Václav Jelínek

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.

Geographic variation in avian clutch size and nest predation risk along a productivity gradient in South Africa

Geographic variation in avian clutch size is thought to be related to the variation in nest predation rate and food availability. We studied predation on artificial ground nests along a large-scale geographic gradient in South Africa characterised by increasing productivity from the deserts in the west to humid savannas in the east, and calculated mean clutch sizes of birds occurring in atlas quadrates surrounding our study sites. Clutch sizes generally increased with increasing productivity and seasonality. The least productive desert site was characterised also by the highest predation rate, whereas all the other sites located in savannas revealed much lower and more or less constant predation rate. We found no evidence for relationship between nest predation rates and clutch sizes of ground-nesting birds. This indicates that food availability is the major factor responsible for geographical variation in bird clutch sizes across South Africa, though high predation rate might also contribute to low clutch size in least productive arid environments.

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.

Lazy males and hardworking females? Sexual conflict over parental care in a brood parasite host and its consequences for chick growth

Due to the costs of parental care, a conflict of interests often arises between mates wherein each prefers the other to invest more. As with parents raising their own offspring, hosts of brood parasites also exhibit negotiations over investment, becoming particularly intensive when parasite demands are high. Lack of cooperation between the partners may eventually affect the condition and fledging success of the young. Here, we investigate the magnitude of sexual conflict over food provisioning in socially polygynous great reed warblers (Acrocephalus arundinaceus) rearing either a parasitic common cuckoo (Cuculus canorus) or their own nestlings and its consequences for chick growth. We found that, overall, males provided less food than females, and that polygynous males provided less food per nest than monogamous males. Moreover, polygynous males provisioning two simultaneous broods supplied their own offspring in relation to age and type (cuckoo/host) of the other brood. Females, unlike males, delivered food amount almost irrespective of social status. The difference in contribution between polygynous males and their mates was most pronounced in nests with a cuckoo. In any case, reduced paternal assistance had no significant effect on growth performance of nestlings. In cuckoos, however, this result may be biased as we could not consider a relatively high proportion of secondary cuckoos that died before their growth parameters could be ascertained. Although not detected in chick growth, host sexual conflict over food provisioning may impose a cost on cuckoos in terms of increased mortality in secondary nests.

Do common cuckoo chicks suffer nest predation more than host nestlings

Nestlings of brood parasites exhibit more intensive begging than offspring of their hosts to gain sufficient amount of food or competitive advantage over host nestlings. This begging behaviour should be costly because exuberant acoustic begging may more likely attract nest predators. However, to date, nobody has explored the survival of nests with and without chicks of brood parasites in the common cuckoo (Cuculus canorus) host system. Here, we analysed an extensive dataset of 817 great reed warbler (Acrocephalus arundinaceus) and 788 reed warbler (Acrocephalus scirpaceus) nests to explore the relationships between nest predation and parasitism status (parasitized vs. non-parasitized), nest contents (cuckoo chick vs. host nestlings) and age of nestlings. We found that although parasitized nests had higher predation rate than non-parasitized nests in the incubation stage, the effect of original parasitism status almost disappeared in the nestling stage. In both host species, nests with younger cuckoo chicks survived similarly to nests with host nestlings of the same age (till the ninth day of age). Later on, however, nest contents influenced nest predation in each species differently. While nests with older cuckoo chicks (from the ninth to the 17th day of age) did not survive worse that host nestlings in the great reed warbler, older cuckoos survived much worse than host nestlings in reed warbler nests. Finally, nest survival decreased with nestling age in all three species. Thus, it seems that common cuckoo chicks can be penalized for more intensive begging only in nests of smaller reed warbler hosts. Significance statementParental feeding of young is in birds frequently accompanied by striking nestlings begging behaviour serving as a signal of their need. Brood parasites exhibit even more intense food solicitation than their hosts which may attract predators to the nest. However, this hypothesis has never been tested in a widely studied brood parasite species—the common cuckoo. Here, we analysed survival of more than 1600 nests of its two main host species. We found that nests containing older common cuckoo chicks were depredated more frequently than nests with host own nestlings only in the smaller reed warbler hosts but not in the larger and more aggressive great reed warblers. This shows that the intensity of begging could be costly in terms of nest predation at least in some common cuckoo host species.