Two conflicting needs affecting predator mobbing by great tits,Parus major

Predator mobbing by great tits, Parus major, was studied by manipulating two contextual variables: the proximity of danger to the nest hole (close versus far), and breeding condition (non-breeding versus nest building versus care of nestlings). In five experiments pitting these contextual variables against each other, great tits were presented with a multi-species mobbing chorus signalling danger to which they responded by typical predator harassment. Based on the ‘move on’ hypothesis of harassment and on context manipulation, harassment levels specific for each of the five experiments could be predictably ordered; the ensuing rank order of scores reflected, by its nature, a series of four strong inference tests. Whereas two latency measures supported the move on hypothesis, two distance-from-danger measures suggested nest hole concealment to be an opposing need; a fifth measure of harassment, comprising both latency and distance, held an intermediate position, thus tying in with the compromise nature of harassment. Considering the ‘phantom’ nature of the unseen danger stimulus applied, the defence levels observed must be regarded as moderately conservative. There are natural danger stimuli (e.g. cats) that more effectively prevent great tits displaying predator harassment.