Wednesday, March 08, 2006

Blakeman on redtails and pigeons

Pale Male with pigeon 3/5/06
Photo by Lincoln Karim

Dear Marie,
Sally Seyal posed once again some cogent questions regarding pigeon-hunting by Central Park red-tails (below). How pigeons are captured in CP is very important. My comments follow Sally's questions.

Sally's letter:
I think I remember reading Blakeman's discussion of the unusual behavior or red-tails taking pigeons as prey in Central Park, whereas they do not normally do so in the wild. He had described the possibility of "brush crashing" into the trees to get roosting pigeons unaware. However, I noted Lincoln's observation of Pale Male chasing down a pigeon and catching it just after takeoff, definitely not brush crashing. Perhaps Central Park pigeons are so used to the commotion and hubbub and people and dogs that they are less wary of true predators and take flight too late? I can imagine a hawk stooping after a bird on the ground that is unaware due to the commotion around it.
Just a thought.

Sally Seyal
Blakeman replies:

Sally's questions touch upon crucial issues. Like all predators, red-tails in Central Park must capture sufficient prey to support their normal life activities. All along, I've had two related questions. 1) What are the Central Park red-tails feeding on? It's certainly not the normal wild rodent prey of rural areas. And, 2) just how are they capturing their Central Park prey? Because they aren't taking many mice, their capture methods differ from those used in the wild.

It's now abundantly clear that NYC red-tails survive on two species, the common Norway rat, which is rather common RT fare where available, and the rock dove, the common urban pigeon. It's this bird that raises all of my questions.

Every authority on the red-tail, and everyone who has watched this species, knows that red-tails seldom, if ever, take healthy, free-flying birds of any kind. Rats are easy catches. Red-tails are "made" to capture rodents of all sizes. The hawks just drop out of the sky on to the rat and quickly dispatch it with piercing talons and a few flesh-ripping bites. Rats are rather nearsighted and can't run very fast. They are easy pickings for red-tails. Early on, I presumed that rats were the primary prey for the CP red-tails. Rats are common in big cities, and I presumed that at least a few would poke their noses out into the daylight, particularly around dusk. I presumed that dusk-active Norway rats would be the primary food for the Central Park hawks. But like so many other Central Park red-tail life history factors, I was wrong on this. Pigeons comprise an unexpected, inordinate fraction of daily prey captures.

None of us who know red-tails in the wild, either from observational studies, or from actually hunting with them, as I do as a falconer, would ever predict that pigeons could be consistently caught by these big, not-so-fast hawks. I still marvel at the pigeon-hunting successes of the Central Park red-tails. Unless it were seen and well-described (as it is now), raptor biologists would have summarily dismissed frequent pigeon killings by red-tails. Red-tails can't capture healthy, free-flying pigeons. period. Pigeons fly much faster than red-tails and are far more maneuverable. A pigeon should be able to easily escape any red-tail pursuit.
But obviously, in Central Park that's not the case. How so?

First, I've never been to Central Park and can only report my secondhand understandings, based upon my extensive experiences with red-tail hunting techniques. It appears that CP red-tails take pigeons in two different ways. The Pale Male video showed one of these, where the hawk soars over a tree that has a pigeon perched within the tree's foliage. The pigeon feels safe there, as the pigeon's ancestral avian predator, the peregrine falcon, will never dive into brush or limbs to capture prey. In doing so, the peregrine would break its wings. Peregrines take prey only in open sky, and pigeons instinctively know this, so for safety, they occasionally retreat to safe inner tree branches.

That keeps them perfectly safe from peregrines. But pigeons are an alien species, "new" to North America originally from the arid lands of the Middle East where peregrines and other large falcons have been chasing them for millennia. There are no red-tailed hawks in the Old World, and pigeons have never had to be concerned about them, so they have no instincts to avoid these big Buteo hawks. Unless directly attacked, NYC pigeons pay no attention whatsoever to the red-tails, as they are not perceived as any threat.

I've noted before, however, that red-tails are cunning, intellectual hunters, and the CP hawks have figured out how to consistently take pigeons. The red-tails are winning. The pigeons are losing (but only a few a day, so it will be a while before natural selection takes place and all the remaining NYC pigeons start paying attention to red-tails).

A red-tail perched high on the side of a Park Avenue building notices a pigeon that flies into a tree far below. The hawk notes exactly where the pigeon sits and the hawk then figures how she can plunge into the tree and snatch the hapless pigeon. As in the video, the hawk drifts over the tree and then executes a typical "brush crash," a unique red-tail hunting maneuver we falconers thrill to. The hawk folds its wings, or more typically flares them, diverting her forward speed directly downward. Powered by both the diverted forward speed (now instantaneously downward) and by gravity, the hawk plunges at 50-70 mph straight down into the foliage of the tree, into the "brush," It's not a random, let's-see-what-we-find plunge. The hawk knows exactly where the pigeon is sitting and exactly how it will drop right on top of it. Combined with its plunging speed and the pigeon's inattention, the hawk plucks off the sitting pigeon.

That's one way CP red-tails take pigeons, classic "brush crashing." I saw that in the video, and I smiled, having seen my own falconry red-tails take rabbits fleeing under brush on the ground in exactly the same way.

But I don't think most CP pigeons are taken by brush crashing, because I don't think many pigeons spend much time perched in trees. I think the CP red-tails have learned something new, devising another method to consistently take rock doves. Here, I'm speculating, and request that anyone who sees this post their observations in detail. This is new red-tail biology, and it needs to be observed and described.
In this, the more frequent pigeon hunting technique, the red-tail likewise begins its hunt while perched. The hawk telescopically zooms its vision down on to flock of pigeons feeding on the ground. The hawk is extremely patient, waiting as much as several hours as it closely studies the individual behaviors of the pigeons. We tend to see just a flock of pigeons, a large, fluid mass of these common birds. But our red-tail looks at every single bird down there, searching for the slightest nuance of disadvantage. In exactly the manner of cheetahs pursuing a herd of fleeing gazelles on the Serengeti, our red-tails will search for the animal that appears to have some slight physical or behavioral disadvantage.

I think the hawk is eager to watch the pigeon flock take off after some alarm, perhaps when a dog wanders too close. The hawk wants to see which pigeon in the flock has an inflated culinary sense, staying too long on the ground trying for the last peck of grain before finally leaping into the air with the rest of the fleeing flock. The red-tail wants to learn which two or three pigeons in the flock are the culinary lingerers, the few who value eating above fleeing.

With this knowledge, the hawk's next move is to figure out a course of attack. After repeated tries much earlier in it's life, usually in its first summer, when the hawk was learning its hunting skills, it has now come to know that it can't just fly up to a sitting flock of birds and catch anything. The hawk is far too slow. The birds just fly up and away, leaving the lumbering hawk embarrassed and empty on the ground. The hawk learned long ago that a bit of avian stealth is required.

Therefore, the hawk drops off its perch, gaining speed in a straight, steep, non-flapping glide. It wants to slice into the pigeon flock from the lowest angle, skimming across the ground at high speed. Again, peregrines can't do this, so instinctively pigeons keep looking up in the sky for falcons. A red-tail zooming in at 45 mph on fixed wings just three feet above the ground isn't so easily seen.
But a few older, attentive pigeons will see the approaching hawk and they will instantly leap away into the air, quickly accelerating to full speed and swing back around to the right or left of the hawk's trajectory, an instinctive trait they use to successfully avoid peregrine dives. Pigeons instinctively want to get above and behind a pursuing hawk.

For the majority of pigeons in the flock, this escape technique works flawlessly with the approach of a relatively slow red-tail. But our hawk lives in New York. It takes smarts to do that, and our bird has been plotting this meal all morning. Our raptor hasn't been sitting up on that building watching merely the general scenery, nor even the general flock of pigeons. It has been focusing -- literally -- on just one pigeon in the flock, a bird that seems to have its head down on the ground most of the time. And when that dog jumped over toward the flock, this pigeon was the last one off the ground as the flock leaped up, swung around, and landed again as the dog's owner called it back. That lone pigeon will be the hawk's target.

With an attack all figured out, the hawk drops off her high perch, glides around a distant tree, dropping again down at increasing speed to just a few feet above ground, and plunging not into the pigeon flock in general, but directly at the lingering target pigeon. In an instant, another fine NYC red-tailed hawk dining experience, at the expense of the pigeon's life.

I believe that most of the pigeons captured in this way are probably young, inexperienced pigeons, newly on the wing in the last few weeks. The CP red-tails are exploiting the inexperience of the young pigeons, particularly those who linger in the restaurant after the other guests have left (in a quarter-second or so).
Again, I'm speculating on all of this from what I know about red-tails and the cursory descriptions of open-ground pigeon captures that have been posted. If anyone there, on site, can post more detailed observations, please do so.

Red-tails will seldom, if ever, capture a pigeon in full flight. If this is ever seen, the captured pigeon had some severe disability. It was either sick or injured. Out in the open sky, no healthy pigeon has anything to fear from a red-tail. A pigeon can fly literal rings around a red-tail. But on the ground, if it's not paying sufficient attention and is tardy in getting into the sky when a red-tail zooms across the ground, the last pigeon up is likely the hawk's target and consequent meal. Pigeon mothers should be telling their offspring in Central Park to never linger on the ground. Jump up into the air instantly with the rest of the flock. To linger is to be lost.

John A. Blakeman