Characterising demographic contributions to observed population change in a declining migrant bird
Populations of Afro-Palearctic migrant birds, which breed in Europe but winter in Africa, have shown severe declines in recent decades. To identify the causes of these declines, accurate measures of demographic rates (e.g. the number of fledglings per season, estimated survival from year to year, and immigration from other populations) are needed to allow effected targeting of conservation and research activities.
A new study by the BTO and the University of Lancaster focused on the Whinchat population of Salisbury Plain (Wiltshire), which has remained relatively stable against the declining European trend. From 2010 to 2014 Whinchats were studied in detail to measure demographic rates. Adults and nestlings were colour-ringed to calculate survival rates for adult and first year birds, while nests were found and monitored to measure breeding success. Intensive surveys to census Whinchats and re-sight colour-ringed birds were also carried out each year, which allowed natal and breeding dispersal to be calculated, i.e. how close to their previous year’s hatching site or nest site, respectively, Whinchats return.
These demographic rates were then used to build a population model to show how both on-site and external factors contribute to population change. Surprisingly, considering the relative stability of the population, the study identified low seasonal breeding success due to nocturnal predation and low apparent first year survival. This indicated a declining population trend. However, this trend was not reflected in the observed census counts, suggesting that high immigration from other UK breeding populations was probably balancing this decline.
The study demonstrates the potential of high quality breeding habitat, such as that on Salisbury Plain, to lower the rate of population decline. However, due to this population’s reliance on immigration, it will remain vulnerable in the future if the decrease in the wider UK population continues to reduce the pool of potential newcomers.
The results suggest that focusing conservation action on improving breeding success is likely to be an important immediate conservation strategy for Whinchats in Europe. In the long term, greater research emphasis is needed on migrant birds generally, as well as the factors that influence their immigration. It should also concentrate on separating first year fledglings’ dispersal and survival rates. Lastly, and importantly, it should focus on understanding the relationship between changes in demographic rates and the population trend in Whinchats, which could have implications for the conservation of wider Afro-Palearctic migrants as a whole.
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