Dear iselpeter.

Black birds of any kind. Or mixed flocks, searching the countryside for food in winter. You've seen the breatakingly beautiful patterns as they fly against the clear winter sky, splitting off and recombining in a airal ballet choregraphed by the gods.

And so within that dance lies our query...

What bird, or what mechanism, determines the direction that the group will take in their communal search for food?

The tropism is indeed interesting.