Population monitoring and conservation implications of intra- and interspecific nest occupation rates in swallows

With the exception of a few groups of birds, such as large raptors and colonial seabirds, direct counts of nests cannot be conducted over very large areas for most of the abundant and widely distributed species, and thus indirect methods are used to estimate their relative abundances and population sizes. However, many species of the Family Hirundinidae (swallows and martins) build their mud nests in discrete, predictable and accessible sites, which are reused across years. Therefore, the direct count of active nests could constitute a reliable method for estimating breeding population sizes and their changes at large spatial and temporal scales. We illustrate the feasibility of this monitoring approach through a single year survey of >2700 nests of three coexisting Old-World species, the barn swallow (Hirundo rustica), the red-rumped swallow (Cecropis daurica), and the crag martin (Ptyonoprogne rupestris), distributed across Portugal and Spain. Our results revealed changes in the use of nesting substrates and increases in interspecific nest usurpation rates over recent decades. While 56% of the nests of C. daurica were located in rocks five decades ago, almost 100% are nowadays located in anthropogenic substrates such as bridges, road culverts, and abandoned buildings, which could have favored the range expansion of this species. Nest occupation rates were surprisingly low (12% in C. daurica, 21% in H. rustica, and 37% in P. rupestris), and the proportion of abandoned nesting sites was very high (65% in C. daurica, 50% in H. rustica, and 27% in P. rupestris). Abandonment rates reflect the population decline reported for H. rustica. Notably, the usurpation of nests of C. daurica by house sparrows Passer domesticus, which is the main cause of breeding failure, has increased from 2.4% in 1976–1979 to 34.7% of the nests nowadays. The long-term monitoring of nests may constitute a reliable and affordable method, with the help of citizen science, for assessing changes in breeding population sizes and conservation threats of these and other mud-nest building hirundines worldwide.