Analysis and interpretation of long-term studies investigating responses to climate change

Effects of recent climate change in mean phenotypic values of different traits in wild bird populations are already apparent, but we are still largely ignorant about the mechanism underlying these changes. Likewise, numerous methodological issues complicating the interpretation of the observed patterns have emerged, but few of these have been widely recognised. Here, we review some of these problems inherent to long-term studies of wild bird populations in the context of understanding climate change effects on mean phenotypes in populations. In particular, we focus on methodological issues such as problems arising from sampling bias, as well as problems of differentiating between phenotypic plasticity, genetic adaptation and gene flow as causes of change in the mean phenotype of a population over time.