Predator-prey relationships abound in the natural world. We tend to think of large charismatic animals such the wolf or tiger when "predator" comes to mind but swallows are predators of flying insects and cod are predators of capelin in just the same way as a wolf is a predator of a moose. Perhaps more akin to tigers than a swallows, the Peregrine Falcon is a bird predator, and its large size and power allow it to take quite large prey such as ducks and gulls. Indeed, the old North American name for the Peregrine Falcon was the "Duck Hawk". However, they take small birds too and when the Semipalmated Sandpipers flock on the shores of the upper Bay of Fundy at this time of year, local Peregrines take advantage of this food source. 

Semipalmated Sandpiper and Peregrine Falcon (1981) Peregrines don't have it all their own way though. The sandpipers have well developed any-predator behaviours such as flocking and fast, erratic flying in large flocks. This provides individual sandpipers with safety in numbers and also confuses the predator. The success rate of a hunting Peregrine on flocking sandpipers is surprisingly low, but it clearly must be worth their while (in other words, the energy and nutrients expended in capturing a sandpiper must be less than the what a sandpiper provides).

At the top of the food web, predators tend to be relatively rare in ecosystems, relying on a large base of prey below them in the food web. A few decades ago Peregrines were even rarer than normal likely because of low breeding success associated with thin-eggshells caused by DDT in the environment. These days, DDT levels are much lower and with the help of captive breeding programs, the species is returning to normal numbers. This is putting more pressure on populations of their prey both through direct predation and the disturbance that it inevitably causes.