Small changes in timing of breeding among subarctic passerines over a 32-year period

Many bird populations in temperate regions have advanced their timing of breeding in response to a warming climate in recent decades. However, long-term trends in temperature differ geographically and between seasons, and so do responses of local breeding populations. Data on breeding bird phenology from subarctic and arctic passerine populations are scarce, and relatively little data has been recorded in open-nesting species. We investigated the timing of breeding and its relationship to spring temperature of 14 mainly open-nesting passerine species in subarctic Swedish Lapland over a period of 32 years (1984-2015). We estimated timing of breeding from the progress of post-juvenile moult in mist-netted birds, a new method exploring the fact that the progress of post-juvenile moult correlates with age. Although there was a numerical tendency for earlier breeding in most species (on average -0.09 days/year), changes were statistically significant in only three species (by -0.16 to -0.23 days/year). These figures are relatively low compared with what has been found in other long-term studies but are similar to a few other studies in subarctic areas. Generally, annual hatching dates were negatively correlated with mean temperature in May. This correlation was stronger in long-distance than in short-distance migrants. Although annual temperatures at high northern latitudes have increased over recent decades, there was no long-term increase in mean temperature in May over the study period at this subarctic site. This is probably the main reason why there were only small long-term changes in hatching dates.