The semipalmated sandpipers arrive in their thousands and spend several weeks resting and mating on the shore, before leaving southern Canada for the warmer climes of South America.
Guy Beauchamp of the University of Montreal in Canada has been studying these birds for 10 years.
When they take to the skies, their flocks can be seen from a great distance. “We are talking about groups up to 100,000 birds,” Beauchamp says.
Such a huge concentration of birds makes the sandpipers attractive targets to peregrine falcons. These birds of prey loiter nearby, waiting for the right moment to swoop in and attack. But several years ago, Beauchamp realised that the falcons were behaving oddly.
Instead of going in for the kill as soon as the sandpipers arrive, the falcons wait and attack them at seemingly random times.
Presumably the falcons are hungry, so Beauchamp wondered why they do not attack immediately.
It pays to be vigilant at times when an attack is most likely
Perhaps, he thought, they purposely delay their attacks so that the sandpipers would never know when to expect them?
To find out if this was true, he spent several weeks each year monitoring how long it took falcons to attack.
He also noted the locations and sizes of the sandpiper roosts, to see how cautious the sandpipers were and how they responded to attacks. It pays to be vigilant at times when an attack is most likely, but they cannot maintain this state for too long because they need to rest and sleep.
Beauchamp discovered that falcons often delayed their attacks. Occasionally they attacked as soon as the roost was settled, but sometimes they waited for more than an hour. This made it impossible for a sandpiper to predict when an attack was coming.