Long-Term Trends in Laying date and Fledged Young of Barn Swallow Hirundo rustica in Two Regions of the Netherlands

Since 1992, Dutch volunteers have monitored the breeding attempts of Barn Swallows on a systematic basis. For two regions – Noord-Veluwe and Twente – a dataset of 27 years (1992–2018) was available. This paper examines the timing of breeding and the number of fledglings per successful breeding attempt. As spring and summer weather in Western Europe became on average warmer during this period, we analysed whether Barn Swallows adapted to this shift. The laying date of first broods advanced on average 2.3 days per ten years. The average advancement of laying date of second broods was 2.9 days per decade. The average number of fledglings per successful breeding attempt was constant over time, but showed large inter-year-variation. First broods consistently produced 0.6 fledglings more than second broods. These results are in line with results from similar studies in Western Europe. Contrary to our expectations, neither large scale nor local weather parameters were found to explain the advancement of laying and the number of fledged young. It is essential to study key species like Barn Swallows during all stages of their life cycle over a long time span and large spatial scale to be able to unravel potential effects of climate change. Our study demonstrates that volunteers can contribute essential demographic data. Ideally, these demographic parameters would be monitored with individually marked (colour-ringed) birds. Frequent quality checks and feedback to volunteers are key to maximize the quality of the data collected through long-term volunteer surveys.