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The Hungry Worm Feeds the Bird

Earthworms (Lumbricidae) are important prey for many birds. Based on their own feeding ecology, earthworms can be separated into two ecotypes: the detritivores that feed on organic material and the geophages that feed on soil particles and organic matter. Detritivores collect their food on the surface during the night when they are exposed to nocturnal predators. Hungry animals tend to show more risk-prone behaviour and may therefore be more vulnerable to bird predation, so we expect well-fed detritivorous earthworms to visit the surface less frequently. In this study, we tested this hypothesis in dairy farmland in the province of Fryslân, The Netherlands. Two uniform grasslands were split, with each half receiving either an early (1 February 2014) or a late (14 March 2014) farmyard manure application. Every two weeks, nocturnal surface activity of earthworms was measured by counting surfacing earthworms from a slowly pushed cart. Furthermore, soil samples were taken for total abundances and to measure individual body conditions of earthworms. As predicted, the density of surfacing earthworms was on average 2.5 times higher in the fields before farmyard manure was applied. Immature detritivores had significantly lower body masses in fields not yet manured, suggesting that these growing earthworms were hungry. Differences in surfacing behaviour and body mass disappeared after all fields had been given farmyard manure. We conclude that hunger forces detritivorous earthworms to the surface. After manure application, they appear satisfied and avoid the risk of depredation by birds by staying away from the soil surface. To promote earthworm availability for meadow birds, spreading farmyard manure on the surface should occur as late in spring as possible. In this way, hungry earthworms are forced to the surface and are available as meadow bird prey for longer periods.