Passerines use of maize crop in addition to reedbed in autumn: abundance, diet and food availability in anthropogenic wetland

Abstract

Wetland habitats experienced a dramatic reduction and fragmentation of biodiversity, because of human activities such as urbanization and agriculture. Now birds, as indicator of this biodiversity, have to breed, winter or stopover in wetlands embedded in a complex and highly altered human matrix. They may concentrate their activities in the wet remnant (wet reedbed) or suboptimal habitats (dry reedbed) and surroundings such as agricultural fields (maize). In a wide wetland area situated south-west of France in a main migration route, we tested if the abundance of passerine species differs among habitats according to their specialization and ecology (wet reedbed vs dry habitats; aquatics vs generalists; migrant vs local), We attempt to identify the underlying mechanisms of observed variation, looking at: arthropod availability in each habitat, bird diet of five insectivorous species and refuelling capacity of birds. Maize crops hosted more invertebrates and biomass than reedbeds for Coleoptera, Diptera, Araneida and Cicadellidae. This may explain why crops were used by aquatic passerines (Bluethroat, Sedge warblers, Reed warblers), migrant or local generalists (Robin, Blue tit, Great tit, Willow Warblers and Nightingale). Bluethroat’s diet was more focused on Formicidae and used the both habitats. In spite of the available food in maize, specialist birds preferred reedbed: Cetti warbler feeding in mainly Araneida and Cicadellidae; Sedge warbler Aphid and Coleoptera. Dry reedbed were better used by Grasshopper Warblers foraging Formicidae. Sedge and Reed warblers were more abundant in wet reedbed. We also noted for this last species youngs refuelling in maize crop. Therefore, the strategy to use maize crop may be different if resident or migrant. Generalist resident may disperse searching for food or transit area in continuity of vegetation; migrant need refuelling, particularly the aquatic trans-saharans more specialist on reedbed than the others. Finally, maize crop provided food resources and suitable shelter for a large group of species. It may be a supplement habitat of the wet and dry reedbeds, but not a substitute. Our study reaffirms to conserve and extend wet reedbed habitats threatened by clogging bush encroachment and drying.