Spring phenology delays in an insular subtropical songbird: is response to climate change constrained by population size?

Abstract

There is irrefutable evidence that spring phenology of birds, such as the timing of migration and reproduction, is advancing in response to ongoing climate change. However, most of the species and populations have been studied in temperate and northern latitudes, where annual seasonality of temperatures is stronger. Here, singing onset phenology in eight populations of the endemic Japanese bush warbler Cettia diphone riukiuensis from the subtropical Ryukyu and Sakishima Islands (24.3–28.4°N) has been studied for the period 1953–2005. Overall, males are singing 13 days later during the last five decades. Temperature increased in the study sites during the study period and most populations showed a negative effect of temperature before singing season. This apparently contradictory phenological response to climate change may be another evidence of the population declines detected in the endemic species of the small southern islands of Japan as a result of habitat loss and degradation. Patterns in the variability among the studied insular populations further confirmed this hypothesis. Those populations showing the strongest delays were those undergoing the higher increase of human population in their islands. However, this effect was strongly influenced by the island area. Phenology of birds in the smallest islands showed the smallest dependence on temperature, and consequently these populations are unable to adapt their responses to rising temperatures. This relationship is probably mediated by the poor genetic variability expected in the small insular populations. These results suggest that insular populations could be threatened by climate change besides particular threats at local scale suffered by each population.