Thursday, April 28, 2011

Fighting eyasses

Though the eyasses are still only a few days old, they are already starting to squabble and peck at each other. Here are the two oldest going at each other on Wednesday.

This can look pretty vicious, but John Blakeman gives us some useful perspective on the developmental need for such sibling fights.

"What you see is quite normal. With Red-tails, it’s never of any concern. No harm will come to any of the eyasses, unless the parents fail to bring sufficient food. If that occurs, a larger eyass can kill a smaller one, after profound hunger. That won’t happen here, by any means.

So, just as little kids “fight,” little eyasses do a bit of this. After all, these birds are born killers. All of what you see is normal nerve and muscle development. Just as with little children, the young eyasses have to learn how to use their muscles in coordination. Poking their beak at another sibling, even sometimes stealing a piece of food it is eating, is all normal and even helpful behavior.

Golden Eagle eyasses do this same thing. But universally, it turns out differently. It’s called the Cain and Abel Effect. Inevitably, the larger Golden Eagle eyass will slay the younger one. Golden Eagles most often lay two eggs, and both hatch. But the larger hatched eyass at some time before fledging will reach over and sink some talons into its brother or sister, killing it.

Our Red-tails are much more respectful and civil!

–John Blakeman


  1. what happened to the nest???? did the camera move? it looks like it is gone? worried :-(

  2. Just the night time reflection from inside the Boardroom - it "covers" the nest. All is well.

  3. I was across the street from the nest today (Mother's day) observing with my father. We spotted the formel on top of the green circle on the far building across from the nest. The building with weird 80's designs on it. She took off and we lost her. Then we saw the tercel with the formel and at least three other redtails. Some juvenile, some adult which all began to kettle up above the corner of the franklin above the nest. We both thought this was interesting behavior as we had never seen such kettle in an urban environment. The formel gave a loud "kee-awww". That's when we spotted the peregrine way up above. It folded up and dove right on the tercel who grappled talons for a breif moment. This happened three more times before the peregrine got bored and flew off. The other redtails left as well and the pair hunted down two rats which they began feeding to the trio of eyasses who were completely unaware of the drama which had unfolded above them. What a day!

  4. This is SO interesting, Erik, especially the presence of the three other red-tails. Did you happen to notice if they actually did have red tails, or were they brown-striped? I wish I could offer some explanation, but this is way beyond my expertise. I will share your observations with John Blakeman for his insights. Thanks so much for posting here.

  5. My fiancee lives in Philly and whenever I visit I stop by to see the Franklin red-tails. This time I took my father along (mother and fiancee went to Reading Terminal Market, haha!) My father and I used to band raptors for fish and wildlife down at Kiptopeeke in Delaware. Are the Franklin birds banded? We couldn't see from the parking area. As for the other red-tails there was a juvenile (maybe second year?) and two other mature birds. The peregrine/red-tail dog fight took place a couple hundred feet up so it was tough to get a solid look at the falcon. Does anyone know of a peregrine nest in Philly?

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  7. There has been a peregrine nest for a few years on the SW corner of City Hall - about half to three quarters of a mile from the Franklin Institute. That's probably what this peregrine was defending.

  8. Erik- Yes, there was at least one known Peregrine Falcon nest at City Hall, as of a couple of years ago:

    They've also been seen sometimes on the Liberty towers: