Phenology-mediated effects of phenotype on the probability of social polygyny and its fitness consequences in a migratory passerine

Environmental conditions influence host-parasite interactions and host fitness in a migratory passerine

The study of host-parasite co-evolution is a central topic in evolutionary ecology. However, research is still fragmented and the extent to which parasites influence host life history is debated. One reason for this incomplete picture is the frequent omission of environmental conditions in studies analyzing host-parasite dynamics, which may influence the exposure to or effects of parasitism. To contribute to elucidating the largely unresolved question of how environmental conditions are related to the prevalence and intensity of infestation and their impact on hosts, we took advantage of 25 years of monitoring of a breeding population of pied flycatchers, Ficedula hypoleuca, in a Mediterranean area of central Spain. We investigated the influence of temperature and precipitation during the nestling stage at a local scale on the intensity of blowfly (Protocalliphora azurea) parasitism during the nestling stage. In addition, we explored the mediating effect of extrinsic and intrinsic factors and blowfly parasitism on breeding success (production of fledglings) and offspring quality (nestling mass on day 13). The prevalence and intensity of blowfly parasitism were associated with different intrinsic (host breeding date, brood size) and extrinsic (breeding habitat, mean temperature) factors. Specifically, higher average temperatures during the nestling phase were associated with lower intensities of parasitism, which may be explained by changes in blowflies' activity or larval developmental success. In contrast, no relationship was found between the prevalence of parasitism and any of the environmental variables evaluated. Hosts that experienced high parasitism intensities in their broods produced more fledglings as temperature increased, suggesting that physiological responses to severe parasitism during nestling development might be enhanced in warmer conditions. The weight of fledglings was, however, unrelated to the interactive effect of parasitism intensity and environmental conditions. Overall, our results highlight the temperature dependence of parasite-host interactions and the importance of considering multiple fitness indicators and climate-mediated effects to understand their complex implications for avian fitness and population dynamics.

Reduced fitness of secondary females in a polygynous species: a 32-yr study of Savannah sparrows

The evolution of mating systems reflects a balance of the often-conflicting interests of males and females. Polygyny, a mating system in which males have multiple mates, presents a fitness benefit to males, but the consequences for females are less clear. Females with polygynous social mates may suffer reduced fitness, especially secondary females who typically receive less male support. We used 32 yr of detailed reproductive data on a population of Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, NB, Canada, to investigate the effects of females' social mating status on 6 indices of female fitness: survival, clutch size, fledging success, number of fledglings produced per nest and annually, and recruitment of offspring. Secondary females produced fewer fledglings per nest and annually than did monogamous or primary females, and their young were less likely to recruit into the breeding population. Yearling secondary females also had lower survival rates than older secondary females. Combined with higher rates of partial brood loss among secondary females, our results suggest that secondary females are unable to provide enough care to consistently fledge all nestlings in their broods, likely due to reduced male provisioning. Given that the sex ratio of breeders in the population is female-biased, we suggest that polygyny persists despite its fitness costs because some females must mate polygynously to "make the best of a bad situation." Our study demonstrates the value of detailed, long-term population monitoring data for understanding mating systems and using multiple indices of fitness to analyze the costs of polygyny.

A Case of Facultative Polygyny in an Enigmatic Monogamous Species, the European Nightjar (Caprimulgus europaeus)

In many socially monogamous bird species with biparental care, occasional social polygyny has been detected. We provide information about a case of facultative polygyny in the European Nightjar (Caprimulgus europaeus). The male nightjar (I96) formed a pair with two females (I95: the presumed primary female with whom he already bred since 2018; M042: the presumed secondary female, an inexperienced yearling). GPS and accelerometer data demonstrate how the male only sang in proximity of the primary nest, while assisting both females during incubation, as well as during the nestling period. When the male came to the nest, the primary and/or secondary female went foraging, but the secondary female received less assistance during incubation than the primary female, and her eggs were often left unattended. However, once the chicks of the secondary female hatched, male assistance suddenly increased, presumably at a cost to the primary female. Being only the second record of social polygyny in the European Nightjar, we do not have a direct explication for the occurrence of this polygynous event. We note that male density at the study site was lower than that observed in previous seasons. The male may have taken over the female that was initially paired to a neighbouring territory holder that then died. Alternatively, the inexperienced female might have mated with an already paired male, either because she was not aware of the mating status of the male, or because she could not find an unpaired male, or because mating with this paired male was better than mating with another unpaired male. In any case, the breeding ecology and mating behaviour of this crepuscular bird species remains little understood.

Long-term annual and spatial variation of polygyny in the white-throated dipper (Cinclus cinclus)

Mating strategies are key components in the fitness of organisms, and notably in birds the occurrence of monogamy versus polygyny has attracted wide interest. We address this by a very comprehensive dataset (2899 breeding events spanning the years 1978-2019) of the white-throated dipper Cinclus cinclus. Though the mating system of this species has been regarded as generally monogamous, we find that 7% of all breeding events were performed by polygynous males (approximately 15% of all pairs). The fraction of polygyny has been stable over the entire study period irrespective of population size. The assumption that polygyny is most common at low population density was not supported. Surprisingly, there was no difference between polygynous and monogamous males with regard to the quality of the territories they inhabited, ranked according to their overall frequency of use. The most common age group, first-year breeders, dominated among monogamous males, while among polygynous males second-year breeders were most common, followed by third and first-year breeders. The primary females were in general older than females mated to monogamous males, also when controlled for their general frequency in the population. The majority of the two females mated to a polygynous male, bred in the vicinity of each other. The probability for a male to be involved in polygyny more than once, was significantly higher than by chance, suggesting phenotypic quality differences among males.

Phenotypic selection on an ornamental trait is not modulated by breeding density in a pied flycatcher population

Most studies of phenotypic selection in the wild have focussed on morphological and life‐history traits and looked at abiotic (climatic) variation as the main driver of selection. Consequently, our knowledge of the effects of biotic environmental variation on phenotypic selection on sexual traits is scarce. Population density can be considered a proxy for the intensity of intrasexual and intersexual competition and could therefore be a key factor influencing the covariation between individual fitness and the expression of sexual traits. Here, we used an individual‐based data set from a population of pied flycatchers (Ficedula hypoleuca) monitored over 24 years to analyze the effect of breeding density on phenotypic selection on dorsal plumage colouration, a heritable and sexually selected ornament in males of this species. Using the number of recruits as a fitness proxy, our results showed overall stabilizing selection on male dorsal colouration, with intermediate phenotypes being favoured over extremely dark and dull individuals. However, our results did not support the hypothesis that breeding density mediates phenotypic selection on this sexual trait. We discuss the possible role of other biotic factors influencing selection on ornamental plumage.

High frequency of social polygyny reveals little costs for females in a songbird

Mating system theory predicts that social polygyny—when one male forms pair bonds with two females—may evolve by female choice in species with biparental care. Females will accept a polygynous male if the benefit of mating with a male providing high-quality genes or rearing resources outweighs the cost of sharing mate assistance in parental care. Based on this rationale, we hypothesise that the population frequency of social polygyny (FSP) varies due to changes in mate sharing costs caused by changing environmental conditions. We predicted that: (1) polygamous females (i.e. mated with a polygynous male) pay a survival cost compared to monogamous females; (2) FSP would be higher in years with better rearing conditions and (3) the difference in survival rates between monogamous and polygamous females would be small following years with higher FSP. We tested these predictions using regression and multistate analyses of capture-recapture data of pied flycatchers, Ficedula hypoleuca, in central Spain collected over 26 years (1990–2016). Monogamous females had a higher mean survival rate than polygamous females (prediction 1), but there was no difference in survival between polygynous and monogamous males. In addition, FSP was positively associated with annual reproductive success (a proxy of the quality of rearing conditions—prediction 2). Finally, following years with high FSP, the survival of polygamous females was similar to that of monogamous females (prediction 3), while the chance of breeding in a polygamous state for 2 years in a row increased for both males and females. Our findings suggest that fluctuating environmental conditions may be a necessary but neglected aspect of understanding social polygyny mechanisms.

Fluctuating selection driven by global and local climatic conditions leads to stasis in breeding time in a migratory bird

The origin of natural selection is linked to environmental heterogeneity, which influences variation in relative fitness among phenotypes. However, individuals in wild populations are exposed to a plethora of biotic and abiotic environmental factors. Surprisingly, the relative influence of multiple environmental conditions on the relative fitness of phenotypes has rarely been tested in wild populations. Identifying the main selection agent(s) is crucial when the target phenotype is tightly linked to reproduction and when temporal variation in selection is expected to affect evolutionary responses. By using individual-based data from a 29-year study of a short-lived migratory songbird, the pied flycatcher (Ficedula hypoleuca), we studied the relative influence of 28 temperature- and precipitation-based factors at local and global scales on selection on breeding time (egg laying) at the phenotypic level. Selection, estimated using the number of recruits as a proxy for fitness, penalized late breeders. Minimum temperatures in April and May were the environmental drivers that best explained selection on laying date. In particular, there was negative directional selection on laying date mediated by minimum temperature in April, being strongest in cold years. In addition, nonlinear selection on laying date was influenced by minimum temperatures in May, with selection on laying date changing from null to negative as the breeding season progressed. The intensity of selection on late breeders increased when minimum temperatures in May were highest. Our results illustrate the complex influence of environmental factors on selection on laying date in wild bird populations. Despite minimum temperature in April being the only variable that changed over time, its increase did not induce a shift in laying date in the population. In this songbird population, stabilizing selection has led to a three-decade stasis in breeding time. We suggest that variation in the effects of multiple climatic variables on selection may constrain phenotypic change.