Zwarte Sterns Chlidonias niger in veenweidemenu van een Torenvalk Falco tinnunculus

In polders in the peat district of Krimpenerwaard in the western Netherlands numbers and breeding performance of Black Terns and Kestrels were simultaneously recorded during the breeding season of 2021. In 1 out of 26 nest boxes occupied by Kestrels in the Krimpenerwaard the remains of at least 16 Black Terns were found (15 adults, 1 juvenile). Within a 1300 m radius of this nest four colonies of Black Terns were located, as well as two other successful Kestrel pairs. At the latter two nests, no tern remains were found, nor at any other Kestrel pair elsewhere in the Krimpenerwaard. Instead, prey remains found in those nest boxes comprised of Sturnus vulgaris (5), Rallus aquaticus (1), Streptopelia decaocto (1), unidentified birds (2), Microtus spp. (7), Sorex spp. (2) and Gryllotalpa gryllotalpa (1). Black Tern predation coincided with a relatively early start of laying of Kestrel TVK-41c, i.e. 1 May, resulting in the terns’ colonies being in full swing when Kestrel food demand was highest (5 chicks to feed). Two Kestrel pairs, one of which breeding even closer to the same tern colonies, started laying much later (24 and 28 May), hence out of sync with breeding activities of local Black Terns; no prey remains of terns were found in their nest boxes (nor any other Kestrel pair, of which mean start of laying was 4 May (N=20). Whether vole numbers influenced tern predation by Kestrels (e.g. by Kestrels compensating poor vole numbers with increased tern predation), remained unknown, as vole density was not quantified. The impact of Black Tern predation by a single Kestrel pair also remained elusive. Even when all 15 killed adult Black Terns had been taken from the nearest colony (of 16 pairs), the colony’s breeding success (1.8 fledglings/pair) was quite high and in fact one of the best of all colonies monitored in the Krimpenerwaard. The possible impact of Kestrel predation on future colony numbers of Black Terns remains to be seen.