A new paper based on studies from the Fraser River estuary in British Columbia finds that hunting pressure from Peregrine Falcons (Falco peregrinus) is changing the behavior of Pacific Dunlin (Calidris alpina pacifica). The peregrine population crashed forty years ago, primarily thanks to pesticides, and has since rebounded. All of those new falcons need to eat, and shorebirds are suitable prey because of their size, numbers, and penchant for foraging and loafing in open spaces. Shorebirds have to adopt evasive behaviors to avoid being eaten.
One way of avoiding predation is to move to areas without much raptor activity. Some dunlin do this by finding safer roosting locations away from where peregrines are hunting, even if it means flying longer distances to reach them. Others utilize a behavior known as "over-ocean flocking": during high tides, they fly out to sea and remain in flight there for an extended period of time. This behavior was rarely noted by observers prior to the late 1990s, but since then it has been observed regularly. According to daily observations from January 2006, "over-ocean flocking" occurred on 15 out of 17 days, and dunlin spent an average 2.8 hours over the ocean at a time.
Most shorebirds build up their body fat reserves to avoid starvation during the winter. The extra weight helps them survive when food supplies are low, but has the trade-off of making it more difficult to escape predators in flight. In the face of increased predation, shorebirds like dunlin will need to carry less body fat to avoid becoming prey. According to bird banding data, that is exactly what happened when peregrines rebounded. Data from the 1970s shows that dunlin mass builds quickly and remains high through November and December before falling in January and February. By the 1990s, the average autumn peak weight had fallen by about 4 grams, or about 7% of their historical peak body mass.
This study is a good example of how conservation actions to benefit one species can have unintended results elsewhere in the ecosystem. In this case, the increased presence of peregrines around coastal areas is forcing dunlin to change their feeding and roosting behaviors.