Tuesday, March 08, 2005

PIGEONS AS REDTAIL PREY --John Blakeman speculates

Marie,

Astute hawk-watcher Donna Browne emailed to me some very fine questions. I responded with the comments below. They border upon the arcane, but I think everyone can ponder them.

Donna,

It's now very clear to me, as I first imagined, that the pigeon is the prime prey for the NYC red-tail. It's becoming much clearer how the hawk captures these birds. The low, ground-skimming ambush is one way. Plucking sitting birds off buildings or from tree perches, and those foraging on the ground are two other ways. Initially, I would have thought these last two methods to be difficult and infrequently successful. Not so. Very clearly, the red-tailed hawks in Central Park have learned to exploit predation vulnerabilities of the rock dove.

For me, the major question now is why do these predation vulnerabilities exist, or even persist. I can understand the continuing vulnerabilities of squabs on nests, and even young pigeons fledglings with reduced flight powers and experiences. Red-tails are programmed to exploit these easy targets. But there are very few squabs in winter, and consequently very few immature fledged pigeons. The hawks are taking adult pigeons.

The key here is that the pigeons simply aren't uniformly aware of either the presence of the red-tails, or of their hunting prowess. A significant number of pigeons don't instinctively recognize their multiple vulnerabilities. We have a classic case of natural selection involving a new predator/prey relationship. The rock dove got itself instinctively programmed to avoid peregrine predation many thousands of years ago. But the pigeon has never been selected for the avoidance of red-tails. Most NYC pigeons are currently oblivious to the continuing threat of the several hunting schemes of the red-tailed hawk.

You know, then, what we will want to watch. Will natural selection by red-tails eliminate the genetically vulnerable pigeons? In a few years (or more), will Central Park be visited only by pigeons as wary of red-tails as peregrines? In coming decades, will the hunting of pigeons by urban red-tails become so difficult as to reduce or eliminate the big hawks from urban areas lacking abundant rodent populations (the normal prey of red-tails)?

Or, are our red-tails merely plucking off an extreme tailing edge of the bell-shaped curve of pigeon behavior? Is there such an abundance of inattentive pigeons that the few taken by our hawks will have no influence on the population-wide distribution of genetic behaviors of pigeons? Could it be that the red-tails are plucking simply dumb pigeons that wouldn't have otherwise produced offspring anyway?

Will the Central Park red-tails change pigeon behaviors there, or not? Here are some very hypothetical numbers that might enter some more definitive equation. Let's assume some maximum numbers, to test probable limits of the question.

First, how many red-tails might prey upon Central Park pigeons? Let's assume three resident breeding pairs, and since 5 immatures have been seen in the park this winter, let's assume that number. That's 11 red-tails consuming pigeons. So how many pigeons are taken each year by these 11 birds?

Let's assume that 75% of their meals are rock doves. (A pigeon is easily a day's food for a red-tail.) Posted data indicates that it's closer to 60%, but let's assume the higher one. Three-quarters of 365 days is about 273. Let's then just assume that each red-tail consumes about 270 pigeons each year. 270 x 11 = 2970. Of course, un-factored here is the prey needed to feed eyasses both in the nest and after fledging.

The next question is how many pigeons reside in or visit Central Park. What is the home range of these pigeons? From what larger area do CP pigeons come from? All of Manhattan, or just the adjacent neighborhoods? What is the size of the total preyed-upon pigeon population? If it's just 10,000 pigeons in the immediate area, our red-tails' killing of 3000 or so inept pigeons will certainly begin to change genetic behaviors. Classic Darwinian survival of the fittest. In time, ever fewer vulnerable pigeons will exist. Ever greater numbers of wary pigeons will carefully feed and roost in Central Park. Finding enough food to support three breeding pairs of red-tails, along with the eyasses and fledglings they produce, may become increasingly difficult.

But what if the pigeon population being exploited is, say, 100,000 birds, not 10,000. Then the annual loss of 3000 birds is an order of magnitude less, and may merely reflect normal losses of "dumb" birds that would not have otherwise contributed their genes to the NYC pigeon gene pool anyway.

To me, this is the final great question. Are Central Park red-tailed hawks changing pigeon behaviors by sufficiently eliminating vulnerable birds? Are they changing the overall distribution of hawk-avoidance behaviors in pigeons? Or, are they only incidentally plucking off pigeons whose vulnerabilities stem from innate insufficiencies that would have prevented them from breeding anyway? If so, our hawks will persist in elevated numbers. If not, the pigeons will eventually become as wary of red-tails as they are of peregrines, thereby reducing, or even eliminating the urban red-tailed hawk.

In no other habitat that I'm aware of do red-tails consistently take pigeons. Any alert, healthy rock dove should always be able to avoid red-tail capture. For a pigeon, red-tails are far more easily avoided than peregrines.

The answer to this question may only be resolved on a decadal time scale. For the last decade, the Central Park pigeons haven't apparently learned much. There are ample dumb pigeons to support not just Pale Male, Lola, and their annual progeny, but at least two other pairs and a handful of seasonal immature hangers-on. In this predator/prey puzzle, the predator currently has the upper hand. In a decade, will that still be the case? Only time (or very careful, evidence-based ecological mathematics) will tell.

Sincerely,

John A. Blakeman