van Oosten H.H. (2025) Clutch size and breeding ecology of multi-brooded European Stonechats Saxicola rubicola in acidic inland heathlands and calcareous coastal dunes. ARDEA 113 (1): 20-20

I studied the breeding ecology of Stonechats Saxicola rubicola in acidic inland heathlands (the Veluwe, 2020–2023) and calcareous coastal dunes in The Netherlands (Noord-Hollands Duinreservaat, NHD; 2015, 2019–2021). Clutch size was on average smaller in the Veluwe because more clutches of five than of six eggs were produced, the opposite of NHD. Clutch size followed the same seasonal pattern in both sites: increasing until about mid-May, followed by a decrease, but clutches were consistently smaller in the Veluwe. The body mass of nestlings was also lower in the Veluwe and varied more in the course of the season, being the lowest between the end of May and early July. In the NHD body mass was lower only in the second half of May. Body mass increased in July in the Veluwe, perhaps because of the abundance of grasshoppers at that time which were commonly provided to nestlings. Nestlings were mainly fed with spiders, grasshoppers, beetles and caterpillars, with a lower fraction of caterpillars and a higher fraction of spiders in the Veluwe, suggesting a ower abundance of caterpillars. The distances at which adults collected food for nestlings did not differ between sites. Irrespective of dietary differences, feeding frequency was the same in both sites and did not change with season. This indicates a lower food quality in the Veluwe compared to the NHD. In addition, the length of the working day of parents feeding nestlings changed in parallel with changing daylength during the season. I suspect daylength to be an ultimate explanation for the clutch size peak in the multi-brooded Stonechat.