Kelderende zangvogeldichtheid op binnenlandse zandgronden in 1970-2020 gaat gepaard met afname van Smellekens Falco columbarius

In two separate regions on sandy soils, some 100 km apart in respectively the central and northern Netherlands (Veluwe and Drenthe) the numbers and winter presence of Merlins were recorded between 1970 and 2020. Prey remains were collected in open landscapes (notably large-scale arable land and extensive heaths), by checking natural and artificial plucking posts; the latter were specifically created to attract Merlins. Simultaneously, the absolute abundance and diversity of birds wintering on farmland and heathland were recorded in fixed plots of 25-100 ha (farmland) and 10-25 ha (heath). In the course of the study, farming practices changed considerably, from mixed farming with a high proportion of cereals to monocultures of maize, potatoes, sugar beets and – later still – lilies and peonies. At the same time, arable fields were increasingly set aside and converted into ‘nature’, often heavily grazed with cows and ponies. The conversion resulted in short-lasting bonanzas of herbage which attracted flocks of seed-eaters. Usually within less than 10 years monocultures of roughly grazed land remained. For the present study, only censuses in January-February were used, except when absent during that period and censuses in December had to be substituted. Most fieldwork on Veluwe took place in 1970-90, and in Drenthe in 1990-2020, but with overlap (1980s in Drenthe, 1991-2015 on Veluwe, but then in both regions with far fewer field days than normally; cf. Appendix 5 for basic data). Altogether, fieldwork in September April encompassed 2958 field days on Veluwe and 4657 field days in Drenthe, resulting in the recording of respectively 162 (33% wintering) and 101 (16% wintering) Merlins. A total of 138 prey remains were collected, of which 1 vole and 137 birds in 21 species varying in body mass between 8 (Long-tailed Tit) and 122 g (Snipe). Twelve bird species were represented with a single prey item. Most important prey species were Skylark and Meadow Pipit on Veluwe (resp. 41% and 17% in numbers, resp. 51% and 10% in biomass) and Meadow Pipit and Starling in Drenthe (46% and 17% in numbers, 23% and 32% in biomass). Prey choice largely reflected local presence and abundance of bird species of <150 g available during winter on arable land and heaths, i.e. the main hunting habitats of inland Merlins. Combined for both regions Merlin numbers (during migration and wintering) showed an increase in the early 1970s, peaking around 1980 and 1990 (dip in between) and steadily and consistently declining numbers from 1990 through 2020. Wintering on inland sandy soils, of regular occurrence in the 1970s and 1980s, has become a rarity later on. The decline in number of Merlin observations is consistent with the depletion of wintering passerines of <150 g on arable land and heaths, on average amounting to -90% in biomass (arable ánd heath) on Veluwe and -82 (heath) to -95% (arable) in Drenthe in the past half century, notwithstanding temporary food bonanzas following conversion of farmland into ‘nature’. The depletion of avian food in winter is thought to be the overriding cause of decline in Merlin numbers on inland farm- and heathland, rather than declining breeding numbers in Fennoscandia and Scotland (for which evidence is scarce or absent, stable numbers more often mentioned). Interspecific food competition with Peregrine Falcon Falco peregrinus is probably negligible, given the small overlap in prey lists; of 275 winter prey of Peregrines 31% weighed less than 150 g, in terms of biomass only 10%. Risk of intraguild predation (both Goshawks Accipiter gentilis and Peregrine Falcons increased in numbers from the 1970s onwards) may be higher nowadays, but no Merlins were found as prey among 22,351 prey remains of Goshawks (Veluwe + Drenthe, 1974-2021) or among 275 prey remains of Peregrines (ditto). In seven instances of prey-carrying Merlins, kleptoparasitic behaviour by other raptor species (including omnipresent Buzzards Buteo buteo) was never recorded. Also, Merlin numbers showed no correlation with local winter severity whatsoever. Of all the possible factors influencing numbers of Merlins on sandy soils in The Netherlands, as migrant and wintering resident, the spectacular crash in winter food supply (birds of <150 g) is suggested as the most likely candidate.