R. Veenstra, MSc

PhD thesis: Sensing the soil - Foraging black-tailed godwits in an agricultural grassland landscape (2026)

Black-tailed godwits are crowned the Dutch national bird, but surprisingly little is known about their foraging ecology on their breeding grounds. In this thesis, which focused on godwits breeding in Southwest Friesland, we used DNA metabarcoding of faeces, field observations and laboratory experiments to develop fundamental knowledge on godwit diet and foraging behaviour, as well as the behaviour of some of their prey, earthworms in particular. We found that, in March, godwits fed predominantly on earthworms. They particularly selected red earthworms – who survive better on traditionally managed grasslands – over grey earthworms. Moreover, godwits were able to find earthworms more easily in wetter soils. A potential explanation for those two results could be provided by our laboratory experiment. This showed that earthworm movements (which godwits may use to detect these prey items), and especially those of red earthworms, could more easily be detected in wet than in dry soils. In June, grassland soils were much drier and harder than in March and earthworms were unavailable to godwits. Hence, they fed mostly on leatherjackets, the larvae of crane flies. In this dry period, more available earthworms could be found in the relatively moist soils of traditionally managed surface-drained fields (‘greppelland’) than in those of conventionally managed, deep-drained fields. This thesis showed that the moisture content of grassland soils mediates the interaction between farmers, foraging godwits and earthworms in important ways, and highlights the relevance of providing moist soils for foraging godwits throughout their breeding season.