The Hungry Worm Feeds the Bird

Earthworms (Lumbricidae) are important prey for many birds. Based on theirown feeding ecology, earthworms can be separated into two ecotypes: the detritivoresthat feed on organic material and the geophages that feed on soil particlesand organic matter. Detritivores collect their food on the surface during thenight when they are exposed to nocturnal predators. Hungry animals tend toshow more risk-prone behaviour and may therefore be more vulnerable to birdpredation, so we expect well-fed detritivorous earthworms to visit the surfaceless frequently. In this study, we tested this hypothesis in dairy farmland in theprovince of Fryslân, The Netherlands. Two uniform grasslands were split, witheach half receiving either an early (1 February 2014) or a late (14 March 2014)farmyard manure application. Every two weeks, nocturnal surface activity ofearthworms was measured by counting surfacing earthworms from a slowlypushed cart. Furthermore, soil samples were taken for total abundances and tomeasure individual body conditions of earthworms. As predicted, the density ofsurfacing earthworms was on average 2.5 times higher in the fields before farmyardmanure was applied. Immature detritivores had significantly lower bodymasses in fields not yet manured, suggesting that these growing earthwormswere hungry. Differences in surfacing behaviour and body mass disappearedafter all fields had been given farmyard manure. We conclude that hunger forcesdetritivorous earthworms to the surface. After manure application, they appearsatisfied and avoid the risk of depredation by birds by staying away from the soilsurface. To promote earthworm availability for meadow birds, spreading farmyardmanure on the surface should occur as late in spring as possible. In thisway, hungry earthworms are forced to the surface and are available as meadowbird prey for longer periods.