The "Domain of Danger," and How to Stay Out of It

[Brown-headed Cowbirds and Red-winged Blackbirds, "balled up" in front of a Merlin near Mauricetown, Cumberland County, NJ on Tuesday. Click to enlarge photos.]

Back in college, about a zillion years ago, I negotiated my way through W.D. Hamilton's seminal 1971 article called Geometry of the Selfish Herd.  You'll understand my choice of words if you hit the link.

Nonetheless, Hamilton's notion of the "Domain of Danger," and how it is reduced for animals that join flocks, is something we see as birders every time we watch a flock of blackbirds or shorebirds evade a raptor. The gist of the notion is that, by simple math, the more of your friends you are with, the less probable it is that you will be the one that gets eaten! Thus, if you have compatriots surrounding you, your personal "domain of danger" is reduced substantially when a hungry Merlin flies by. That's not to mention the confusion factor, that being the difficulty for a predator to select one target out of a dense, swirling flock. There is an extensive body of literature on the pros and cons of flocking before and since Hamilton, of course, with numerous other hypotheses as to why it all works, but no one has seemed to have figured it out completely. No one, except the birds - because if flocking and "balling up" didn't work, the behavior wouldn't persist!

[This Merlin along Buckshutem Road north of Mauricetown never did figure out how to excise a blackbird from the flock.]