Monday, April 24, 2006

Ideas about the nest failure

Many letters focus attention on the new structure put up after the nest was removed in 2004 -- the so-called "cradle". Here's one , followed by my response, which in turn is followed by John Blakeman's response:

Dear Marie;

....from the information I've gleaned about PaleMale & Lola's nesting situation, it seems to me that the single element that separates them from other successful urban nesting couples, and also separates them from their own past successes, is the underdraft to their nest - specifically the egg-nest part. No matter how warm their breasts would keep the eggs, the open air from underneath would take temperature from the eggs from the bottom, by convection. ...

I don't know if there is a viable way to insert a windblock...perhaps the architectural firm that designed and installed the support/spikes could get out their specifications and make one for a perfect fit, and after the birds give up on this group, get in there as quickly as possible and insert and secure the windblock to the nest support.

Elizabeth Kyle
Galveston, Texas

Here's what I answered, followed by John Blakeman's additional comments:

Here's the problem with your idea: a nest in a tree [redtails' usual site] also wouldn't have a completely solid base as they happened to have on the window ledge. In a tree nest there are all sorts of chinks and holes where air can get in from underneath. But redtails are hardwired to line their nest in such a way that they can tell that no air is getting in from underneath. They'll just keep adding lining material until everything is perfect.

But...I agree that it MUST be something about the "cradle" they put up there, something that gives them a false impression of completion when the nest still needs more sticks or more lining materials. And another possibility, maybe there's a problem with the cradle material, which is not wood but steel. I'm going to pass this on to John Blakeman, your question and my response, and see if he has a response.

Thanks for writing!

Here's John Blakeman's response:


Your response to Elizabeth Kyle's concerns was virtually identical to what I would have stated, only to add that I've seen a great number of new red-tail tree nests that were so poorly built I could look up through the nest with binoculars and see bits of the sky above. Of course, none of these were successful. First attempt nests are frequently thin and leak air through them.

But more experienced adults create thick (10 to 20 inches or more) nests in trees. The bowl or cavities of these get densely packed with fine lining materials that seal out the rising cold air beneath.

The shallowness of the 927 nest is a concern, but it appears that the eggs there are warm enough to develop and pip. If they eggs were dead or unfertilized, the adults would just sit on them and continue to incubate into April. They wouldn't stand at the edge and look down into the nest at anything. Because they appeared to do that this year, it's probable that the eggs developed and initially pipped or hatched (but perhaps too quickly, causing the eyasses to die from lung desiccation).

The 927 nest is shallow, as you stated, because the adults stop adding sticks when they feel that everything is solid and firm, the result of the pigeon prongs which hold everything together.

Once again, it's good to have such intelligently questioning readers.


--John A. Blakeman

Several readers have written to ask about possible rat poison or pesticides that may have affected the egg development.


About chemically testing the eggs, I still think it is highly suspicious that the failures have happened since that underneath structure went in. Could a bird expert learn anything from the fragments about that?

Of course, I agree they shouldn't be retrieved early. Sure wish there were a camera up there so we could see if the eggs actually hatched, that would be a clue.

Karen Anne Kolling

Another letter:

Hi Marie,

Thanks for all of your web updates concerning the hawks. I'm happy to see that the times reported on this. I'm also very happy to see that you (and others) are planning on examining the nest/eggs when PM and Lola decide to abandon the nest. I find this particularly necessary since nobody examined the nest last year (as far as I know).

Aimee Van Dyne

And another:

Hi Marie,

What about pollution/toxic accumulation as being a problem with fertility?

Waiting with much anticipation and anxiety,

amy campbell

John Blakeman, in regard to testing the eggs for pesticides, expresses a diffent opinion about when to retrieve the eggs:


I saw the posting suggesting the retrieval of an egg for pesticide analysis. If it were done, I'm absolutely certain nothing untoward would be discovered. First, red-tails never were affected by DDT. Their prey, in this case, squirrels, rats, and pigeons, just don't accumulate pesticides. Yes, a stray rat may have some powerful rodenticide that
could affect the hawk, but the affect would be death, not a contaminated egg. And Junior and Charlotte on the South are eating the same foods.

On the other hand, don't be concerned about any disruption of the pair by someone entering the nest. If red-tail pairs could be disrupted by someone entering their nest spaces, the species would be in trouble. In rural areas, farm machinery, cattle, and just plain hikers commonly move right up beneath a nest tree and almost always drive off the sitting adult. We are talking here of nests just 50 ft above the humans on the
ground, not a nest 12 stories (sp?) high. The birds always -- always -- return and resume their normal activities.

I've been involved in any number of incursions into active red-tail nests, and it's always a spectacle. As the nest is approached while climbing the tree, or while exposing ones self at a nearby elevated observation blind, the birds begin to scream loudly. As the nest is approached, they begin to make awesome diving plunges at the climber. When we reach into the nest to temporarily take out the eyasses for examination and banding, the adults become even more incensed. They dive at us with talons outstretched as they shoot past our heads just a few feet away.

But they seldom actually contact the intruding researchers (except in California, where the red-tail population is noted for attacking nest intruders -- out there, the researcher is wise to wear a leather, talon-proof jacket, and a helmet, preferably with two big eyes painted on the back of it to help divert the hawks' attacks).

In all cases, the hawks quickly return to their normal activities once we leave the nest and the nest tree. This apparently disruptive episode doesn't threaten the pair bond, fidelity to the nest or territory, or result in any other untoward happenstance. If anything, an infrequent "nest raiding" experience actually strengthens the pair in their
cooperative defense of the nest.

Red-tails commonly encounter and know instinctively how to react to nest incursions. It's not overly disruptive. They can withstand such an event perfectly. There is no harm done, if conducted by someone with knowledge and experience.

Throwing a saddle on a colt for the first time is probably more disruptive.

Now, back to your more important point, the presence of the steel at the bottom of the nest. Could this be conducting away sufficient heat to preclude successful incubation? Perhaps. The nest is already very shallow, a result of the "supporting" spikes, which trick the adults into believing that the shallow nest they have constructed is sufficiently firm. Without the prongs, a large pile of loose sticks must be assembled to create the feeling of a firm nest.

But if the steel bottom of the present new structure were conducting heat away from incubation, we shouldn't have seen the birds standing up and looking down into the nest. Because the eggs would have been dead early on, they would have remained in the nest and incubation would have gone on without disruption or change, for probably six or seven weeks. With dead or infertile eggs, the birds just sit, interminably, until they give up as hormone prompts tell them to start doing other things in their daily lives.

But am I correct in understanding that the adults stood around the 927 nest and looked down into it, as though new eyasses (or a broken egg) had appeared? If so, that's evidence that an egg had developed and something hatched or fell out of an egg.

I don't feel that the metal bottom could have cooled the eggs. The sitting female would have discerned the coolness in the nest and would have continued to tuck in new lining until everything felt warm enough for her babies. That's a crucial element in successful nesting. It would have been attended to by this older female.


John A. Blakeman

A new angle is brought up in the following letter:

Dear Marie,
Like so many others who love them, I have been spending the last week processing my disappointment over Pale Male and Lola's egg not hatching.
Someone I work with told me that other species of birds roll unhatched eggs out of the nest. I was hoping that you or John Blakeman could tell me what happens to the unhatched hawk eggs.
Thanking you for any information you can provide.
Joan Corr

Now Donna Browne adds a comment based on her experience with doves:

Hi Marie,
Remember in past seasons how it was said that the reason the Central Park South Hawk's nest had failed was because the eggs had rolled out of the nest?
Then you posted an email from John Blakeman who said no self respecting Red-tail would allow their eggs to just roll off. There was talk about the high winds up there on the corbel and the possibility that perhaps the corbel was slanted just a little for drainage.
I wondered at the time if Charlotte, as do the female doves I have at home who are mateless and therefore lay infertile eggs, had rolled the nonviable eggs out of the nest herself. Knowing eventually, in some way, that they were duds and therefore clearing the decks for a second clutch. But that brought up the question as to why in failed years the Fifth Avenue hawks had not done the same. Did Charlotte know something that Pale Male's mates didn't? Is there a difference in the nests or location that allows the procedure in one but not the other? Is Charlotte "wired" as are my doves for this behavior but then why aren't the other RT females in the park?