The impact of an invasive exotic bush on the stopover ecology of migrant passerines

Migration is highly energy-demanding and birds often need to accumulate large fuel loads during this period. However, original habitat at stopover sites could be affected by invasive exotic plants outcompeting native vegetation. The impact of exotic plants on the stopover behavior of migrant bird species is poorly understood. As a general hypothesis, it can be supposed that habitat change due to the presence of exotic plants will affect migrants, having a negative impact on bird abundance, on avian community assemblage, and/or on fuel deposition rate. To test these predictions, we used data obtained in August 2011 at a ringing station in a coastal wetland in northern Iberia which contained both unaltered reedbeds (Phragmites spp.) and areas where the reedbeds had been largely replaced by the invasive saltbush (Baccharis halimifolia). Passerines associated with reedbeds during the migration period were used as model species, with a particular focus on sedge warblers (Acrocephalus schoenobaenus). The saltbush promoted a noticeable change on bird assemblage, which became enriched by species typical of woodland habitats. Sedge warblers departed with a higher fuel load, showed a higher fuel deposition rate, and stayed for longer in the control zone than in the invaded zone. Invasive plants, such as saltbush, can impose radical changes on habitat, having a direct effect on the stopover strategies of migrants. The substitution of reedbeds by saltbushes in several coastal marshes in Atlantic Europe should be regarded as a problem with potential negative cons equences for the conservation of migrant bird species associated with this habitat.