Friday, January 07, 2005

THE RAISIN-BREAD HYPOTHESIS

THE RAISIN-BREAD HYPOTHESIS

A few days ago one of the Central Park hawkwatchers reported that she had seen Lola at the Azalea Pond with an immature redtail in close proximity who was "mantling" a squirrel [Mantling means covering the body of some previously caught prey with the wings] The hawkwatcher assumed that the bird was one of the young from the 2004 nest.
"How did you know that the immature hawk was one of the 2004 kids?" I asked, "Did you recognize it by some field marks?"
"No," answered the hawkwatcher, "But it HAD to be one of the kids because Lola tolerated it in her territory. I assume that she would chase away any unrelated bird."

I wondered what John Blakeman might make of this report, and asked him , by e-mail, whether it was odd to have one of last year's young still around in the Ramble at the beginning of January.
He answered that it would be highly unusual, even "weird", and the next day followed up with the letter below:

Marie,

The chance that the recent immature is one of last year's offspring would be only that, pure chance – and rather unlikely.

There is no way that Lola or Pale Male could (or would) recognize the new bird as one of their progeny. Yes, that happens with social mammals, where parents imprint to the smells or other identifying clues of the young. But that sort of thing doesn't happen in hawks. Lola didn't fail to confront the new bird because she recognized it as one of her beloved offspring. “Belovedness” rapidly evaporates in mid- to late summer when eyass feeding behaviors are no longer hormone driven.

I believe I mentioned in an earlier note that RTs are famous for becoming slightly social in early winter, meaning that adults will allow other RTs, mostly unpaired young adults (“floaters” we call them) and first year birds to occupy prime winter hunting habitats.

I think that's what's being seen in Central Park. Lola just sat there while the immature mantled her catch. While doing so, Lola was looking around and she also had seen numerous rats and squirrels in the park. She was also completely fattened from abundant food that she paid no attention to the intruder. Remember, for a hawk, food is life.


In my Western studies in Nevada, raptor biologists out there have a wonderful habitat concept called the “raisin bread” theory. In early summer there are abundant ground squirrels that provide abundant food for nesting RTs. But mid-summer drought sends these mammals back underground to pass the drought in a hibernation-like state. By late summer and into the fall, there are few prey animals for the RTs to capture, and except for the very capable old adults, immatures get hungry (well, they start to starve) and that always compels young hawks to migrate.

But scattered across the Idaho and Nevada deserts are isolated desert marshes. In the raisin bread theory, these are the “raisins.” As the RTs (and others) encounter these islands of refuge, they drop in to hunt the abundant voles there. Hundreds of diurnal raptors can be seen at some of these in early winter.

The same phenomenon may occur at NYC. Central Park is a raptor raisin, a winter raptor food oasis, an area with a large, available prey supply within a larger region devoid of prey. This new intruder just happened to see the food, too, so she* decided to stay and take advantage of the mammalian offerings. Lola, fat as she was, paid no attention. This is rather common, as I said, at this time of winter.

But in a few weeks, that will change altogether. As the sex hormones take effect, Lola will not allow any intruder in her territory. It's always something of a spectacle in watching an adult resident drive off an intruding bird. There is seldom, if ever, any physical contact (an event falconers call “crabbing”). Usually, the adult simply flies over and displays her body in a way that is easily discerned by the intruder. The immatures almost always get the body language of the adult. If the adult lands nearby, she will bow her head and lift her wings a bit. This is called an intra-specific threat display, and the young bird recognizes instantly. It's the look every Catholic nun used to give errant 8-year old boys when they should have been doing their arithmetic lessons. The youngster responds rather quickly.

But the real story here is that Lola isn't making any threats. She's comfortable allowing the intruder her hunting discretions. This is further testament to the ample prey in Central Park. The pair produced two and three eyasses for nearly a decade. That, alone, indicates the large supply of food animals. Lola's seasonal disregard for this new intruder is another one.

I doubt that the intruder was a bird hatched at 927. And even if it was, the parents would not have recognized it as such.

Hope all of this brings some light.

(*The sex of unknown hawks are always female, a grammatical convention of falconers that extends from Shakespeare's time. The word “falcon” specifically applies to the female peregrine; the male is the “tiercel.” And in red-tails, 55-60% of fledged first year birds are females.)

Sincerely,

John A. Blakeman