Sunday, April 06, 2008

Female Peregrines Battle Over Nest Box

Peregrine Falcon
Photograph Courtesy of The U.S. Fish and Wildlife Service

Frequent blog contributor Karen Anne Kolling, sent in some questions about a Peregrine battle she witnessed on line.

Froona, who has a web site that aggregates web cams of peregrines from various places, reported a territorial battle in Nijmegen in which an invading female tried to displace a female who was incubating four eggs.

The resident female was successful. The male apparently just stood around and did zip.

That must have been terrifying for the resident female, I know they fight to the death, plus she had eggs to protect. The eggs were scattered at the end of the video. I hope they are okay.

Is it usual for a female peregrine to actually displace an incubating one? I had only heard of this before laying eggs started, usually when one of a pair had not returned from migrating yet. Apparently the intruding female returned a second time, and was also defeated.

Might she give up then? With Pale Male and Lola, they seem to cooperate to drive off intruders?

(They do cooperate but there are definite differences in how that manifests itself. Depending, I think on the category of Red-tail intruder.

Sometimes only one of them will take on an intruder. Sometimes the “ushering out” is a kind of gentle herding, other times it can become quite hostile.

Then there are the occasions when Pale Male will be ushering an unwelcome visitor out of the territory and Lola will take off from the nest like a flaming Valkyrie, leaving the eggs as she shoots away to light a fire under the visitor in a major way.

Usually in these instances though, within a few minutes, Pale Male will have come round and will stand on the nest looking extremely tough while raking his talons across the twigs of the nest.

I don’t have a definitive explanation but I think that the intruder may be a female Red-tail when Lola gets that energized.

Pale Male absolutely would not stand by while another bird scattered the eggs. Or at least he’s never shown any behavior that suggested he would—ever. D.B.)

Froona's blog is at

(Karen finds the original link. It's posted further down. D.B)

And here is John Blakeman’s extremely interesting response.


I don't know Peregrines as well as I do Red-tails. But here's my take on this.

For some reason, the intruding female intruded. She had to have had some reason to do this, and some sort of previous attachment with the territory was almost surely a factor, probably the only one. The bird may have been at the territory last fall, or even in some winter or migration past, at a time when the territory wasn't being well defended. This time, the bird came back when the resident had eggs. As I would have expected, the resident mother was able to drive off the intruder.

The inaction of the tiercel is, I think, definitive. Had this been an utterly foreign, unknown intruder, the tiercel would have driven her off with the intensity that the female did. But I think the intruder exhibited "I live here and own this territory" behaviors, which the male read and therefore didn't react. As humans, we can't so well read these nuanced animal behaviors, but the hawks can. For some reason, the intruder thought that she "had rights" and could enter. The male respected her display and presumption of such rights. The sitting female, of course, would not recognize this intruder's sense of territorial rights.

And as happens in virtually every case like this, the real resident wins. The intruder saw the intensity of the sitting female's defense and finally retreated.

As you may know, there are many cases where one of these battles leads to a death, either to an aged, less athletic resident, or to a too-intrusive new intruder.

All of this is more pronounced with the falcons, as they become particularly attached to aeries, ledge nest sites. We think that the falcons are actually more "mated" to the nest site and territory than they are to each other. It's a long, convoluted set of circumstances, which I won't detail here, but it's best not to try to think of or understand or explain these territorial battles in simplistic terms. The behaviors are very complex, and very non-mammalian.

Sincerely,John A. Blakeman

Wait there’s more... a response from Karen

Hi, John,

I had just found the original webcam site
and there does appear to be a history of intrusion.

If you click on the English version, it says:"The nesting box was placed in 1995. From 1997 through 2002 there were successful hatches with 16 fledglings. In 2003 and 2004, several hatching attempts failed because of females rivalling for a place to nest inside the nesting box." I can't tell if the resident female from 1995-2008 is one bird.

Too bad they cannot put up a second box sufficiently far away but still within noticing range of the "intruder" bird.

Regards, Karen
Karen, I’m with you. It sounds like more boxes could be worth a try.
Who knows? Even though the second female may be bonded to the site, maybe, just maybe, after enough battles she might be willing to settle for a second box. Unless of course, the tiercel in question is the only male currently around.

Donegal Browne


Karen Anne said...

I am not sure exactly how the eggs got scattered, since the video is a series of photos taken ten seconds apart. The eggs are together near the end of the video at webcam time 18:32 and then it looks like the resident female, who is highly agitated, rolls them and they wind up quite spread out at 18:38. I don't see the intruder there at that time, although she could have been just outside the nest box. I'm assuming that when eggs are rolled in normal circumstances, they are together at the end so they can all be covered. Or maybe they are normally rolled apart a bit to cool off?

Donegal Browne said...


In my experience, the eggs even when turned end up reasonably close together and certainly close enough to be covered by the sitting bird.