Climate change and bird phenology: a long-term study in the Iberian Peninsula

Many studies in recent years have demonstrated long-term temporal trends in biological parameters that can only be explained by climate change. Bird phenology has received great attention, as it studies one of the most conspicuous, popular, and easily observable phenomena in nature. There are many studies of long-term changes in spring arrival dates, most of which concur with earlier records from the last few decades. However, few data are available for autumn departures or length of stays. Furthermore, existing data offer an equivocal picture. In this study, we analysed a huge database of about 44 000 records for five trans-Saharan bird species (Ciconia ciconia, Cuculus canorus, Apus apus, Hirundo rustica and Luscinia megarhynchos). Data were collected from over 1300 sites around Spain during the period 1944-2004. Common spring arrival patterns were found in all species. Spring arrival dates have tended to advance since the mid-1970s. Current dates are similar to those from the 1940s (except for C. ciconia). Thus, the advance of spring migration over the last three decades could be seen as a return to the initial timing of arrival dates, after abnormally delayed arrivals during the 1970s. A strong negative relationship with temperature in Spain at arrival time was observed in all species. A negative relationship with the Sahel Index (a measurement of precipitation in the African Sahel area during the rainy season) for the previous year was also found in C. canorus, A. apus and H. rustica. Regarding autumn departures, all species showed common interdecadal fluctuations, but only H. rustica is leaving earlier Spain at present. All species departed earlier in years that had higher temperatures during their reproductive period. However, only for H. rustica the relation between Spanish temperatures at departure time and the last sightings of individuals was significant. A heterogeneous temporal response for the length of stay was also found: C. ciconia increased, A. apus did not change and H. rustica decreased its stay. This is the first study, based on an extensive bird phenology observational network covering a large region, that shows the most complete and thorough analysis available for the Mediterranean region.