Phenological data for the article: 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.

Methods
We studied a wild blue tit population in the Corsican forest of Pirio (France, lat: 42.38, lon: 8.75) dominated by evergreen holm oaks (Quercus ilex, Fagales: Fagaceae, see Blondel et al., 2006 and Charmantier et al., 2016 for an extensive description of the population characteristics).

From 1976 to 2019, the team weekly monitored the nests boxes -provided in abundance- during the reproduction season, between April and June (67 boxes in 1976, 187 now, see Charmantier et al. 2016, for further details). Laying date was deduced from the number of laid eggs, as females lay one egg per day. The team captured and banded parents, and estimated their ages at the nest boxes, when chicks were at least 10 days-old. The offspring were banded when 15 days-old, and one week later, the number of chicks surviving (fledgling success) was estimated for each brood.