Aquatic subsidies supplement the diet of Western Yellow Wagtails in an intensive agricultural landscape

With intensification of agriculture, it has become increasingly difficult for farmland birds to find food. The chicks of many farmland bird species depend on insects, yet terrestrial insects have shown steep declines in agricultural environments. Here we test whether aquatic emergent insects may provide a subsidy for farmland bird chicks. We studied the diet of Western Yellow Wagtail ( Motacilla flava ) nestlings in intensively managed flower bulb fields (90% agricultural area) surrounded by ditches (3% land cover) in the Netherlands. We combined camera observations at the nest, stable isotope analysis and DNA metabarcoding of nestling faeces to determine diet composition and the relative contribution of aquatic and terrestrial prey. We found 26 arthropod species from 6 orders using DNA metabarcoding. Diptera occurred in 75% of samples, mainly representing Tipulidae (43.6%) and Chironomidae (23.1%). Camera observations showed that Diptera were most abundant (77.1%), followed by Hymenoptera (4.2%) and Odonata (3.4%). Most prey biomass estimated from camera observations was terrestrial (43.8%), versus 21.4% aquatic prey. Stable isotope analysis indicated 47–69% terrestrial, 17–24% aquatic and 14–33% prey of unknown origin in the diet. DNA-metabarcoding showed 37% (semi-)aquatic origins of identified species and 63% terrestrial, based on larval ecology. We conclude that Yellow Wagtail nestling diet contained more insects of terrestrial origin, but was substantially subsidised by (semi-)aquatic insects. Our findings underscore the ecological importance of aquatic–terrestrial interfaces in enhancing food availability in intensive farmland to support conservation of breeding insectivorous birds.