Thursday, March 31, 2005


I'm posting these again up here, since John's answers have come in. They are below.

Dear Marie,

...Not only am I monitoring Pale Male and Lola's nest, I'm also "incubating" three eggs (as of this writing) with the peregrine falcons in Harrisburg PA. The various web cam views of the Harrisburg raptors include an up close view inside the nest box. Comparing peregrine nesting activity with that of PM/Lola, I noticed that the female peregrine (hen?) often leaves the eggs and nest unattended for periods of time, and the tiercel does not sit on the nest when she leaves. I noticed this behavior after the first egg was laid, and it has remained unchanged with the arrival of each additional egg. This behavior is the complete opposite of that of PM and Lola.

I have a few questions regarding eggs and incubation that I hope either you or John can answer on your web site:
1) The peregrine eggs appear to be colored a reddish hue with dark speckles; what color is a RTH egg?
2) The daily observations always include egg rotation by PM and/or Lola. I have not observed the peregrines doing the same with their eggs. How does egg rotation benefit the embryo?
3) Are RTH's the only bird where the male shares incubation duties?

Thanks again for your courtesy, and for sharing your wonderful web site with the world.

Best regards,
Marilyn Fifer



I appreciate Marilyn Fifer’s questions, as they match red-tails to peregrines, always an interesting comparison between two species that are very different but still diurnal raptors. A synoptic (broadly encompassing) view of raptors is best. One can easily concentrate on only a single species (as I tend to do with my beloved red-tails) and fail to see and understand much wider hawk phenomena.

The colors of peregrine and red-tail eggs? I've never seen a live peregrine egg, so I'd have to look that one up. Marilyn’s description is accurate, I'm sure. Red-tail eggs tend to be of a light brown background color, with darker thin blotches or spatterings of darker brown. But this can vary from almost unmarked, to very blotchy. I'm not sure of the causative variables here. I think overall egg colors can vary geographically, perhaps as a result of different foods.

Egg rotation is very important. As the embryo develops, it is supported in the egg by a number of loose, film-like membranes. If the egg is allowed to develop unturned, these membranes can grow together or fuse, distorting and eventually killing the embryo. We do something of the same thing while sleeping. Even in deep sleep we turn over every few minutes. We don't have membranes that will stick together, but you get the idea. If you're a hawk, it’s just helpful to have the force of gravity come from various angles while still in the egg. It evens things out. The peregrine should begin to rotate the eggs.

Virtually all raptor males share incubating duties. Red-tails aren't unique in this behavioral trait. The mother needs to get up and eat, and especially to defecate (to “slice,” as the falconers say about hawk defecation; falcons “mute.”), so the male often takes his minor turn and sits for a while. He'd rather be out hunting, but he does his duty. It’s sort of like a new human father. We try to change the diapers and tuck the little one into bed, but for human dads, not much of this is instinctive. For mothers, it comes naturally. Same for the hawks. I'll never forget watching the female in my red-tail breeding trials ceremoniously settle down on her eggs. Nothing arbitrary or awkward at all. But for the male, when giving the hen her moments of relief, his incubation was just a bit contrived. He did his duty, but not always with any developed parental ease. As a male myself, I understood.

Here’s one very interesting observation, alluded to by Marilyn with her mother peregrine. She noted that the peregrine female occasionally left the eggs exposed for some length of time.. This apparent neglect has been frequently noted in many raptors, and no one knows for sure what’s happening. Here’s the best explanation, albeit a bit conjectural. We've watched red-tails leave eggs exposed for up to 20 minutes or so in cool March or April air. There is no doubt that the eggs have cooled significantly in this period of time. Doesn't this, then, kill or retard the developing embryo?

Here’s what we think may be happening (and how the parent can discern this is a mystery). If you remember back to first year chemistry, you will recall that the solubility of gases in water increases as water temperature decreases. It’s possible that it may be helpful for a warm egg to cool down once every day or so, allowing increased oxygen to diffuse across the porous egg shell into the egg fluids. This cooling period may actually be a way of restoring ample dissolved oxygen levels in the egg fluids and embryonic blood system of the developing hawk. This periodic cooling may be required to bring sufficient oxygen into the egg. An egg that stays persistently at body temperature may run out of oxygen. But by allowing a brief period of cooling, oxygen crosses the porous egg shell, recharging egg fluids. That’s the best explanation I've encountered for this parental inattention to incubation duty.

But there is one other form of inattentive incubation. Often, the female will fail to sit closely on the eggs until the very last one is laid. We know that newly laid eggs can be taken from the nest and held safely at refrigerator temperatures for some time. By leaving the first eggs cold in the nest, incubation can begin at essentially the same time for all eggs. If there is a three or four day difference between the first and last eggs laid, and if the first egg got solid incubation from the start, the unfortunate last egg would hatch to find it’s older siblings much larger and more competitive for food. This occurs in many species, but seems to be minimized in red-tails. This is why the red-tail female sits rather high over the eggs until she has laid her last egg. She then sinks low into the nest and begins a fair, equal-start incubation for all of her future eyasses.


John A. Blakeman