Study area
The study was conducted in central Hungary, ca. 25–60 km south of Budapest, at around the settlements Alsónémedi (47°18′; 19°09′), Apaj (47°06′; 19°05′), Kunszentmiklós (47°01′; 19°07′) and Tass (47°01′; 19°01′) during the 2020 and 2021 breeding seasons. We also used heterospecific controls with Eurasian collared doves for comparisons conducted in the year 2016. In this study area common cuckoos can be found in high densities in their breeding season (May and June). They almost exclusively parasitize great reed warblers (Acrocephalus arundinaceus) locally, a large host which breeds in narrow reed-beds along small irrigation and flood-relief channels47.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. Local animal ethics regulations and agreements were followed for fieldwork. All work complied with the Hungarian laws, and the Middle-Danube-Valley Inspectorate for Environmental Protection, Nature Conservation and Water Management, Budapest, provided permission for research (permit no. PE/KTF/17190-3/2015).
Playback files
We used cuckoo calls recorded in May between 2016 and 2019. Recording were made with a Telinga Universal parabola dish, equipped with a Sennheiser ME-62 microphone, a K6 powering module, a FEL MX mono preamp, and a Marantz PMD-620 MKII recorder (sampling rate: 48 kHz, 24-bit quality)30.
We constructed ten different sound files for playback from the basic “cu-coo” calls:
Heterospecific (negative) control
(1) The calls of a neutral species from the local avifauna, the Eurasian collared dove, were used for interspecific vocalization control.
Natural (positive) control
(2) Normal (natural) “cu-coo” calls.
Experimental treatments; one-note calls
(3) Deleting the second note, i.e. contained “cu”, only.
(4) Deleting the first note, i.e. contained “coo”, only.
Two-note calls
(5) Reversal of the basic “cu-coo” call, i.e. “coo-cu”.
(6) Repeating the first note, and deleting the second note, i.e. “cu-cu”.
(7) Repeating the second note, and deleting the first note, i.e. “coo-coo”.
Three-note calls
(8) Repeating the first note, i.e. “cu-cu-coo”.
(9) Repeating the second note. i.e. “cu-coo-coo”.
Three-note natural
(10) Normal (but rare and context specific) “nat. cu-cu-coo”.
The experimental 3-note variant of the calls (“cu-cu-coo”; call type No. (8)) differs from our natural 3-note calls (“nat. cu-cu-coo”; call type No. (10)) in two out of the three acoustic parameters (length: F1,18 = 79.258, P < 0.001; maximum frequency: F1,18 = 58.243, P < 0.001) as an ANOVA revealed (Table 3). The natural variant has a shorter duration and higher maximum frequency than our experimental call type, which was constructed directly from the 2-note call by repeating the first element (Fig. 4). However, in the minimum frequency we did not find any deviation (F1,18 = 0.159, P = 0.695, Table 3).
Altogether 104 experimental trials were conducted and analyzed, 10–12 per trial types (see separate sample sizes per categories in Fig. 2, where we regarded the number of trials per playback type as sample size). Most of the trials (calls (5)–(10)) were played back in the 2021 breeding season, between May 11 and 26. Regarding the constraint in the size of the study area, the accessibility of birds, we also used trials of calls (2)–(4) played back between May 4 and 18, 2020, together with other categories not used in the present study36. Controls with collared dove calls were done in 2016 following the same protocols. Each playback file lasted 2 min, and it contained six calls for 10 s, repeated two times. This 30-s section was followed by a 15-s pause, and repeated two times (30 + 15 + 30 + 15 + 30 s), similarly to the file structure used by30. To reduce pseudo-replication (sensu48,49), in all types of playbacks (cu-coo, cu, coo, coo-cu, cu-cu, coo-coo, cu-cu-coo, cu-coo-coo, and dove control) we used each acoustic file for only one playback trial, except for one playback type. As the natural 3-note calls (“nat. cu-cu-coo”) typically overlap together with females’ bubbling calls (“duetting”35), we selected such calls without the bubbling calls, to broadcast clear signals. We had such natural “cu-cu-coo” calls non-overlapping with female bubble call from seven individuals, only. For this reason, in three cases we used two different call sequences from the same individuals in the “nat. cu-cu-coo” playback type.
All syntax error types generated here were heard and recorded by us at least once in the field (Fig. 1b,c), except the two-note monotypic calls (“cu-cu” and “coo-coo”).
Field experiment
Playbacks were carried out under similar weather conditions (no rain and wind, between 7 and 11 h in the mornings). A playback site was selected where a male cuckoo was observed within 50 m, sitting on tree and calling. A JBL Xtreme 40 W loudspeaker was set on a tree about 1.5 m height, 20 m from the hide of the observer, and it was connected with an audio cable to a Lenovo TAB 2 A7 tablet, containing the playback files in wav 16-bit format. During playbacks cuckoos were followed continuously, and observations were dictated on a sound recorder (Tascam Dr05). We collected the following data: starting distance (the position of the focal bird from the speaker; m), the closest distance of the focal bird from the speaker during the 2-min playback (m), and latency (sec) as the time when the cuckoo started to approach the speaker during the playback. Cuckoos’ starting distances (the distance between cuckoo’s perch site and the loudspeaker) were measured with the help of a Bushnell Yardage Pro 800 rangefinder. Specifically, we estimated the closest distance between the flying cuckoos and the loudspeaker by sight, after personally having trained on visual assessment of distances with this rangefinder. We regarded an approach to the loudspeaker when the cuckoo left the tree where it was perched on at the start of a playback and flew toward the speaker. In contrast, when the focal bird changed position in the same tree at the starting point, or, rarely went into the other direction, these were not considered as approach movement. The next trial point was selected at a distance of about 500 m36, or more if the focal bird was continuously calling at the same site with known position, otherwise at least 1 km (c.f.50) to exclude the repeated use of the same individual cuckoo as a focal bird. The type of playback was drawn randomly at each site.
Statistical analyses
We applied two fixed effect linear models when analysing the data. In the first model the response variable was reaction (Y/N), referring to whether or not the focal bird approached the speaker during the 2-min playback. In the model we used treatment as a predictor factor, and starting distance as a covariate. In the second model we replaced the binary response variable (Y/N) to closest distance (m) when cuckoos approached the speaker.
We used binary logistic regression for the comparison of male common cuckoos’ responses (Y/N; dependent variable) with the independent variables of 3-note playbacks (“cu-cu-coo” and “cu-coo-coo”), relative to natural 3-note cuckoo calls (“nat. cu-cu-coo”; reference category) and starting distance (m). For logistic regression the method “enter” was applied.
We ran one-way analysis of variance (ANOVA) to reveal if latencies to the different playback call types were different. As cuckoos showed no or almost no (0 ≤ n ≤ 2) response to a few playback categories (“coo”, “coo-coo”, “nat. cu-cu-coo”, and “dove control”), we excluded these categories from the comparison of latencies. For pairwise comparisons of cuckoos’ responses to different playback types we also used Tukey’s post hoc test after ANOVA. One-way ANOVA was also useful for the comparison of acoustic parameters of 3-note cuckoo calls, i.e. the natural “nat. cu-cu-coo”, experimental “cu-cu-coo” and “cu-coo-coo” (see above in “Methods”).
All statistical analyses were conducted using the SPSS ver. 17 package (SPSS Inc., Chicago, IL, USA).
Source: Ecology - nature.com
