Responses of floaters to removal experiments on wintering chickadees

Summary

In response to natural mortality in a local population of wintering black-capped chickadees, Parus atricapillus, high-ranked flock regulars are usually replaced rapidly by flock switchers, while low-ranked regulars are not replaced. A series of removal experiments was done to examine the replacement process. A large outdoor aviary was used to house the removed birds, all of which were returned to their flocks no more than 4 days after removal. Of 10 birds taken, all 6 of the high-ranked ones (3 males, 3 females) were replaced by flock switchers. All inserting switchers made sudden jumps in rank; each was seen to supplant regulars of its sex the day after removal. All 10 removed birds, including the 6 that had been replaced by switchers, regained their former status immediately upon release. The 6 switchers that had inserted into the artificial openings were all driven away, and stayed away from the flock for an average of 15 days. Four removed birds were not replaced. All were males from the bottom-ranked pair in their flock. This is consistent with field observations: of 58 low-ranked regulars disappearing naturally over 7 winters, none was replaced by a switcher, although to do so would be an apparent jump in rank for any switcher.