Why did the formel attack the escaped cage bird?

Sunday's blog entry detailed the formel's dramatic and gory capture of an escaped cage bird. Because she is now in full incubation mode, and mostly relies on her mate to bring her food, I asked John Blakeman whether she had to hunt because the tiercel wasn't keeping her sufficiently supplied with food, or whether it took a few days for the "Stay on the eggs all the time" mode to switch on for her after the third egg was laid.

Here is his fascinating reply. Who knew that there is a No-Kill Zone around the nest?

From John:

Here are some important understandings of the formel's capture of the escaped cage bird, whatever it might have been.

First, she quickly spotted the bird because it flew or acted in a manner the formel had never seen. Red-tails are always looking for or noting anything different in their territory, especially any small potential prey item.

In this case, the bird probably fluttered its wings in such a manner that it appeared at a disadvantage. As some sort of tropical bird, it may have been at a thermal disadvantage in Philadelphia in March. It may have been quivering to stay warm. Even from three blocks away, the hawk would have seen that instantly and recognized the bird's vulnerability.

The formel nailed this non-native bird for reasons entirely unrelated to the tiercel. She had been sitting in the tree for a long time, and was able to take a break from the nest to preen, to slice (defecate), stretch, and otherwise make herself comfortable for a time in her more usual posture.

But when she saw the small bird, she was instantly after it. Couldn't resist.

Note that the captured bird was some distance away. This is important. We know that nesting red-tails and other raptors have an interesting response to prey when sitting on eggs or tending to hatched eyasses. The adults' hunting and killing impulses are markedly suppressed within a certain radius of the nest. I've seen house sparrows nesting inside the sticks of a large, well-used and active red-tail nest. The haggards pay no attention to these otherwise fine prey birds, although they could easily jamb a foot down into the sparrow nest and pull out a fistful of baby sparrows.

That's because if they attacked them, or any other prey animal near the nest, they could also in an unguarded, impulsive instant just reach out and kill a downy eyass. For those of us who have watched falconry red-tails kill game, we always marvel at the restraint of the haggards' normal killing impulses at the nest. If they found a similar downy bird on a nest elsewhere, they would pounce instantly on it.

Pale Male and all of the New York City nesting red-tails have learned to visually scrutinize the walls of Manhattan buildings, looking for pigeons nests on window sills and building ledges. They take nestling squabs (baby pigeons) with ease, delight, and with no restraint. A pigeon squab at the start doesn't look much different from a newly-hatched eyass.

Around the nest, within a few hundred yards or so (it varies greatly from nest to nest), prey animals are safe. There is a respected no-kill zone around each nest. Rats and mice and pigeons within this zone will not be preyed upon during the nesting season, so as to never harm an eyass, who would make as fine a meal as any other prey animal.

In this case, our formel saw a vulnerable bird far outside the no-kill zone, and she shot out and took it. Even though she's spending most of her hours sitting on three eggs and keeping them warm, she's still looking out over the landscape next to her nest and is watching all the animals and activities out there. When given a chance, when an animal is seen to be both beyond the no-kill zone and vulnerable, and when she's able to get up off the eggs because the tiercel takes his brief turn on them, she'll be on the attack. After all, she's a hawk, an avian predator of the highest order. She lives by hunting and killing. When prey is seen and the opportunity arises, she will make a kill."

--John Blakeman