Alfréd Trnka

The effectiveness of hawk mimicry in protecting cuckoos from aggressive hosts

The close resemblance between the common cuckoo, Cuculus canorus, and the Eurasian sparrowhawk, Accipiter nisus, is often viewed as Batesian mimicry evolved by the cuckoo in response to the aggression of its host. However, the effectiveness of such mimicry is poorly known. We examined cuckoo–hawk discrimination ability in the great reed warbler, Acrocephalus arundinaceus, a well-known and aggressive cuckoo host. We measured the responses of birds to three combinations of simultaneously presented taxidermic mounts of the cuckoo, sparrowhawk and turtle dove, as a harmless control, placed near their nests. Great reed warblers clearly discriminated the two enemies from the innocuous species. They always attacked cuckoos and sparrowhawks more often than turtle doves suggesting they considered both a danger to their broods. However, when we confronted the tested birds with the simultaneously presented mounts of these species, the parents attacked the cuckoo more frequently than the sparrowhawk. The results revealed that although great reed warblers attacked both cuckoo and sparrowhawk mounts near their nest, they were able to discriminate between them. This may suggest that if cuckoos had evolved the visual mimicry of a sparrowhawk to avoid host attacks, this mimicry may be unsuccessful not only because it is imperfect and hosts can learn to discriminate but also because of the generalized nest defence of more aggressive hosts.

Uncovering dangerous cheats: how do avian hosts recognize adult brood parasites?

Background Co-evolutionary struggles between dangerous enemies (e.g., brood parasites) and their victims (hosts) lead to the emergence of sophisticated adaptations and counter-adaptations. Salient host tricks to reduce parasitism costs include, as front line defence, adult enemy discrimination. In contrast to the well studied egg stage, investigations addressing the specific cues for adult enemy recognition are rare. Previous studies have suggested barred underparts and yellow eyes may provide cues for the recognition of cuckoos Cuculus canorus by their hosts; however, no study to date has examined the role of the two cues simultaneously under a consistent experimental paradigm. Methodology/Principal Findings We modify and extend previous work using a novel experimental approach – custom-made dummies with various combinations of hypothesized recognition cues. The salient recognition cue turned out to be the yellow eye. Barred underparts, the only trait examined previously, had a statistically significant but small effect on host aggression highlighting the importance of effect size vs. statistical significance. Conclusion Relative importance of eye vs. underpart phenotypes may reflect ecological context of host-parasite interaction: yellow eyes are conspicuous from the typical direction of host arrival (from above), whereas barred underparts are poorly visible (being visually blocked by the upper part of the cuckoos body). This visual constraint may reduce usefulness of barred underparts as a reliable recognition cue under a typical situation near host nests. We propose a novel hypothesis that recognition cues for enemy detection can vary in a context-dependent manner (e.g., depending on whether the enemy is approached from below or from above). Further we suggest a particular cue can trigger fear reactions (escape) in some hosts/populations whereas the same cue can trigger aggression (attack) in other hosts/populations depending on presence/absence of dangerous enemies that are phenotypically similar to brood parasites and costs and benefits associated with particular host responses.

Color plumage polymorphism and predator mimicry in brood parasites

BackgroundPlumage polymorphism may evolve during coevolution between brood parasites and their hosts if rare morph(s), by contravening host search image, evade host recognition systems better than common variant(s). Females of the parasitic common cuckoo (Cuculus canorus) are a classic example of discrete color polymorphism: gray females supposedly mimic the sparrowhawk (Accipiter nisus), while rufous females are believed to mimic the kestrel (Falco tinnunculus). Despite many studies on host responses to adult cuckoos comprehensive tests of the “hawk mimicry” and “kestrel mimicry” hypotheses are lacking so far.ResultsWe tested these hypotheses by examining host responses to stuffed dummies of the sparrowhawk, kestrel, cuckoo and the innocuous turtle dove (Streptopelia turtur) as a control at the nest. Our experimental data from an aggressive cuckoo host, the great reed warbler (Acrocephalus arundinaceus), showed low effectiveness of cuckoo-predator mimicry against more aggressive hosts regardless of the type of model and the degree of perfection of the mimic. Specifically, warblers discriminated gray cuckoos from sparrowhawks but did not discriminate rufous cuckoos from kestrels. However, both gray and rufous cuckoos were attacked vigorously and much more than control doves. The ratio of aggression to gray vs. rufous cuckoo was very similar to the ratio between frequencies of gray vs. rufous cuckoo morphs in our study population.ConclusionsOverall, our data combined with previous results from other localities suggest polymorphism dynamics are not strongly affected by local predator model frequencies. Instead, hosts responses and discrimination abilities are proportional, other things being equal, to the frequency with which hosts encounter various cuckoo morphs near their nests. This suggests that female cuckoo polymorphism is a counter-adaptation to thwart a specific host adaptation, namely an ability to not be fooled by predator mimicry. We hypothesize the dangerousness of a particular model predator (sparrowhawks are more dangerous to adult birds than kestrels) may be another important factor responsible for better discrimination between the gray cuckoo and its model rather than between the rufous cuckoo and its model. We also provide a review of relevant existing literature, detailed discussion of plumage polymorphism in cuckoos, methodological recommendations and new ideas for future work.

Infanticide or Interference: Does the Great Reed Warbler Selectively Destroy Eggs?

Infanticide, the killing of offspring by adult conspecifics, has been demonstrated in many insect, mammal and bird species. In contrast to selective pressures influencing infanticide in other species, egg destruction in birds is thought to primarily represent interference competition for food, nest sites or predator-free nesting space. In the case of the great reed warbler, two opposing hypotheses exist for the explanation of its egg destruction behaviour. Our study tested these hypotheses separately by manipulating the presence of real conspecific, familiar and unfamiliar eggs and nests inside polygynous and monogamous great reed warbler territories. Out of 147 experimental nests placed in the vicinity of 49 active great reed warbler nests, only conspecific eggs were preyed upon by great reed warblers. Furthermore, significantly more great reed warbler nests were destroyed in polygynous than monogamous territories. These results support the hypothesis that egg destruction behaviour in this species is motivated intraspecifically and most probably intrasexually.

Why do grebes cover their nests? Laboratory and field tests of two alternative hypotheses

Egg predation is a common feature influencing the reproductive success of open nesting birds. Evolutionary pressure therefore favours building cryptic, inconspicuous nests. However, these antipredatory pressures may be in conflict with thermoregulatory constraints, which select for dry nest material maintaining optimum temperature inside a nest cup during the absence of incubating parents. Here we examined possible trade-offs between nest crypsis and thermoregulation in Little Grebes (Tachybaptus ruficollis), which lay their eggs in floating nests built from wet plant material. As this species regularly covers its eggs with nest material, we experimentally examined (1) the rates of egg predation on covered and uncovered artificial nests and (2) possible thermoregulatory costs from nest covering by comparing temperature and relative humidity changes inside the nest cup. Results revealed that covering clutches is beneficial in terms of deterring predators, because uncovered eggs were more vulnerable to predation. Moreover, covering clutches also had thermoregulatory benefits because the mean temperature and relative humidity inside nest cups covered by dry or wet materials were significantly higher for covered compared to uncovered treatments. Covering clutches in Little Grebes therefore does not pose thermoregulatory costs.

Using nested RT-PCR analyses to determine the prevalence of avian influenza viruses in passerines in western Slovakia, during summer 2007

The prevalence of avian influenza virus (AIV), together with the distribution of different AIV subtypes, was studied in migratory waterfowl and terrestrial birds caught in western Slovakia during summer 2007. Both oropharyngeal and cloacal swabs were collected. Screening of samples revealed that 18% of oropharyngeal and 18% of cloacal samples were positive for AIV. Samples from both the oropharynx and cloaca were positive in only 6.6% of cases. A total of 10 different subtypes of haemagglutinin (H2, H3, H4, H6, H7, H9, H10, H11, H12, and H13) and 4 different subtypes of neuraminidase (N1, N2, N3, and N5) were detected in 32 samples from this location. The most abundant subtypes of HA in the samples were H12 and H9 (25% each), followed by H11 and H10 (15% each), and H13 (9%). There were 3 cases where different AIV infections were detected in oropharyngeal and cloacal samples originating from the same bird (H13N1 and H12N5; H13N3 and H9N5; H10N2 and H9N5 in the oropharynx and cloaca, respectively).

Reedbed structure and habitat preference of reed passerines during the post-breeding period

Post-breeding habitats of nine passerine species were studied in the permanently flooded reed beds of ponds near the village Pusté Úlany (SW Slovakia) in 2002. Structural features of reed beds and the abundance of all birds studied were sampled at two sites and eight study plots. The reed bed structure and abundance of four passerines differed considerably between the individual study plots. The variations in vegetation structure were also caused by winter reed burning in the SE part of study area. Stems in burnt reed beds were smaller and thinner than unburnt. Abundance of the reed warbler and to a lesser extent sedge warbler was higher in tall reed beds, while the bearded tit preferred thick reed stems. Position of reed beds along the shore — open water gradient was an important factor in terms of abundance at the study plots in the case of the reed bunting. This species showed a strong preference for the reed bed edge with open water. Horizontal distribution of other species seems to be random, however, the birds with the exception of Savi’s warbler preferred the upper stratum of reed bed in vertical distribution. Our field data corroborate with those found in experimental studies concerning with ecomorphological characteristics of birds. Some inconsistencies, however, could also be caused by other factors (e.g. foraging preferences). Comparison of breeding and post-breeding habitat requirements of the studied bird species confirmed a more or less random distribution of the majority of species in the post-breeding period. Some variation was found in the cases of only the reed warbler and bearded tit.

Testing for correlations between behaviours in a cuckoo host: why do host defences not covary?

Fitness costs associated with brood parasitism have led host species to evolve several lines of defence. The first two lines of defence, aggression against adult parasites and egg rejection, are present at varying levels in almost all hosts. However, it remains unclear how these two fundamental defences covary at host individual level, with previous studies suggesting both positive and negative correlations. A theoretically critical yet empirically untested scenario is that variation in host antiparasite behaviour may relate to individual variation in host behavioural types or personalities. Here we examined whether host aggression against adult brood parasites and egg rejection behaviour were correlated with host behaviours displayed outside the context of brood parasitism. We selected the great reed warbler, Acrocephalus arundinaceus, a favourite cuckoo, Cuculus canorus, host as a suitable model. Only females reject foreign eggs and show high individual repeatability of both aggression towards cuckoos and nest guarding. We found that female behaviours in different situations (nest guarding, nest defence, handling in the net) were strongly correlated with each other. This is the first empirical evidence on correlation between individually consistent antiparasite adaptation (female nest defence) and behaviours that are not directly related to brood parasitism. In contrast, egg rejection/acceptance responses and latency to these responses did not correlate with any of the female defence/guarding behaviours and behaviour during handling. Proximately, this may be because nest defence and egg recognition represent cognitively and behaviourally completely different tasks. These patterns were not affected by female mating status in this polygynous cuckoo host. We hypothesize that differences in host behavioural types, rather than host egg discrimination ability, may predict host nest defence behaviour against adult brood parasites in general.

Prevalence of avian influenza viruses, Borrelia garinii, Mycobacterium avium, and Mycobacterium avium subsp. paratuberculosis in waterfowl and terrestrial birds in Slovakia, 2006

The prevalence of Borrelia, Mycobacteria and avian influenza virus (AIV) infections, together with the distribution of different AIV subtypes, was studied in migratory waterfowl and terrestrial birds trapped in three localities in Slovakia during 2006. Samples obtained from waterfowl captured in the Senianske Ponds area of Eastern Slovakia showed the highest diversity of AIV isolates. A total of 13 different subtypes were detected in 19 samples from this location (H1N2, H2N2, H3N2, H6N6, H7N6, H9N2, H9N5, H9N6, H10N5, H10N6, H12N6, H13N6, and H16N6). H3N5 virus was detected in 50% of passerines testing positive for AIV in the Parizske Wetlands, with H7N2, H9N2, H9N5, H12N1, and H13N2 infections also recorded at this locality. H9N5 virus predominated in passerines captured at Trnava Ponds, with isolates H1N6, H6N5, H7N2, H7N6, H10N3, and H10N6 also detected at this location. There were five cases where different AIV infections were detected in oropharyngeal and cloacal samples originating from the same bird (H13N6 and H1N2; H10N5 and H12N6; H9N5 and H6N5; H10N6 and H7N6; and H9N2 and H3N5 in the oropharynx and cloaca, respectively). Between 21% and 52% of captured birds tested positive for Borrelia burgdorferi sensu lato, with the proportion infected depending on bird species and locality. Samples were characterized by polymerase chain reaction-restriction fragment length polymorphism analysis and identified as Borrelia garinii species (either B/B′ or R/R′ pattern). Mycobacteria were detected in 42% and 26% of waders captured at Senianske Ponds and marsh-dwelling passerines captured in the Parizske Wetlands, respectively. Interestingly, forest-dwelling passerine species caught in the Trnava Ponds region were tested negative for Mycobacteria.

Interacting Effects of Vegetation Structure and Breeding Patterns on the Survival of Great Reed Warbler Acrocephalus arundinaceus Nests

We studied the rate of predation on artificial and natural nests of Great Reed Warbler Acrocephalus arundinaceus in two contrasting vegetation types, reed Phragmites australis and reed mace Typha angustifolia beds. Reed provided thinner and taller stems to attach a nest to than reed mace, and reed mace provided more cover in late spring, but not in early spring. Nest density, the distance of nests from the water edge, and the timing of breeding differed considerably between vegetation types. However, there were no differences in survival rates of natural nests between both habitats. Artificial nests, on the contrary, were more frequently depredated in reed beds. Based on peck marks left on plasticine eggs, 74% of identified nest predators were large birds in both reed and reed mace beds. There was no correlation between the predicted predation rate of natural nests (derived from a logistic regression model using artificial nests) and the observed predation rate of natural nests, suggesting parental nest defence behaviour or subtle differences between actual nest sites and artificial nest sites might account for the observed discrepancy between natural and artificial predation rates. We suggest that interactions between vegetation structure, nest site choice and parental behaviour may have influenced nest predation rates in the Great Reed Warbler.