Thursday, July 07, 2005

Pre-fledging speculation about the Trump Parc nestlings: Blakeman on sexual dimorphism

<>Photo by Lincoln Karim
July 5, 2005

Boy or Girl?

It's time now to start speculating on the sexes of the two Trump Parc eyasses. Of course, for a while, it will be only speculation, until the birds are ready to leave the nest, to fledge. But an interesting developmental observation has been made by biologists and falconers who closely watch eyasses on nests.

As many here know, there is a marked sexual dimorphism between the sexes in virtually all raptors. When fledged, females are larger than males. When the birds are about to fledge, it should be easy to "sex" the individuals (to determine the sex, in biological parlance). If there is a noted size difference, the larger bird will be a female and the smaller a male. If both birds are the same size, they will have to be sexed in comparison to the size of the parents.

But even that is not always so successful. Red-tails in their first-year plumage are actually dimensionally larger than adults. They look bigger because they are. Their flight muscles aren't fully developed when they leave the nest, so they require slightly longer wing and tail feathers to easily hold them in the sky. Consequently, first-year red-tails appear larger than adults. By weight, they are not as large, but their tails can be an inch or more longer than adults, also with longer wing feathers. A female fledgling can appear to be a giant, larger than either parent. But under her feathers, she's a relative weakling.

Our eyasses in the nest, however, can present a curious contrast. Although females in the end are markedly larger than tiercels (males), it's been observed that tiercels in the middle third of nest occupation can be actually larger than a sister hen eyass. Consequently, if a size dimorphism can be observed in the next week or two, it's not likely to hold till fledging. At this stage, tiercel eyasses tend to spurt ahead in size, causing the unaware to label them as hens. But the females soon catch up and overtake the size of the eventually smaller tiercels.

Therefore, watchers of the Trump Parc eyasses should try to keep some track of the differential growth patterns of the birds, should they be of different sexes. Soon, with emerging flight feathers, astute observers should be able to discover minute feather patterns that identify each bird. This will be helpful for dentification. Mention was made, I believe, of a patch of white feathers on the back of the heads of the eyasses. This is a well known eyass feather pattern. Later, dark feathers will be seen emerging here. This curious feather pattern seems to be deeply ingrained in the genetics of red-tails and other related hawks and eagles. I once trapped and studied an adult female who was "leucistic" or albinistic, a nearly all white partial albino red-tail. Most of her body feathers were pure white, but she retained dark pigmentation in this back-of-the-head patch.

Lastly, I wouldn't be surprised if fledging actually deviates from the published, understood common time ranges for such. Remember, this is New York City, Central Park in particular, and everything here related to red-tails is different from the events, forces, and processes acting on typical rural and wild red-tails. So don't be alarmed if the birds fledge earlier or later. We are dealing with brand new red-tailed hawk biology that so far, is academically undescribed. All of this is leading edge, unknown raptor biology. Others, appropriately, have been excited by the discovery of remnant ivory-billed woodpeckers, a species formerly thought extinct. That story is about an old species being rediscovered. But in the heart of Manhattan we have an old species doing utterly new and unexpected biological things -- a story that for me prompts exactly the same ornithological excitement that the rediscovery of the ivory-billed woodpecker did for others.


John A. Blakeman

Wednesday, July 06, 2005

Blakeman answers questions about nest maintenance

Christopher Lyons, a Bronx birder, sent me a letter which he hoped I would forward to John Blakeman. I did so but added my own observations.. I sent both letters to John Blakeman. His response is below:

Christopher Lyons wrote:

In the course of observing two different Red-Tailed Hawk nestings, one in Van Cortlandt Park last spring, and one at Fordham University in the Bronx over the past several months, I have noticed something.

After the young had fledged from the Van Cortlandt Park nest, which was in a wooded area, we checked all around the base of the tree for pellets and remnants of prey items, remembering how in past years we had found a great wealth of the same underneath Great Horned Owl nests (which were usually built by Red-Tails, and either inherited or taken over by the
owls). However, we found nothing--not so much as a bone or a feather. We knew hawks don't cast pellets the same way owls do, but we were still surprised to find no evidence of the vast number of pigeons, squirrels, and chipmunks we knew the parents had brought to the nest over the past several months.

Moving ahead to February of this year, I was astonished to see a pair of Red-Tails building a nest in an oak tree just a few hundred feet from the library at Fordham University, where I work. In this case, there was no point in waiting for the young to leave the nest tree (which they did in Mid-June) before venturing closer--the hawks had picked a tree that had people all around it, all the time, and human presence seemed to bother them very little, if at all.

I passed that tree at least 15-20 times a week, at all times of the day, and never failed to look to see if there were any pigeon carcases or squirrel bones underneath. The well manicured lawn beneath the tree would have left no doubt if any such items had ever been present. The maintenance staff at the university is very diligent, but not that diligent, and in fact they don't seem to have ever had to clean up after the hawks.

As has been remarked upon by Lincoln Karim regarding the Manhattan hawks, the parents seemed to always take "the trash" out when they left, carrying the uneaten portions of the prey animals with them. Is this standard operating procedure for Red-Tailed Hawks? If so, leaving aside the fact that it casts certain accusations made by a certain 5th Ave. Co-op Board in a dubious light, what would the purpose be of this scrupulous cleanliness? It could be to avoid disease, but that doesn't really explain the need to carry the leftovers away from the nest. Why not just let them drop? Concern for the squeamish sensibilities of humans does not seem a reasonable explanation.

It occurred to me that Red-Tails might be more concerned than Great Horned Owls about nest predation, and go to greater pains to avoid drawing attention to their nests. I've read that raccoons sometimes rob Red-Tails nests, risking the wrath of the parents. Raccoons are too large to be prey for Red-Tailed Hawks, and a hungry one might decide a good meal was worth risking injury for. And in other areas where Red-Tails nest, there would be even more formidable tree-climbers to contend with.

Obviously a lot of partly eaten animals underneath the tree would attract attention from any meat-eating animal with an acute sense of smell. Therefore, by carting away the portions of prey animals that their chicks don't eat, the hawks are acting on a survival instinct that may not be terribly relevant to birds nesting on a building in Mid-Manhattan, or a leafy urban campus, but which might well save them a lot of potential problems when nesting in a city park, or a suburban neighborhood, with plenty of potential nest-robbers around. Since the behavior is instinctive, and still relevant to most Red-Tail habitats, there would be no reason for Midtown Red-Tails to abandon it.

As to Great Horned Owls, the first one I ever had a good look at was roosting in a tree in Van Cortlandt Park. At the base of the tree was the half-eaten carcase of an adult raccoon--with talon punctures in its back. GHO's have a lot less to fear from nosy neighbors, and may simply not have evolved this behavior (as indeed they never evolved the skills needed to build a nest of their own).
Be curious to know what your observations have been with regard to this matter, and whether mine are even remotely close to the mark.

I replied to Lyons::

I'll forward your letter to John Blakeman, but I know the answer too, from my own experience with the Fifth Ave. birds. They carry everything away, --food remnants, carcasses and the fecal sacs of the young --though not necessarily very far. Regina Alvarez, who was the Zone Gardener at the model boat pond back in 1994 and 95 told me then that she often found carcasses on the lawn just inside the park wall in her area. That is, right across Fifth Ave. from the nest.

I'm pretty sure redtails don't leave stuff at the base of the nest tree [or whatever ] to avoid attracting the attention of nocturnal predators -- raccoons abound in CP and I've even seen them on Fifth Ave outside the park!. And you are absolutely right about the complaints of the 927 Fifth management about rat and pigeon carcasses littering the sidewalk. Complete hogwash. Only the poop of the pre-fledge hawklets on the building's canopy sullied their entranceway-- easily cleaned up with a hose. I did once find a redtail pellet -- at the base of a tree in the park where Pale Male was eating a pigeon. He never ate at the nest, and I'd guess that during incubation when the female did eat at the nest she cast her pellets and defecated in the park during her periodic breaks.

>And here is John Blakeman's response to both Chris's and my observations:

In relation to the questions (and answers) you and Christopher Lyons asked me about, both of you were correct on all accounts.

The questions related to the observed lack of food remains at red-tail nests, both those in Central Park and elsewhere. Your suggested answers were correct, that red-tails have an instinct to carry away food remains from their nests.

Red-tails, including both adults and eyasses, can't consume the entirety of larger prey animals. They swallow their beloved voles, large gerbil-sized rodents, along with any smaller house or deer mice they capture. But squirrels, pigeons, and adult rats have large, long bones that can't be easily broken and consumed. Large rodents also have copious amounts of skin and fur, which provide no nutritional value. The feathers of birds, especially the copious fine feathers of pigeons, are a culinary nuisance. If you get a chance, watch a red-tail pluck off the feathers of a pigeon before it gets down into the delectable flesh. The hawk can spend inordinate efforts in plucking pigeon feathers. The columbids, pigeons and doves, are famous for having large numbers of feathers. This helps them escape capture by smaller hawks such as the Cooper's and sharp-shinned hawks, which have smaller feet that sometimes are able to clutch merely a fist-full of feathers.

The discarded feathers of a red-tail's pigeon lunch easily blow away. But the other remains, usually the carcass of a head and two wings (depending on the feeding habits of the individual bird, which varies greatly from bird to bird -- Europeans use knives and forks differently from Americans, too) are inedible leftovers. If the meal was consumed away from a nest area, the carcass is typically dropped and left. But carcasses are seldom, if ever left on or near an active nest, for the reasons both Marie and Christopher Lyons mentioned.

If a red-tail allowed prey carcasses to drop to the ground beneath a nest, roving mammalian predators would be easily tipped off to the nest's presence by rotting odors. By carrying the dead prey items away from the nest, smell-guided predators are diverted.

And yes, the primary such predator is the raccoon, an animal that expends inordinate energies trying to find and consume eggs of any kind, either avian or reptilian. Now that there is no longer any significant commercial trapping of raccoons, their populations have exploded, along with their egg depredations. Quail, pheasants, and other ground-nesting bird populations are severely restricted by raccoon nest and egg predation. Raccoons, like urban (and in many areas, rural) deer, have no natural predators today and they have saturated the countryside. In Ohio and in most of the raccoon's range, the primary population control of the species is the motor vehicle, the primary killer of raccoons.

Don't get me wrong. Raccoons are wonderful, interesting, and native animals. But they are also extremely successful predators and scavengers. Red-tails have a lot to be concerned about regarding this species. Both of you were correct. Raccoons are far too big for a red-tail to easily kill. A raccoon that ascends the nest tree of a sitting red-tail is not likely to be much challenged. The sitting female, at the last instant, will abandon the nest, leaving the eggs or little eyasses to be eaten by the raccoon. In many cases, it will be a pair of raccoons on the prowl.

There was a case here in Ohio at a nearby bald eagle's nest where a raccoon was seen driving off a sitting eagle before consuming its eggs. It would be wise not to try to defend a nest against a marauding raccoon, for either an eagle or a red-tail. The best defense is to present no olfactory clues to scavenging raccoons, and that's accomplished by conveniently carrying the carcasses some distance from the nest, an instinctive trait all successful red-tails exhibit.

So, no, the residents at 927 Fifth Avenue, nor anyone at Trump Parc, need concern themselves with dropped or discarded carcasses beneath the nest. Any red-tail that did that in the past merely donated it's egg lipids and proteins to the ever-present raccoon, another case of Darwinian natural selection.

Christopher Lyons noted, however, that great-horned owl nests are often festooned with rotting carcasses. The ground beneath them is often littered with mangled and rotting food leavings. That's because the great-horned owl is a remarkable killer. Unlike the red-tail and the eagle (which don't see at night any better than we do), the great-horned owl will instantly, even lethally, attack anything it perceives to be a predation threat at the nest. It can kill a raccoon expertly, so it has no need to carry away food leavings. It just instantly sinks four 2-inch talons through the cranium of an approaching raccoon. Next to motor vehicles, great-horned owls are probably the second-most common killers of raccoons. Tigers of the air they've been called.

Those of us who band wild red-tails on the nest know that the adults will scream and dive at us, but seldom, if ever actually attack while we are at the nest. (Some California red-tails, however, do attack at the nest, so appropriate care must be rendered.) But when banding great-horned owlets, the bander better be wearing a thick leather jacket, a very thick neck scarf, and a helmet. An adult great-horned will try to knock the intruder from the tree, hitting the person with remarkable force. (We've discovered that if we paint two large eyes on the back of the helmet, the owl thinks it's being watched and the attacks are sometimes diverted.)

Once again, the NYC red-tails are operating with their wild, rural genes. To them, Central Park is only a nice big green area with everything a red-tailed hawk population needs. As in the wild, they don't want to be attracting any raccoons to their nest sites, so they keep a clean nest by carrying out the garbage each day.


John A. Blakeman