Reproduction of the common buzzard at its northern range margin under climatic change

A changing climate induces shifts in the location of biomes. Tracing such a shift may pose problems for life history traits adapted to the prior conditions, so that, e.g. the timing of reproduction and the time with sufficient resources for rearing hatchlings do not match. We show that the timing of breeding of Finnish common buzzards Buteo buteo, has advanced with over ten days as a response to the warming of early spring, during 1979-2004. During the same period the isoclines of the onset of breeding have moved about 200 km to the north-east. However, the reproductive performance of the common buzzard has not increased as a response to these changes. Despite increasing temperatures during early spring, the temperatures of early summer have remained the same since the 1970s. Combined, the early onset of breeding and the unchanged temperatures of early summer have lead to decreased post-hatching temperatures. Under these circumstances, common buzzard offspring now face a higher risk to hatch into less favourable weather conditions than three decades ago. Furthermore, summer precipitation, harmful for nestlings, has been predicted to increase in the future, thus possibly further worsening the circumstances for breeding common buzzards. Our results demonstrate that even if common buzzards in Finland breed at the northern limit of the species' distribution, and could therefore be expected to gain advantage from a warming climate, the opposite is the case.