Raptors on the Hunt

Many birds of prey (especially buteos and larger owls) depend on small mammals for much of their diet, and snow can record dramatic encounters between predator and prey. We see such a story in the image above: the trail of a white-footed mouse ends abruptly where it was snatched by a raptor. The mouse was bounding from the left, and the bird hit it a little to the right of center, leaving a deep depression. Some light striations around the disturbance show where the bird’s wings stroked the snow. It’s hard to say with certainty whether the lucky hunter was a hawk or an owl, but there are a few observations that point to hawk. First, hawks hunt by sight, usually locating their prey on the surface, and that’s what happened in the clash shown in the photo. Owls usually detect prey by sound and often attack prey under the snow surface. Second, hawks tend to carry prey with their talons while owls take the victim in their beaks (although both deliver the killing wound with their claws). The disturbance where the mouse met its end doesn’t show signs of repositioning from talons to beak. A less reliable feature is the sharpness of the wing marks. Owl wing feathers have soft edges designed to reduce the sound of the wings, while hawk flight feathers have sharp, well defined edges. Under ideal conditions owl feather marks are more diffuse than those of hawks, but this feature varies depending on the texture of the snow. The wing marks in the photo are relatively crisp, so at the very least they don’t rule out hawk as the hunter.

In the next photo you see another attack on a mouse. This time the mouse trail enters from the right. The bird also came in from the right, and the deep disturbance where it hit the mouse tells of the violence of the impact. One talon made a gouge in the snow going down into the depression from the right, and both wings made marks on the left side of the photo as the bird lifted off.

A closer look reveals a spot of blood in the depression, visible in the center of the photo. It also looks like there was some repositioning, either in the talons or from talons to beak. The surface attack suggests hawk, but the repositioning tends more toward owl, and the feather marks could have been made by the softer feathers of an owl.

In both of the encounters I just described the hunter succeeded in capturing the prey, but in the photo below you see a different situation. The snow was deep and had an icy crust covered by about an inch of new snow. Sounds of an animal (most likely a white-footed mouse or a meadow vole) under the snow had drawn an avian predator, and the bird had hit the surface hard with its talons. The crust hadn’t yielded, and the only signs of the attack were the impressions of the talons and a few feather marks in the snow. This was definitely an owl since a hawk would not have detected prey hidden under the snow. And this time there’s additional evidence in the form of tracks. The K-shaped talon prints in the lower part of the photo indicate owl–probably a great-horned owl based on the field-and-hedgerow habitat. The feather marks in the upper right part of the photo give some insight into the variation associated with this kind of evidence. The left-most stroke is sharply defined but the ones to its right are less crisp.

In the next photo you see another attempt by an owl, this time in soft snow. Sounds of a small animal under the surface had drawn the attack, and the quarry had been able to evade the owl’s talons at least for a time. I wasn’t able to get close enough to look more closely, so I don’t know who came out on top of that struggle.

Many potential victims are never attacked (consider how often we find the undisturbed trails of mice and voles in the snow), and many that are attacked manage to escape. Estimates of hunting success vary widely, but the highest I’ve found is about 50% from a study of red-tailed hawks in Missouri. Other estimates go as low as 15%. And remember, this is the percentage of actual attacks that result in prey capture. Before an attempt is even made, a bird of prey must spend time hovering, soaring, or perching, in an effort to detect a potential prey animal. There’s a finely tuned balance between predators and their prey. Raptors display impressive hunting skills, and the small animals they prey on have effective ways of evading capture and high rates of reproduction. When we come across signs of a raptor attack we experience a vivid illustration of the complex interplay between predator and prey.