When migratory birds leave cats, foxes and other predators behind, the are sometimes greeted by a faster threat: bird-killing raptors.
By Rex Graham
This avian “arms race” between prey and predator sometimes unfolds in the binocular views of scientists and birdwatchers. We witness life and death based not just on speed, but also on tactics.
Indiana State University Biology Professor Steven Lima once witnessed a Cooper’s Hawk ambush a Mourning Dove in one of these duels to the death. The dove plunged breast-first, full-speed into the ground, only to immediately “resume flight in the opposite direction before the hawk had a chance to respond and resume chase.”
It might not seem like an effective survival tactic, but it worked.
Avoiding avian predators
Foraging ducks also dive to avoid predators, but unlike Lima’s dove, they use a softer surface: water. Woodpeckers under attack simply hop to the opposite side of the tree trunk. Some fleeing pigeons fly under utility lines to gain separation from diving-bombing falcons.
“All animals must eat to survive, and virtually all are potential food for other animals,” says Lima. He has observed dozens of deadly encounters.
In nearly all cases, the best defense for a bird is to avoid detection. However, camouflage is no help during migration. Any bird is easier to spot in the air.
Mudflat Merlin menace
Many migrating shorebirds prefer featureless mudflats. These rich sources of worms and other invertebrates often peak in biomass just as the birds arrive. In this case, the fewer trees, the better: approaching Merlins and Peregrine Falcons are easier to detect.
Merlins are particularly fearsome predators for most shorebirds. They and Peregrine Falcons stoop directly at flocks of shorebirds. Often the highest ranking shorebirds feed in the middle of the flock. The less fit or less socially connected look for food on the margins: they are are more vulnerable there.
The initial attack is designed to panic and separate the flock. Those fleeing have limited options. They can try to escape by climbing, fly horizontally as fast as possible, or try a last-second turn or dive.
Eleonora’s Falcon climbing speed
“Generally, a smaller bird will out-climb a larger predator, however the Eleonora’s Falcon has a very high rate of climb for its size,” said Lund (Sweden) University ecologists Anders Hedenstrom and Mikael Rosén. “Prey species with an equal or higher capacity to climb fast, such as the Common Swift, usually adopt climbing escape when attacked by Eleonora’s Falcons.”
Eleonora’s Falcons nest along the shore of the Mediterranean Sea and its islands. They spend winters in Madagascar and Mozambique. Hedenstrom and Rosén used tracking radar and optical range finders to record the falcons’ predation tactics over the Mediterranean.
One species they recorded was the Common Swift, an exceptionally fast climber that breeds in Europe and Asia. It winters in southern Africa. The swifts’ fall migration takes them past Eleonora’s Falcons, but the raptors rarely attack swifts. Maybe they know it would be a waste of their time and energy.
Dunlins, Red Knots and Arctic Terns also are fast, powerful climbers, and Eleonora’s Falcons usually let them pass unobstructed.
Last-second speed dive
Another escape tactic is flat-out horizontal speed. However, outrunning a falcon is risky, too. Many falcon species win these races. Most waterfowl species fly at 40-55 mph (64-88 kph), but the Red-breasted Merganser is exceptional: tracking radar revealed that it reaches 96 mph (155 kph) in a shallow gliding dive.
While Peregrine Falcons, Northern Goshawks and other raptors can dive faster than the gliding dive of the Red-breasted Merganser, those diving raptors decelerate significantly near the end of their dives. “By adopting a moderate stooping speed, raptors may gain in hunting precision,” said the University of Lund’s Thomas Alerstam.
Many songbirds, shorebirds, waterfowl and other migratory birds form cohesive, fast-moving flocks. Under attack, rapid synchronized turns can keep all the members of the flock safe.
Shorebirds that spend more time well offshore, decreasing their body weight in order to maneuver faster. Closer to land, shorebirds tend to increase the size of their pectoral muscles in order to outrun their pursuers.
Difference between life and death
Attack and evasion tactics can change quickly depending on circumstances. As a Merlin closes the gap with a Dunlin in the estuaries of western Washington it may suddenly shift tactics. “The most common evasive measure used by isolated birds was a lateral dodge,” Washington and California researchers studying Merlin-Dunlin encounters said in The Wilson Bulletin.
Gazelles also use a tight turn at the last second to escape cheetahs. The cats can run faster, but the gazelles can execute tighter turns.
The same “turning gambit” is used by some moths trying to escape faster-flying bats in southeastern Arizona. On paper, the bats should win: they accelerate more quickly, fly faster and generally turn faster than their moth prey. However, some moths can make very tight turns at low speed, and then accelerate in a straight line toward a safe zone. “In many cases this could mean the difference between life and death,” said biologists from Wake Forest University who conducted the research.