Seasonal variation in bird species richness and abundance in riparian galleries in Southern Portugal

Abstract

Riparian ecosystems are critical in maintaining biodiversity on a regional scale, which is particularly important for open agroforestry systems. We evaluated bird species richness and abundance in three different distances from the stream (0, 125 and 250 m) in Cork and Holm Oak forest systems (Montado) in southern Portugal. We used 5-minute point-counts to survey birds in two daily periods (morning and afternoon) of three different seasons (breeding season, summer-autumn migration and winter), to describe seasonal and daily variations in the use of riparian galleries and adjacent areas by birds. To assess whether birds move actively from the surrounding matrix into the riparian gallery, we installed mist-nets in mid-summer, autumn migration and winter periods, in two sites adjacent to streams, and recorded flight direction of all passerines trapped in the mist-nets. Both species richness and bird abundance were significantly higher in the riparian gallery than in the adjacent matrix. Species richness was significantly higher during the summer-autumn migration period, and bird abundance significantly lower during the breeding season. Apart from the Short-toed Treecreeper Certhia brachydactyla, Nuthatch Sitta europaea and Chaffinch Fringilla coelebs, all other species (e.g. Blackbird Turdus merula and Sardinian Warbler Sylvia melanocephala) were generally more abundant closer to the stream than at 250 m away. A significantly higher percentage of birds moved from the surrounding matrix into the riparian gallery in mid-summer, but not during the autumn migration and winter, which suggests that microclimatic conditions are important to explain observed seasonal differences. This study shows the importance of considering seasonal variation for the management of passerine bird populations in riparian galleries of Mediterranean areas. A well conserved riparian gallery appears to be a keystone structure exerting a strong influence on the number of bird species associated with surrounding agro-forestry systems such as the Montado.