Heung Ying Janet Chik , Catalina Estrada, Yiqing Wang, Priyesha Vijendra, Alex Lord, Julia Schroeder

In the plant-insect-insectivorous-bird system, changing climates can result in mis-timing in bird reproduction, potentially impacting chick survival. To adapt to earlier prey emergence, birds can make use of phenotypic plasticity, which can be characterized by reaction norms. Despite gaining focus in research, studies on avian reproductive reaction norms as traits are scarce, particularly on laying-date-budburst-date and clutch-size-laying-date reaction norms. Here we examined the possibility of evolution of these two reaction norms from a quantitative genetics viewpoint, and tested whether 1) there is among-individual variance in reaction norms properties (intercept and slope); 2) there is selection on these reaction norm properties; and 3) variances and selection pressures differ between the two reaction norms. Data of oak (genus Quercus) budburst and blue tit (Cyanistes caeruleus) reproduction were collected from a wild population for 18 years. We used bivariate random regression models with a Bayesian approach to test for among-individual variance in reaction norm properties and their covariance with fitness. Individuals significantly differed in intercepts and slopes of both laying-date-budburst-date and clutch-size-laying-date reaction norms, and directional selection was present for an earlier laying date and a larger clutch size, but not on either plasticity. Results suggested that variation in reaction norm properties can be attributed to genetic and environmental effects, and that stabilizing selection on plasticity could be tested as a next step. This study compliments previous research on the evolution of reaction norms and helps shed light on their genetic structure, the context of their selection, and their mediation in birds.