A pint-sized Northern Pygmy-Owl, not much bigger than a pine cone, hoots from a tree-top on a winter morning. Before long, this diurnal owl - a determined predator of small birds and mammals - will attract a mob of a dozen or more small birds. Mobbing may be a collective response to danger. But it's not certain if the "mobbers" hope to drive away the predator, or simply draw attention to the threat.

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