Predation risk on incubating adults constrains the choice of thermally favourable nest sites in a plover

Birds are thought to choose nest sites that meet two main functions: providing security to both nest contents and incubating adults, and providing an appropriate microclimate for incubation. Many shorebirds nest in sites with no or little cover. In a lake in southern Spain, nearly 70% of the nests of Kentish plovers, Charadrius alexandrinus, were in sites with little or no cover, where ambient temperatures might be more than 50(degree)C during very hot days, thus causing the incubating adults to suffer from heat stress. We tested the hypothesis that Kentish plovers nest mainly in exposed sites because this may allow the incubating birds to detect approaching predators early, and thus to reduce predation risk. When we occluded the view that incubating adults had from their nests, they took longer to detect approaching predators than when the view was unrestricted. Incubating adults were also more frequently killed by mammals in covered than in exposed nests. Females that nested in covered sites were in lower body condition than those nesting in exposed sites, possibly because they were unable to withstand the high ambient temperatures in exposed sites. Thus, the benefits of thermally favourable nest sites are reduced by the constraints of predation risk.