Gut microbiota in a host-brood parasite system: insights from common cuckoos raised by two warbler species.

An animal's gut microbiota (GM) is shaped by a range of environmental factors affecting the bacterial sources invading the host. At the same time, animal hosts are equipped with intrinsic mechanisms enabling regulation of GM. However, there is limited knowledge on the relative importance of these forces. To assess the significance of host-intrinsic vs environmental factors, we studied GM in nestlings of an obligate brood parasite, the common cuckoo (Cuculus canorus), raised by two foster species, great reed warblers (Acrocephalus arundinaceus) and Eurasian reed warblers (A. scirpaceus), and compared these with GM of the fosterers' own nestlings. We show that fecal GM varied between cuckoo and warbler nestlings when accounting for the effect of foster/parent species, highlighting the importance of host-intrinsic regulatory mechanisms. In addition to feces, cuckoos also expel a deterrent secretion, which provides protection against olfactory predators. We observed an increased abundance of bacterial genera capable of producing repulsive volatile molecules in the deterrent secretion. Consequently, our results support the hypothesis that microbiota play a role in this antipredator mechanism. Interestingly, fosterer/parent identity affected only cuckoo deterrent secretion and warbler feces microbiota, but not that of cuckoo feces, suggesting a strong selection of bacterial strains in the GM by cuckoo nestlings.

Unveiled feather microcosm: feather microbiota of passerine birds is closely associated with host species identity and bacteriocin-producing bacteria.

The functional relevance of microbiota is a key aspect for understanding host-microbiota interactions. Mammalian skin harbours a complex consortium of beneficial microorganisms known to provide health and immune-boosting advantages. As yet, however, little is known about functional microbial communities on avian feathers, including their co-evolution with the host and factors determining feather microbiota (FM) diversity. Using 16S rRNA profiling, we investigated how host species identity, phylogeny and geographic origin determine FM in free-living passerine birds. Moreover, we estimated the relative abundance of bacteriocin-producing bacteria (BPB) and keratinolytic feather damaging bacteria (FDB) and evaluated the ability of BPB to affect FM diversity and relative abundance of FDB. Host species identity was associated with feather bacterial communities more strongly than host geographic origin. FM functional properties differed in terms of estimated BPB and FDB relative abundance, with both showing interspecific variation. FM diversity was negatively associated with BPB relative abundance across species, whereas BPB and FDB relative abundance was positively correlated. This study provides the first thorough evaluation of antimicrobial peptides-producing bacterial communities inhabiting the feather integument, including their likely potential to mediate niche-competition and to be associated with functional species-specific feather microbiota in avian hosts.

Host genotype and age have no effect on rejection of parasitic eggs.

Egg rejection belongs to a widely used host tactic to prevent the costs incurred by avian brood parasitism. However, the genetic basis of this behaviour and the effect of host age on the probability of rejecting the parasitic egg remain largely unknown. Here, we used a set of 15 polymorphic microsatellite loci, including a previously detected candidate locus (Ase64), to link genotypes of female great reed warblers (Acrocephalus arundinaceus), a known rejecter, with their egg rejection responses in two host populations. We also tested whether host female age, as a measure of the experience with own eggs, plays a role in rejection of common cuckoo (Cuculus canorus) eggs. We failed to find any consistent association of egg rejection responses with host female genotypes or age. It seems that host decisions on egg rejection show high levels of phenotypic plasticity and are likely to depend on the spatiotemporal variation in the parasitism pressure. Future studies exploring the repeatability of host responses towards parasitic eggs and the role of host individual experience with parasitic eggs would greatly improve our understanding of the variations in host behaviours considering the persistence of brood parasitism in host populations with rejecter phenotypes.

Brood parasites lay eggs matching the appearance of host clutches.

Interspecific brood parasitism represents a prime example of the coevolutionary arms race where each party has evolved strategies in response to the other. Here, we investigated whether common cuckoos (Cuculus canorus) actively select nests within a host population to match the egg appearance of a particular host clutch. To achieve this goal, we quantified the degree of egg matching using the avian vision modelling approach. Randomization tests revealed that cuckoo eggs in naturally parasitized nests showed lower chromatic contrast to host eggs than those assigned randomly to other nests with egg-laying date similar to naturally parasitized clutches. Moreover, egg matching in terms of chromaticity was better in naturally parasitized nests than it would be in the nests of the nearest active non-parasitized neighbour. However, there was no indication of matching in achromatic spectral characteristics whatsoever. Thus, our results clearly indicate that cuckoos select certain host nests to increase matching of their own eggs with host clutches, but only in chromatic characteristics. Our results suggest that the ability of cuckoos to actively choose host nests based on the eggshell appearance imposes a strong selection pressure on host egg recognition.

Blue-green eggshell coloration is not a sexually selected signal of female quality in an open-nesting polygynous passerine.

It has been proposed that blue-green egg colours have evolved as a post-mating signal of female quality, selected by males allocating their parental effort in response to the strength of this signal. We tested two main assumptions of the sexually selected egg coloration hypothesis: (1) whether the intensity of eggshell blue-green chroma (BGC) reflects female quality; and (2) whether males make their decisions on the level of parental care that they provide according to the intensity of eggshell BGC. As a model species, we chose the facultatively polygynous great reed warbler (Acrocephalus arundinaceus). In this species, females simultaneously paired with the same male, compete for his nest attendance and could benefit from signalling their quality through egg coloration. However, we found no association between the variation in eggshell BGC and the measures of female quality (physical condition, mean egg volume and age). Moreover, great reed warbler males did not adjust their investment (as measured in terms of nest defence against a brood parasite) in relation to the eggshell BGC. We conclude that blue-green egg coloration in this open-nesting passerine is unlikely to have a signalling function. Rather, the large colour variation among clutches of individual females may depend on yearly fluctuations in environmental conditions.

Sex-specific defence behaviour against brood parasitism in a host with female-only incubation.

Nest protection against intruders is an indispensable component of avian parental care. In species with biparental care, both mates should evolve nest defence behaviour to increase their reproductive success. In most host-parasite systems, host females are predicted to have more important roles in nest defence against brood parasites, because they typically are primarily responsible for clutch incubation. Male antiparasitic behaviour, on the other hand, is often underestimated or even not considered at all. Here we investigated sex-specific roles in four aspects of great reed warbler (Acrocephalus arundinaceus) nest defence against a brood parasite-the cuckoo (Cuculus canorus), namely (1) mobbing, (2) nest attendance/guarding, (3) nest checking and (4) egg ejection. Using dummy experiments, simulating brood parasitism and by video-monitoring of host nests we found that males took the key roles in cuckoo mobbing and nest guarding, while females were responsible for nest checking and egg ejection behaviours. Such partitioning of parental roles may provide a comprehensive clutch protection against brood parasitism.