ORIGINAL ARTICLE Spring phenology delays in an insular subtropical songbird: is response to climate change constrained by population size? Oscar Gordo • Hideyuki Doi Received: 8 March 2010 / Revised: 4 August 2011 / Accepted: 23 August 2011 / Published online: 27 September 2011 Ó Dt. Ornithologen-Gesellschaft e.V. 2011 Abstract There is irrefutable evidence that spring phe- nology 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 ende- mic 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 appar- ently 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 pop- ulations further confirmed this hypothesis. Those popula- tions 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. Keywords Japan Á Cettia diphone Á Singing phenology Á Long-term study Á Warming Zusammenfassung Verzo ¨gerungen in der Fru ¨ hjahrspha ¨nologie bei einem inselbewohnenden subtropischen Singvogel: ist die Reaktion auf den Klimawandel durch die Populati- onsgro ¨ße eingeschra ¨nkt? Es gibt unwiderlegbare Beweise, dass die Fru ¨hjahrspha ¨- nologie von Vo ¨geln, wie zum Beispiel das Timing von Zug und Fortpflanzung, als Antwort auf den anhaltenden Klimawandel verfru ¨ht ist. Die meisten Arten und Popula- tionen wurden jedoch in gema ¨ßigten und no ¨rdlichen Brei- ten untersucht, wo die Temperaturen im Jahresverlauf sta ¨rker schwanken. Hier haben wir die Gesangspha ¨nologie in acht Populationen des endemischen Japan-Buschsa ¨ngers Cettia diphone riukiuensis auf den subtropischen Inseln Communicated by T. Friedl. Electronic supplementary material The online version of this article (doi:10.1007/s10336-011-0750-x) contains supplementary material, which is available to authorized users. O. Gordo Department of Zoology and Physical Anthropology, Faculty of Biology, Complutense University of Madrid, 28040 Madrid, Spain H. Doi Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky University Oldenburg, Schleusenstrasse 1, 26382 Wilhelmshaven, Germany H. Doi (&) Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan e-mail: doih@hiroshima-u.ac.jp 123 J Ornithol (2012) 153:355–366 DOI 10.1007/s10336-011-0750-x Author's personal copy