Janne Ouwehand, Claudia Burger and Christiaan Both
Migratory birds have evolved life cycles that let them exploit food peaks at their breeding sites and then avoid harsh winter conditions by migrating to more hospitable environments. They face advances in the phenology of their habitats caused by climate warming, meaning that the timing of the appearance of important food sources has shifted forward. How do migrants with their complex annual cycles advance their spring arrival time to facilitate earlier breeding and maintain synchrony with the food peak? We studied this in pied flycatchers Ficedula hypoleuca breeding in the Netherlands. This insectivorous migrant winters in West-Africa and breeds in temperate forests across Europe and West-Asia. They start their spring migration without direct information on the progress of spring and food conditions at their breeding sites.
We consider two possible and not mutually exclusive mechanisms that can facilitate adjustments in their time schedules to climate change across generations: 1) hatch date directly affects timing later in life, and hence advancing hatch dates result in advancing migration dates, 2) Genetic variation in migration timing exists, and changes over time result from evolutionary changes.
We first describe seasonal variation in hatch date in relation to subsequent spring arrival and laying dates of offspring (2010-2014). Secondly, we experimentally delayed hatch dates by one week during three years (2009-2011) to test for the direct effect of hatch date, and measured if experimentally delayed young arrived later and if females laid their eggs at later dates in subsequent springs compared to control chicks.
In three out of five study years, naturally later born offspring also arrived later at the breeding sites in spring than earlier born offspring, but no effects were found on egg laying. The experimental delay in hatching resulted in both delayed arrival and laying in a single year relative to controls. This experimental effect was found only in the first year of life. Early hatching did not lead to early spring arrival dates in all return years, suggesting that individual variation in time schedules does not always become visible as arrival date differences at the breeding grounds. Moreover, effects of hatch date were only observed on spring arrival dates and not on egg laying dates, most likely because environmental conditions encountered at the breeding grounds affect the time interval between arrival and egg laying.
Our mixed results plead against a direct effect of hatch date as a major driver of advances in breeding dates in this species. Moreover, direct effects were only visible in first year birds. Hence, we consider it more likely that genes for the timing of migration caused most of the observed date effects, and that evolutionary change drives responses to climate change. Our results also suggest that the speed of adaptation is probably slowed down by environmental noise that obscures the genetic variation on which selection can act.
Image caption: Pied flycatcher chick before fledging, by RG Bijlsma