First to answer a question many readers have written in to ask: what did I mean when I wrote that when the eggs were examined they showed "no evidence of embryonic material "? The answer, quite simply, is that there was no sign within any of the eggs of a hawk chick developing, none whatsoever.
Now, here are some thoughts about this finding by John Blakeman, These are followed by a question I sent JB today, and his answer.
There can be only three explanations for the lack of any embryonic development in the eggs. They are:
2. Chemical Toxicity.
3. Egg Cooling.Infertility
. Would it be possible that Pale Male is now too old and no longer has viable sperm? Could be. We don't have any data that I know of concerning reduced spermatogenesis in older red-tails. The bird shows absolutely no other geriatric complication, and he copulated frequently and normally this year. I doubt that this is the cause. If he wasn't producing copious volumes of sperm, I doubt that he would have copulated so normally.Chemical Toxicity
. Lab results are awaiting. But I doubt this is the explanation, either. Any chemical toxicant powerful enough to kill the egg or embryo almost surely would have also zapped the adults, or at least appeared in their behaviors, which were quite normal. I'm betting that pesticides or pollutants won't be discovered in the eggs. On the other hand, we need to account likewise for the failure of the Trump Parc eggs this year, and perhaps some infrequent or uncommon poison got both of these clutches. The lab chemistry should affirm or deny this cause. Egg Cooling
. Lastly, if the eggs were cooled for any length of time in the first few days in the nest (after incubation begin in earnest), the microscopic zygotes would have died. Without other evidence, I'm leaning strongly for cooling to be the explanation. Yes, Lola sat diligently and normally, so prolonged cooling wasn't her fault. But if the eggs were in direct contact with the metal pigeon prongs in the cold of March, sufficient heat could have been conducted away from the eggs by these heat sinks. The metal prongs are attached to the metal nest cradle beneath the nest lining material. In the cold weather of late March, a few degrees of heat could have been continuously conducted down through the spikes and out of the eggs, no matter how tight the nest lining was.
But how might this have been different from the many previously successful nestings of Pale Male and his mates on earlier nests, all of which had pigeons spikes?
One explanation would be that for whatever reason, those developed nests were thicker, elevating the eggs entirely above the spikes, with the eggs shielded from the metal by sufficient sticks and nest lining material. Did the new nest cradle structure cause the birds to construct a more shallow nest, through which the spikes protruded? I believe that must be considered, especially if egg chemistry shows no toxins.
I hope that detailed telephoto pictures of the nest were taken when the eggs were retrieved. These might reveal any involvement of the pigeon spikes, particularly if they could be seen directly below or adjacent to the eggs. It will be recalled that one of my other prognostications was that the eggs might have been lethally pierced by the prongs toward the end of incubation, when egg shells are known to thin from the carbonic acid produced by the respiring bird in the egg. Of course, the retrieved eggs show that no such piercing occurred. The eggs also reveal that there were never any developing, respiring birds in the eggs, so the normal egg shell thinning couldn't have happened.
For those who might think me to be casting about in random directions for an explanation, perhaps I am. But that's how field biology must be done, awaiting final evidence and data. The lack of an on-site digital camera that would reveal all nest dimensions, structures, and activities mandates such informed speculations. At best, I can be only one third "correct." I could be wrong altogether, when some other, unknown lethal factor is discovered. But I hope all of this prompts others to examine the evidence and come up with their own explanations. We await a full report of both the lab findings and whatever detailed visual inspection of nest was conducted.
Postscript: My question to Blakeman and his answer:
John, a few months ago you wrote
"When Pale Male begins to age out, I think other things will first become evident. He's likely to be less active in hormone-driven nesting behaviors in January and February. He's likely to spend most of his reduced energies merely hunting, not in breaking off twigs and carrying them to the nest. He's less likely to ascend and engage in courtship dives"
Could you steer me to any literature on this idea-- ie that there is some connection between a bird's observed behavior and fertility? Blakeman replied:
There is no literature on this. There is virtually no cogent articles on any of the attendant matters. No one any where has studied a persistent red-tail nest or pair. All of the studies are population ones, not of continued single nests or single pairs. In these studies, no one can possible know the age of full adults. Once again, this is why the Pale Male nest is so important. The exact age of the tiercel is known. His previous breeding successes are known. We know what he eats. We know (sort of) what he captures and feeds his mate and eyasses. More is known about Pale Male than any other single wild adult red-tail.
But I'm (as so often) I'm still speculating, based upon my personal experiences with single red-tails over many years, one bird I had for 13 years, and another for 16. I've known falconers who've had even older red-tails. There is one falconry account of a red-tail that lived, poorly, into its twenties. But the poor thing had arthritis, couldn't fly, and had trouble picking apart its food. It should have been put down. Instead, heroic measures were extended just to keep it alive after it lost all of its normal functions.
It's from these experiences that I proposed that if Pale Male was becoming impotent other behavioral anomalies would be seen. And none this year were seen. The fact that he copulated as frequently as before suggests to me that his hormones were still at normal levels. Copulation is driven by the same sex hormones that cause spermatogenesis, suggesting that if a hawk is copulating it also has viable sperm. The sperm are produced by the same hormones that prompt the copulation. The two reproductive elements are hormonally linked.