Long-Term Decrease in Coloration: A Consequence of Climate Change?

AbstractClimate change has been shown to affect fitness-related traits in a wide range of taxa; for instance, warming leads to phenological advancements in many plant and animal species. The influence of climate change on social and secondary sexual traits, which are associated with fitness because of their role as quality signals, is, however, unknown. Here, we use more than 5,800 observations collected on two Mediterranean blue tit subspecies (Cyanistes caeruleus caeruleus and Cyanistes caeruleus ogliastrae) to explore whether blue crown and yellow breast patch colorations have changed over the past 15 years. Our data suggest that coloration has become duller and less chromatic in both sexes. In addition, in the Corsican C.c. ogliastrae, but not in the mainland C.c. caeruleus, the decrease is associated with an increase in temperature at molt. Quantitative genetic analyses do not reveal any microevolutionary change in the color traits over the study period, strongly suggesting that the observed change over time was caused by a plastic response to the environmental conditions. Overall, this study suggests that ornamental colorations could become less conspicuous because of warming, revealing climate change effects on sexual and social ornaments and calling for further research on the proximate mechanisms behind these effects.

Genetic variance in fitness indicates rapid contemporary adaptive evolution in wild animals.

The rate of adaptive evolution, the contribution of selection to genetic changes that increase mean fitness, is determined by the additive genetic variance in individual relative fitness. To date, there are few robust estimates of this parameter for natural populations, and it is therefore unclear whether adaptive evolution can play a meaningful role in short-term population dynamics. We developed and applied quantitative genetic methods to long-term datasets from 19 wild bird and mammal populations and found that, while estimates vary between populations, additive genetic variance in relative fitness is often substantial and, on average, twice that of previous estimates. We show that these rates of contemporary adaptive evolution can affect population dynamics and hence that natural selection has the potential to partly mitigate effects of current environmental change.

Phenotypic plasticity drives phenological changes in a Mediterranean blue tit population.

Earlier phenology induced by climate change, such as the passerines' breeding time, is observed in many natural populations. Understanding the nature of such changes is key to predict the responses of wild populations to climate change. Genetic changes have been rarely investigated for laying date, though it has been shown to be heritable and under directional selection, suggesting that the trait could evolve. In a Corsican blue tit population, the birds' laying date has significantly advanced over 40 years, and we here determine whether this response is of plastic or evolutionary origin, by comparing the predictions of the breeder's and the Robertson-Price (STS) equations, to the observed genetic changes. We compare the results obtained for two fitness proxies (fledgling and recruitment success), using models accounting for their zero inflation. Because the trait appears heritable and under directional selection, the breeder's equation predicts that genetic changes could drive a significant part of the phenological change observed. We, however, found that fitness proxies and laying date are not genetically correlated. The STS, therefore, predicts no evolution of the breeding time, predicting correctly the absence of trend in breeding values. Our results also emphasize that when investigating selection on a plastic trait under fluctuating selection, part of the fitness-trait phenotypic covariance can be due to within individual covariance. In the case of repeated measurements, splitting within and between individual covariance can shift our perspective on the actual intensity of selection over multiple selection episodes, shedding light on the potential for the trait to evolve.

A new westward migration route in an Asian passerine bird.

The evolution of migration routes in birds remains poorly understood as changes in migration strategies are rarely observed on contemporary timescales.1-3 The Richard's Pipit Anthus richardi, a migratory songbird breeding in Siberian grasslands and wintering in Southeast Asia, has only recently become a regular autumn and winter visitor to western Europe. Here, we examine whether this change in occurrence merely reflects an increase in the number of vagrants, that is, "lost" individuals that likely do not manage to return to their breeding grounds, or represents a new migratory strategy.4-6 We show that Richard's Pipits in southwestern Europe are true migrants: the same marked individuals return to southern France in subsequent winters and geo-localization tracking revealed that they originate from the western edge of the known breeding range. They make an astonishing 6,000 km journey from Central Asia across Eurasia, a very unusual longitudinal westward route among Siberian migratory birds.7,8 Climatic niche modeling using citizen-science bird data suggests that the winter niche suitability has increased in southwestern Europe, which may have led to increased winter survival and eventual successful return journey and reproduction of individuals that initially reached Europe as autumn vagrants. This illustrates that vagrancy may have an underestimated role in the emergence of new migratory routes and adaptation to global change in migratory birds.9,10 Whatever the underlying drivers and mechanisms, it constitutes one of the few documented contemporary changes in migration route, and the first longitudinal shift, in a long-distance migratory bird.

Bacterial microbiota similarity between predators and prey in a blue tit trophic network.

Trophic networks are composed of many organisms hosting microbiota that interact with their hosts and with each other. Yet, our knowledge of the factors driving variation in microbiota and their interactions in wild communities is limited. To investigate the relation among host microbiota across a trophic network, we studied the bacterial microbiota of two species of primary producers (downy and holm oaks), a primary consumer (caterpillars), and a secondary consumer (blue tits) at nine sites in Corsica. To quantify bacterial microbiota, we amplified 16S rRNA gene sequences in blue tit feces, caterpillars, and leaf samples. Our results showed that hosts from adjacent trophic levels had a more similar bacterial microbiota than hosts separated by two trophic levels. Our results also revealed a difference between bacterial microbiota present on the two oak species, and among leaves from different sites. The main drivers of bacterial microbiota variation within each trophic level differed across spatial scales, and sharing the same tree or nest box increased similarity in bacterial microbiota for caterpillars and blue tits. This study quantifies host microbiota interactions across a three-level trophic network and illustrates how the factors shaping bacterial microbiota composition vary among different hosts.