Influence of the local spring warming on the breeding phenology in blackcap (Sylvia atricapilla) in Croatia.

Recent papers have shown that climate change affects many species, including birds. Several papers from different areas have devoted attention to the negative correlation between the dates of breeding and year, and also negative correlation between the dates of breeding and air spring temperatures. Recent work give some firm evidence for postulating causal relationship between spring temperature and laying dates. We used 31 years (1979-2009) of data from blackcap Sylvia atricapilla in Mokrice area, Northwestern Croatia to assess whether there has been any systematic change in breeding phenology through time.Among environmental factors possibly affecting the breeding date, consideration was given to mean monthly air temperatures (April-May). Over the past three decades, the breeding date of blackcap in our study population has changed significantly: They started breeding progressively earlier (11.77 days). Two regression analysis with laying date as criterion variable showed that spring temperatures can significantly predict variation of laying date and that there are also some other unknown factors which significantly explain variation of laying date. We conclude that blackcaps across Northwestern Croatia are breeding earlier and that mean air spring temperatures is probably the most important factor causing it, among other factors.