Effects of hay cropping on invertebrate biomass and the breeding ecology of Savannah Sparrows (Passerculus sandwichensis)

In hay fields in the northeastern United States, Savannah Sparrows (Passerculus sandwichensis) whose first nests fail as a result of the harvesting of hay renest in the same field in a dramatically altered habitat. We evaluated availability of invertebrate prey in two treatments (harvested and unharvested hay fields) in Vermont's Champlain Valley and assessed potential effects of food resources on clutch size, food provisioning by adults, and growth of nestling Savannah Sparrows. A relative measure of invertebrate biomass (sweep-net samples) showed a 36-82% decline after harvesting, compared with continual increases throughout the nesting season on unharvested fields. Clutch sizes were not significantly different between treatments. Nestling provisioning rates obtained through video observations differed between treatments, with birds on unharvested fields delivering 73% more food than those on harvested fields (P = 0.02). However, food provisioning differences did not translate into differences between treatments in either average nestling mass or the mass of the lightest nestlings within nests. A measure of total biomass (vacuum samples) on harvested fields showed a 28-56% decline after harvesting; this reduction was insufficient to induce food limitation based on the energetic requirements of Savannah Sparrows. Our results suggest that adult Savannah Sparrows must compensate for reduced food availability on harvested hay fields, possibly by increasing the total time spent foraging.