.5) 2 (0–93) 0.5 (0–1) 0 (0–23.5)
0 0 1 1 0
(0–0) (0–0) (0–4) (0.5–3) (0–0)
U
P
187.5 232.5 341.5 180.5 272
