Wednesday, April 02, 2008

John Blakeman on "Leaving The Eggs Uncovered"

Pale Male's 2007 eggs. Note the height of the nest walls and the small fragment nest lining, both of which would, in normal circumstances, protect the eggs from drafts and sudden cooling during a parental break from the nest.
And as promised, John Blakeman on raptors, and leaving eggs on the nest unattended--

I, too, saw the posting at noting the 11 minute absence of Lola from incubation on the nest. This would seem to be a lengthy period of the bird's absence, during which it might be presumed that the warm eggs would cool and perhaps die.

But those of us who have studied rural Red-tail nests have seen this often. When first noticed, we all are greatly concerned. The sitting bird gets up off the eggs, stands on the edge of the nest, and starts to preen, stretches her legs, and looks around at the landscape's scenery. Frequently, she will also "slice," defecate in the typical Red-tail manner, backing her tail over the edge of the nest and expelling her feces ("slices") away from the nest. In most cases, after just a minute or two of all of this, she then dutifully walks ever so carefully (with folded talons) back over to the eggs and settles in again.

But from time to time, as noted in this case, instead of promptly resuming incubation, the bird inexplicably flies off, leaving the eggs un-warmed. In this case, it was for 11 minutes or so. Here, in our Ohio nests, we've seen incubation lapses of up to 20 minutes.

When seen, this is unnerving, especially in late March or early April, when air temps are still quite cool. One doesn't have to do any thermodynamic calculations to understand that the eggs must actually cool somewhat in these absences. But in virtually every case, the eggs go on to hatch without incident.

We've seen this often enough to believe that periodic, short and mild coolings of the eggs are actually beneficial, for perhaps the following reason. One learns in first-year chemistry that the solubility of gases in liquids in increased as the temperature of the liquid declines. Cold water in a trout stream holds a lot more oxygen than the warm waters of the lower Mississippi. So, could it be that these periodic and lengthy incubation absences are actually methods to allow increased oxygen diffusion through the porous egg shell into the fluids of the egg, thereby providing an episode of more-available oxygen?

That's all prognostication or hypothesis. Someone needs to create an artificial Red-tail egg that detects motion (the egg turning routines) and temperatures, all of which would be broadcast to a nearby antenna and recorded. At the same time, the activities of the sitting adult should be simultaneously recorded on a digital movie camera. Then, we'd get the real story of all of this.

There is still a great deal of scientific ignorance about Red-tailed Hawks. We still can't completely explain everything that this regal species does. Leaving the eggs un-warmed is one of those.

--John Blakeman

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