Determinants of variation in productivity, adult survival and recruitment in a declining migrant bird:the Whinchat (Saxicola rubetra)

摘要

Populations of many species of Afro-Palearctic migrant birds, including the Whinchat (Saxicola rubetra), have shown severe declines over the last few decades. Habitat change on the breeding grounds, especially agricultural intensification, is thought to be the main driver of the decline in Whinchats. However, recent evidence also suggests that the decline may have a common source, such as low over-winter survival, which affects the whole UK population. To better understand the declines, this study investigated the key demographic parameters driving the breeding Whinchat population change on Salisbury Plain, UK, which is an area of agriculturally unimproved grassland where Whinchats are still relatively common. Territory settlement and nesting attempts of colour-ringed individual Whinchats were monitored intensively during 2012-2014. Pairs were significantly more likely to breed in sheltered valleys with long, grassy, structurally diverse vegetation and a high density of tussocks. Territories with an abundance of perches, for use in foraging, were also preferred. The first occupied territories, by returning birds in spring, tended to have higher invertebrate availability, and the order of territory occupancy was positively correlated between years, which suggests that territory quality was consistently perceived. Nestling starvation was rare because food availability did not limit reproductive output. Neither did the availability of suitable breeding habitat apparently limit the population, but nest productivity was lower than expected, mainly because of a high rate of nest failure due to nocturnal predators. Adult apparent survival was high relative to other open-nesting passerine migrants; however, the survival and recruitment of Whinchats in their first breeding year was low. This low apparent survival could partly be explained by natal dispersal, which was greater than breeding dispersal. From the reproductive output, survival and recruitment quantified in this study, it is apparent that the Salisbury Plain population is not currently self-sustaining.