Herman van Oosten, Christian Kampichler & Hubert Kivit (2025) Increase of insectivorous birds in Dutch coastal dunes since 1982. LIMOSA 98 (2): 67-79.

Strong insect declines may lead to a decline in insectivorous birds. Such declines have indeed been reported at large spatial scales, but interpretation of the underlying causes remains challenging due to the diversity of the landscapes involved. Here, we analysed trends in the abundance of insectivorous birds occurring in a nature reserve in the Dutch coastal dunes (Noordhollands Duinreservaat), between 1982- 2020 using 31 monitoring plots. For comparison we also analysed trends in granivorous birds in the same reserve. Over the 38-year monitoring period, the total number of insectivores increased by 72% and their summed body masses by 12% (Fig. 2). Nation-wide species numbers increased by 18%, but their body mass decreased by 14%. Granivores declined by 16% in numbers and by 63% in total body mass over the same time period (Fig. 2). Trends varied by habitat: numbers of insectivores tripled in shrubland (Fig. 3; body mass +113%), increased by 70% in open dunes (body mass -27%) and increased by 40% in forests (body mass +58%). Trends were not the same for birds with different body masses. We conclude that total numbers of insectivorous birds in our study area strongly increased since 1982 in spite of widely reported insect declines, but their summed body masses did not always follow in parallel.

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