Peeling bark for nest lining
Pale Male stripping bark on March 1, 2007
Photo from http://www.palemale.com
When I saw the photo posted 3/2/07 of Pale Male stripping bark for the Fifth Ave nest, I sent John Blakeman a note asking if a different hormone kicks in when hawks move from bringing twigs to bringing nest lining materials, like bark. Here's his answer:
These are the only photos I know of red-tail bark stripping. Remarkable. There have been incidental and anecdotal written reports of this, but no one that I know of has ever photographed this.
As always, right in Central Park, of all places.
Yes, there is a sequence in nest building behaviors, but I don't think they are driven by different hormones. It's controlled or prompted by two factors, the first being hormone dosages the various endocrine glands are excreting. And that's almost completely a factor related to increasing day length.The curve of ascending day lengths is right now about as vertical as it can get. Back in January, day lengths were increasing, but minutely. In March, at our latitude (I'm pretty much straight west of NYC), we are gaining about 4 minutes of additional day light each day. For the red-tails, that's a breeding turn-on of the highest order.
The second factor in nest construction is the actual state of the existing nest. If it's somewhat dislodged or flimsy, the first order of business will be to bring in new sticks and twigs and thrust them together to make a firm structure.
Sticks will be brought to the nest throughout the entire nesting period, from January (or earlier) through May and June. That's just instinct, a ritualistic part of breeding. The real question is, are the sticks being tucked into the actual pile, to fill in the underlying stick pile? Or, are they just being ritualistically dropped on the surface or edge of the nest?
After the stick pile is firmly consolidated and feels secure, the birds next work on the lining, starting now, as in the remarkable photos at PaleMale.com. The birds start with more coarse lining materials, usually strips of bark, as shown in the photos. Progressively, the materials will get finer, with clumps of grass or leaves as the final, top layer. The female (primarily, with occasional help from the tiercel) will finally pluck off a few molting body feathers and add those to the lining material.
This sequence is all prompted by the same sexual hormones. The behaviors vary by the conditions at the nest, not be varying hormones. The coarser lining materials are early-March items (at our latitude). In a week or so, grass and weed bunches are likely to turn up.
One point everyone should recall here, lest anything apparently aberrant occur. It appears that the south end pair, last year on the Trump Parc building, is working on a nest elsewhere. Red-tail biologists really have no clue on why RT pairs simply abandon nests that they've use and start all over elsewhere. Some pairs seem to stay pretty stuck on a particular site, as with Pale Male at 927. But other pairs just fly away and build a new nest nearby every two or three years. I have a pair I've been watching for almost 20 years (new members of the pair, I'm sure) that flip from one woodlot to another every two or three years. I can't explain this.
But I wouldn't be alarmed if next year (it's not going to happen in 2007) Pale Male and his consort build a nest somewhere else in Central Park. If they do, it's not because they sat next to each other and had a serious talk about trying something new, in new digs. That's a degree of rationality these birds don't have.
John A. Blakeman