Temperature and density influence survival in a rapidly declining migratory shorebird

Planning effective conservation strategies for migratory birds can be tricky. It is crucial to identify and understand the potential risks to survival on the species’ breeding and wintering grounds as well as on their migration routes. Furthermore, conservation efforts need to be coordinated across all these locations (which often cross international boundaries) and target the pressures that are likely to have the largest impact on population trends.

For Curlews, a globally near-threatened species, any increase or decrease in their numbers is likely to be affected by how long they survive as an adult, as they live for 10-15 years on average. As with other migratory species, much of the previous research into the factors affecting their numbers has focused on the breeding season. However, in reality the breeding season is a small proportion of a Curlew’s annual cycle, and it is possible that pressures they face during other parts of their cycle could be having a greater impact on overall population trends.

Using data from the BTO Bird Ringing Scheme, BTO researchers have collaborated to uncover potential pressures on Curlew at different stages of their life cycle. By using established analysis techniques to take into account not only the likelihood of a bird’s survival, but also other factors such as the probability of someone reporting a bird’s colour ring, and the likelihood of recovering it deceased, the research team looked at data from over 15,000 ringed Curlews to determine what pressures (and at what life stage) may be most affecting population declines.

In line with previous research, the results showed that cold winter weather and increased population density negatively affects survival rates of Curlew wintering in the UK. However, overall adult survival rates are high and have actually increased in recent years – and the work here shows that, in fact, it is the low breeding success that is negatively impacting the global decline of the species. From these data, it is estimated that on average, Curlew are only successfully raising 0.25 chicks per nest. To stabilise and reverse Curlew declines, this number would need to be increased to 0.43 chicks per nest.

This work highlights that the international conservation focus for Curlew should be on increasing nest productivity, as well as maintaining survival rates for the adult population.