DOI: 10.5061/dryad.bnzs7h4jw
title: Host nest defence does not act as selective agent against plumage polymorphism in brood parasites

description: Batesian mimicry in brood parasites is often viewed as an evolutionary
 strategy to mitigate host aggression. Female common cuckoos (Cuculus
 canorus) exhibit two morphs: the hawk-like grey and the rufous one,
 potentially maintained by apostatic selection. It was hypothesized that
 the grey morph's predator-like appearance deters host defences, while
 the rufous morph benefits from its rarity by evading host attention.
 Previous research predominantly utilized static cuckoo dummies, lacking
 insights into real-world interactions. We investigated the effectiveness
 of the cuckoo morphs in accessing great reed warbler (Acrocephalus
 arundinaceus) nests under natural conditions. Analysing video-recorded
 cuckoo attempts, we found no significant difference in nest-access success
 between the morphs. Both experienced a similar probability of physical
 attacks when hosts were present, and the rufous morph did not increase
 success in the nest-access attempts in the absence of hosts. These results
 fail to support the assumptions of (1) Batesian mimicry that hawk-like
 mimicry enhances nest access or reduces host aggression, and (2) apostatic
 selection that the rarity of the rufous morph confers an advantage in the
 successfully accessing the host nest. Future research should aim to
 identify stages in the cuckoo's life cycle or host interactions where
 colour polymorphism provides an evolutionary benefit.
description: <em>Data collection</em> Data were
 collected during the breeding seasons 2020‒2022 within the fishpond area
 situated between Hodonín (48°51'0"N, 17°07'0"E) and
 Mutěnice (48°54'0"N, 17°02'0"E), in the Czech
 Republic. We employed a video-recording setup of one camera focused on the
 nest from a close distance of &lt;1 metre and another camera installed
 on a tripod approximately 4–10 metres from the nest, monitoring the nest
 surroundings. Most of the nests were video-recorded from the day the nest
 was completed throughout the egg-laying period. In a minority of nests,
 the filming started anytime during the egg-laying period. The
 closer-positioned camera was a Carmedien STO-IR rear-view camera
 (Carmedien, Tschernitz, Germany) with IR illumination connected to a
 miniature Mini DVR CH-HD0065 digital video recorder (Shenzhen Chu-Tech Co.
 Ltd., Shenzhen, China) placed in a water-resistant box or a custom HD
 camera with inbuilt recorders without IR illumination. The cameras were
 powered by 12 V/100 Ah gel batteries placed on the bank. In 2021, these
 cameras were replaced with motion sensitive BUNATY micro 4K trail cameras
 powered by AA batteries and placed about 1 metre from the nest. The camera
 capturing the wider area around the nests was a Sony HDR-CX 240 powered by
 a power bank (GoGEN 20000 mAh) with a scanning range of about 10 m from
 the nest on each side, again depending on reed density. Daily checks were
 conducted to record the nest status, replace the power bank and SD card.
 All the recording equipment was camouflaged by green cloth and reed stems
 to prevent disturbance of nesting birds. The use of simultaneous filming
 allowed us to analyse cuckoo-host interactions around the nest and at the
 nest, determining the outcome of cuckoo activity (unsuccessful access or
 successful access to the nest; and in the latter case further: parasitism,
 predation or only nest checking event). We considered nest access
 successful when a cuckoo made physical contact with the host nest. The
 location of the nest in the reeds sometimes prevented us from detecting
 cuckoo activity using a more distant camera, so we used only recordings
 from a closer camera that captured the nest.
 <em>Analyses</em> Firstly, we
 examined if the hosts feared the hawk-like grey cuckoo more than the
 rufous morph. We specifically tested if the probability of physical attack
 by host (binary response variable; yes or no) differed between cuckoo
 morphs (categorical; grey or rufous). In this model, we additionally
 tested the effect of the number of host parents present (1 or 2), the
 clutch size at the time of event (0–5), the time of day (hour), and the
 ordinal date within the breeding season (1 = 1st January). To account for
 repeated cuckoo visits to the same nest, we included a random intercept
 for nest ID in our model. We assessed potential multicollinearity using
 the variance inflation factor (VIF), which was satisfactory for all
 predictors (VIF &lt; 2.0). We employed a Bayesian model with Bernoulli
 distribution and logit link, which effectively solved the problem by
 setting a weakly informative prior for fixed effects to be the normal
 distribution of the mean set to 0 and the standard deviation to 10. For
 this purpose, we used the package brms (version 2.20.4) in R software
 (version 4.3.2). Next, we tested whether the rarer
 rufous morph evades the host attention more often compared to the grey
 morph. This should be indicated by a higher proportion of rufous morph
 observed at the nests, where hosts are absent, compared to the nests with
 hosts present. Again, we employed a Bayesian mixed-effects model with
 Bernoulli distribution with response variable of cuckoo morph and
 predictor of the presence of host parents during a cuckoo visit (at least
 one host present or hosts absent). The additional predictors and model
 structure have been identical as in the previous model. Further, we aimed
 to identify the factors that influence the success of cuckoos in accessing
 host nests (binary response variable; yes or no). We primarily examined
 the potential effects of the presence of host parents during a cuckoo
 visit (yes or no), effect of cuckoo morph (grey or rufous) and then
 included the same additional predictors as in the above models. In this
 model, there was a problem with a complete separation (all the cuckoos
 successfully reached the nest if no host was present; n = 29). We
 effectively solved this problem by employing a Bayesian model and setting
 a weakly informative prior for fixed effects to be the normal distribution
 of the mean set to 0 and the standard deviation to 2.
 Finally, we used the same Bayesian mixed-effects model to explore
 the effect of the number of host parents present (1 or 2) on the success
 of cuckoos to access host nests (binary; yes or no). In this model, we
 excluded all cases where no parent was present during the cuckoo attempts
 and applied the same model structure and settings as in the previous
 model. The only exception was setting a weakly informative prior again to
 the mean of 0 and standard deviation of 10.
description: # Host nest defence does not act as selective agent against plumage
 polymorphism in brood parasites
 [https://doi.org/10.5061/dryad.bnzs7h4jw](https://doi.org/10.5061/dryad.bnzs7h4jw) The datasets provide field data on the effectiveness of both cuckoo morphs in reaching the nests of great reed warblers (*Acrocephalus arundinaceus*). The data were collected in the Czechia fish ponds and include detailed information extracted from video recordings. ## Description of the data and file structure The dataset, "dataset_defense_final_submit.csv", is structured with multiple variables describing in detail each cuckoo's attempt to reach the host nest. nest_ID: Unique identifier for each nest date_real: Date of observation in YYYY-MM-DD format date_real_ordinal: Ordinal date of the year (1-365) real_time: Time of observation in HH:MM:SS format real_time_h: Hour of observation (13–21) cuckoo_morph: Colour morph of the cuckoo (grey or rufous) cuckoo_success_nest: Whether cuckoo succeeded in accessing the nest (succeeded or failed) host_attacked_cuckoo: Whether host physically attacked the cuckoo (yes, no, host was absent, or unclear due to e.g. reed cover) host_presence: Whether host was present (yes or no) host_count_present: Number of host individuals present (0,1 or 2) nest_content: Number of eggs in the nest