Scott Stephens, PH.D
Imagine a living missile armed with long, sharp hooks and guidance systems locked onto you, coming out of the sky at nearly 200 miles per hour. That's exactly what the ducks I was watching one sunny January day were worried about as they nervously eyed the peregrine falcon circling high over their pond.
The amazing eyesight of raptors is difficult for humans to comprehend. In fact, these birds may have the keenest vision in the animal kingdom. Hawks have, on average, five times more photoreceptors in their eyes than humans, giving them visual acuity estimated to be 10 times greater than our own. That means something that we can see at a distance of 100 yards, a raptor can see from a mile away—or a mile up. For waterfowl, that's a serious concern.
The threat posed by raptors can affect waterfowl behavior in many ways. For example, researchers observed that gadwalls wintering in Louisiana increased their feeding activity at night to avoid harassment by northern harriers (once called marsh hawks) during daylight hours. Waterfowl also select wetland habitats that provide them with greater protection from raptors and move to new habitats where raptors are less prevalent.
Waterfowl use a variety of tactics to elude raptors. When threatened by bald eagles or northern harriers, green-winged teal and cackling geese take to the air, make several tight circles as a flock, and then return to their original location when the perceived danger has passed. Similar to schooling fish, this flocking behavior confuses raptors by making it more difficult for them to focus on a single target. While effective, this strategy consumes valuable time and energy. And if raptors are sufficiently abundant or persistent, individual birds can become separated from their mates or family groups.
Returning to those ducks I was watching on the pond in January, a peregrine falcon causes the opposite reaction. Peregrines (once called duck hawks) prefer to grab or knock down birds (as large as mallards and pheasants) in the air, so the instinctive response of waterfowl is to stay on the water when one of these falcons is spotted. Capable of reaching speeds of 200 miles per hour while in a "stoop," peregrines are the avian equivalent of a Ferrari. The ducks I watched that day simply refused to fly, even when I walked to the edge of the pond. They were much less worried about me than about the falcon.
Of course, one of the best ways to avoid becoming prey is to avoid being seen in the first place. Studies show that the brighter the plumage of potential prey, the greater their chance of being spotted and taken by a raptor. Because female ducks are particularly vulnerable to predation while incubating their eggs, having drab, brown plumage helps them avoid detection from above. Males of many duck species also lose their bright coloration and assume hen-like eclipse plumage while they are flightless during the summer molt.
Hens of many duck species also avoid detection by nesting in dense stands of tall grass or shrubs that provide overhead cover. Early breeding pintails, however, often nest in short grass or crop stubble, where they are more easily seen by raptors. One recent study in Prairie Canada found that red-tailed hawks were the primary cause of mortality in hen pintails from May to mid-July.
Raptors come in all shapes and sizes, and most are not a threat to healthy adult waterfowl, even small species like teal. Of the many factors that influence what raptors eat, size is the most important. The sizes of raptors and their prey are directly related. For example, with a wingspan of seven feet, golden eagles are the largest raptors in North America and can easily take cackling geese, while smaller bald eagles cannot. At the other end of the spectrum, green-winged teal see northern harriers as enough of a threat that they take flight when these medium-size raptors approach. Mallards, which are significantly larger and heavier than greenwings, typically show little concern when harriers pass by.
While it's energetically advantageous for raptors to take large prey, the potential benefits must be balanced with the risk of going after something too big. Researchers in California recounted observing a golden eagle that attempted to grab a Great Basin Canada goose, somewhat larger than the cackling geese these raptors readily prey on. In this instance, after the eagle failed to secure the goose by the back, the goose grabbed the eagle with its bill and gave it several good whacks with its wings. The two birds parted ways with neither showing signs of injury, but if the goose had squarely landed a blow and broken one of the eagle's hollow wing bones, the raptor would have likely perished as a result.
Being hunters ourselves, man has long been in awe of the enviable predatory skills and aerial capabilities of raptors like eagles and falcons. While many children dream of becoming a fighter pilot when they grow up, most of us never realize that ambition. But regardless of our age, we can still live the dream by thoughtfully watching and vicariously flying along with the raptors that we see while visiting wetlands and other wild places.
Raptors, Ducks, and Habitat
While it would be easy to assume that fewer raptors would mean more ducks, no studies have indicated that these birds take enough waterfowl to significantly depress their populations. Waterfowl and raptors have shared the same habitats for hundreds of thousands of years, and in healthy ecosystems both groups of birds thrive in "the balance of nature." Like most predators, raptors tend to take the small and the weak. For example, a study of crippled mallards in Wisconsin found that most of the birds were killed by predators, and raptors including red-tailed hawks, northern harriers, and great horned owls accounted for about half of the predation.
By managing healthy ecosystems, we can help ensure that the balance of nature is maintained. Some specific management practices include:
- Minimizing encroachment by trees on the prairies, where raptor numbers were historically limited by a lack of perching sites.
- Managing healthy prairie uplands and wetlands to provide secure cover for nesting hens and broods.
- Maintaining shrubby vegetation like buttonbush in forested wetlands to provide cover for wood duck ducklings.
- Avoiding habitat loss and wetland drawdowns that concentrate waterfowl and expose them to increased risk from raptors and other predators.