Responses to climate change in avian migration time—microevolution versus phenotypic plasticity

While the evidence for advancement of spring phenology of animals and plants in response to recent climate change is overwhelming and undisputed, formal meta-analyses of avian migratory phenologies in response to climate change have not been conducted. Likewise, attempts to evaluate the relative roles of phenotypic plasticity versus evolutionary responses of observed advances in arrival times have been few. We conducted a meta-analysis of published data on timing of avian spring migration, with particular emphasis on evaluating whether the observed patterns are consistent with evolutionary explanations. In addition, we compared the observed rates of advancement with the theoretically expected rates of ‘sustainable evolution’. The meta-analysis confirmed a general advancement of avian migration time and that this advancement is correlated with climatic parameters. However, large-scale geographical patterns and relationships with age at first reproduction—a proxy of generation time—were less clear. The average rate of advancement expressed in haldanes (h = 0.07) was within theoretically predicted limits of ‘sustainable evolution’. All in all, while the results are in clear agreement with the assertion that birds are advancing their migratory schedules, they do not unambiguously support or refute the possibility that the observed responses would be genetic, rather than examples of phenotypic plasticity.