Arthropods and novel bird habitats: do clear-cuts in spruce plantations provide similar food resources for insectivorous birds compared with farmland habitats?

Arthropods, and insects in particular, constitute important food resources for several higher trophic levels like birds. Their abundance and diversity is likely to differ between habitat types depending on the local conditions and resources. This may have important consequences for arthropod consumers that occupy structurally different habitat types. Most bird-focused studies address, however, habitats at the structural, vegetation-based level and disregard the presence of sufficient quantities and qualities of arthropod prey items. Here, we compare the quantity and quality of ground-dwelling and above-ground arthropods as food resources for early-successional birds between two structurally different human-modified habitat types sharing similar bird assemblages: low-intensity farmland areas and plantation forest clear-cut areas in the south of Belgium. Forest clear-cut patches constitute a novel habitat for so-called ‘farmland’ birds. Our results show that arthropod abundance is substantially higher in farmland than in forest clear-cuts, although arthropods are slightly larger in clear-cuts. Higher arthropod abundance is associated with higher ground-level temperature in farmland. Although both habitat types host the same spectrum of arthropod species, forest and farmland management practices induce different conditions for food quantity and, to some extent, food quality for insectivorous birds. We discuss the mechanisms behind the observed pattern of arthropod abundance and the fitness-related consequences of contrasting food availability in farmland and forest clear-cut habitats for early-successional bird species.