Mate protection and winter pair-bonds in black-capped Chickadees

Black capped Chickadees (Parus atricapillus) that bred together in the spring had significantly higher winter association indices than birds thai did not breed together, indicating winter pairing. Most members of winter flocks were paired, and pair members had the same within-sex dominance rank in nine out of ten cases. These results confirm the findings of other studies of wintering chickadees. We report the first quantified example of mate protection in a North American parid. From observations of chickadees visiting feeders in winter, we identified three benefits for females paired to alpha males that are consistent with the hypothesis of mate protection: (I) these females were subject to less intense aggression from their mate, compared with that between other dominants and subordinates. When accompanied by their mate, these females experienced (2) less frequent aggression and (3) maintained a higher feeding rate than other subordinates in the presence of dominant flock members. Chickadees commonly form a 'pair-hierarchy' within a flock. We suggest that mate protection has a role in the formation of pair-hierarchies, and that mate protection is the main cause of the overlap in survival rates between the sexes. These results, and this view, suggest that a wintering female's pair bond status may have a greater effect on her fitness than her dominance rank.