Friday, March 11, 2005

JOHN BLAKEMAN on INCUBATION

On Thursday, March 10, Lola began to spend many hours sitting on the nest. She also didn't leave it at the end of the day, as she has been doing previously, but spent the night on the nest. This led many hawkwatchers to conclude that incubation has begun. In years past we assumed that the day the female began to spend nights on the nest marked the onset of incubation [Don't forget we cannot see into the nest to set an exact date by the presence of eggs.]

However, I have come to the conclusion that we have been jumping the gun, in previous years. If you check Lincoln's chart of previous years' important events, you'll see that there are often 38, 39, or 40 days between the day we have assumed that incubation has begun and the day the eggs hatch [We figure hatch day from changed behavior too. When the female no longer sits all day, but now frequently stands at the end of the nest and does little up-and-down motions with her head, as if she were feeding something tiny in the nesty. Also when Pale Male begins to bring food to the nest.]

But there cannot be a 40 day incubation period. That's too long. Scientists tell us that the incubation period of RT Hawks is 28-32 days. A few stretch that to 28-34 days. But no longer. And here is John Blakeman's response to Donna Browne's question to him about our observations of changed behavior at the nest, with Lola sitting long hours and spending the night:

3/11/05 -- Blakeman's Reply

I think this is likely all to be egg laying preliminaries. Females will spend long periods of time
sitting on the nest before laying. This can happen frequently during the week or so before eggs are laid. I doubt that an egg was laid, because if it was, you
probably would have seen egg-turning behaviors (although these can occur a bit after all eggs are
laid, depending, I think, on the particular
female).

No doubt, however, things are entering a new
stage. Egg laying and incubation are at hand.

Wednesday, March 09, 2005

JOHN BLAKEMAN ON THE PIGEON vs. RAT DEBATE

Marie,

Donna Browne sent me another wonderfully provocative email, pondering if pigeons might not be such a prominent part of the CP red-tails’ diets. She carefully noted that Pale Male seems to be bringing large numbers of rats to Lola. Rats are appearing as nuptial food offerings more frequently than expected. This prompted my response below.

Donna,

In my red-tail heart of hearts, I think CP red-tails really prefer rats over any other food item. They will take any vertebrate that is easy to catch, but I don't think we should so quickly write off the Norway rat, as perhaps we are presently doing with the preponderance of recent pigeon capturing data
I'd be really careful in presuming that what we see the birds capturing is actually representative of what they take altogether. Yes, a bunch of pigeon captures were surely witnessed. But has anyone seen how, where, and how frequently rats are taken? Rat captures are likely to be much more discrete, even almost secretive. Red-tails can capture rats with great success, if the rodents pop out during the daytime. I had a falconry red-tail that reveled in early-evening rat captures, so I've seen this close at hand. The details are too lengthy to describe here, but the red-tailed hawk's cunning and prowess at taking these large rodents is superb.

I've watched with awe as my falconry red-tail captured Norway rats in a rural ditch. Their ability to see where the rats are running, and their ability to calculate and be at a perfect interception point for capture is remarkable. Pale Male and Lola, while sitting around Central Park, certainly have noted both rat holes and the behaviors of emerging rats. Like my falconry bird, they have learned to wait any length of time until the rat is sufficiently away from cover before attempting a pursuit. In their hunt, they will almost always dive toward the rat hole or cover, not directly at the rat. They have learned that when alarmed the rat shoots right back into the cover, using a known rat walkway. The hawks have seen these and use the rats' innate escape behaviors to their hunting advantage. It's not a wild goose chase. It's a well-calculated lethal game the hawks play with the rodents -- and more often than not, our birds win.

Continue to track pigeon captures. By any measure, pigeons are a major food source. But let's try to get a better handle on the rat factor. It’s likely to be larger than we currently think. The best way to do this will be to record as many food sightings at the nest as possible. Let's see what the ratio of pigeons to rats to squirrels to others is at the nest. I think this will be more representative of normal day-to-day hunting patterns (at least during the breeding season).

Where are these rats being hunted? When are they being captured? Are they full-sized adults, or half-sized new pups?

We continue to elucidate the remarkable ecology and behavior of the Central Park red-tails. Keep me posted.

Sincerely,

John A. Blakeman

Red-tail Sleep and Temperature Control

Red-tail Sleep and Temperature Control


I asked John Blakeman for some information about how and where Red-tailed Hawks sleep, and about their night roosts. Here is a part of his answer. [I'll post another part soon.]

First, how does my falconry red-tail Savanna spend her nights? She stays out doors year round, especially in the winter. Red-tails are a bit put off with room-temperature warmth in the cold seasons. After all, they wear a remarkably effective down coat year-round. Beneath the outer covert feathers, the ones we see, is a thick layer of the finest, insulating down. This is what keeps the birds warm. The outer covert feathers are oiled daily by diligent preening, and this repels water rather well. The down feathers retains heat. To be kept indoors all winter is somewhat like wearing a thick winter coat indoors.

Actually, the bird could do this. It can be parked indoors at room temperature with little problem. Red-tails, along with other similar hawks, have a remarkable way of coping with temperature changes, which also often relate to food availability.

Red-tailed hawks are "homeotherms," otherwise known as "warm-blooded," like all mammals and birds. But unlike humans, who tend to keep body temperatures narrowly near 37 degrees Celsius (the classic 98.6 degrees Fahrenheit), red-tail body temperature can vary greatly, which allows them to sleep as they do, changing body temperatures with the weather and body food reserves.

In my undergraduate years at Bowling Green State University (the Falcons, where I kept the university mascot), the Ohio Division of Wildlife donated a wing-injured red-tail that was permanently disabled. I was able to "man" it, to "tame" it, for some body temperature research BGSU's ornithology professor, Dr. Eldon Martin, wanted to investigate. He, too, was interested in the maintenance of body temperature as weather changed. He had a graduate student talented in electronics. This fellow devised a peanut-sized radio transmitter that could be surgically embedded in the abdominal cavity of a large bird. This was long before modern digital electronics. This little device could transmit data for only a few days, and the receiving antenna had to be just a foot or so away from the bird.

The transmitter broadcast frequencies of beeps that varied by temperature. Dr. Martin surgically implanted the transmitter in the body of the red-tail, which was housed inside a climate-controlled environmental chamber, a large room. Daily seasonal temperature regimes were created, and the results were unexpected.

In summary, deep body temperature varied by as much as 10 degrees Fahrenheit. When highly active, the bird's temperature would rise to 105 degrees or more. But at night, particularly in cold weather temperatures, body temperature would drop to the low 90s. I believe that this was one of the first studies to show such wide temperature variation in hawks. It is known in other birds, but this settled it for red-tails.

This means that when sleeping, red-tails can turn down the furnace, conserving fuel during the inactive night period. In winter, at New York and Ohio latitudes, night periods can approach 16 hours.

But back to the question, where do the birds roost? Here's one place. On the way to school each morning, I used to watch resident pairs I knew of. There were several tall utility poles way out in the middle of several hundred acres of row-crop fields, upon which I frequently saw a red-tail perched. This was just at dawn. The bird surely had taken the perch the previous evening. I saw this hundreds of times, and was able to discern that red-tails will never perch out on these poles when windy weather might be approaching. The birds know what the next 16 hrs of winter weather is going to be when they elect to spend the night on those poles.

So where were they at night during inhospitable weather? I don't know. I have never seen a wild red-tail roosting as you people so frequently have done in Central Park. I simply presumed that the birds fly into a woodlot and park themselves on the downwind side of a tree trunk. For me, your Central Park observations are new and helpful.

One other consideration, one that presently isn't a concern in Central Park. In rural areas, the great-horned owl occupies exactly the same habitats as the red-tail. This owl is larger and even more muscular. It can kill anything it chooses to attack, including skunks, house cats, and other very large prey. When banding young at a great-horned owl's nest, one must wear a leather jacket, helmet, and probably a face guard. The bird will almost always attack with fury.

The bird could therefore easily dispatch a sleeping red-tail. Red-tails seem to be able to see at night just about as well as humans. Owls, of course, see well, and a sleeping red-tail would appear to be a tempting meal vulnerably sitting on a pole or branch.

When sleeping, red-tails turn their heads back over the shoulder and bury it in the feathers of the back. They look utterly decapitated. They certainly can't see any approaching hungry great-horned owl.

But great-horns rarely, if ever, kill sleeping red-tails. They seem to abide their nocturnal presence. This is probably an evolutionary outcome. Great-horns are incapable of building a nest. Most frequently, they expropriate a recent nest of a local red-tail, forcing the red-tail to build a new nest. And this relates to what we've seen with the 927 nest. This is why I was not overly concerned about the "disruption" at the site. Many rural red-tails have their nests taken over by great-horned owls every year, and they have to build new ones. Red-tails are genetically programmed to do this. Those that weren't able to re-nest in new, adjacent locations, simply didn't reproduce and their genes were lost. Established red-tail pairs such as Pale Male and Lola can withstand a great deal of nest disruption, probably a result of the ecological interactions of great-horned owls over many millennia. The ecological interplay great-horned owls and red-tailed hawks is a close and delicate one. Conversely, a red-tail could easily snatch a young owl from a nest during any unprotected moment in the daytime, just as our birds pluck pigeons out of Central Park. But red-tails seldom pester the owls. There is a continuing, delicate detente between the species.

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

Monday, March 07, 2005

JOHN BLAKEMAN ON INCUBATION

"If the Hawk Bench folks watch carefully, they will be
able to see when incubation begins in earnest. The
parents, especially the female, will first sit
with a slight elevation. Then, when she decides to attend
solely to incubation, she simply melts down into
the nest, showing a markedly lower profile. When you see
this, incubation is underway.

Also, a close reading of Lola's physical deportment at
the nest site can reveal that an egg is descending
through her single fallopian tube. In the day or hours
before laying, the female will often just stand at
the nest edge, or on a perch nearby, with an almost
glazed look in her eye. There may be some discomfort
involved in creating and passing an egg through her reproductive organs. I recall seeing this. The
female expressed a restrained, almost contorted
deportment that I've never seen since. She even walked
strangely around the nest rim, almost in the manner of a
duck. She didn't seem to want to pick up her feet.

All of that should be in another week or more."