Heterosexual affiliation involves behavioral mechanisms that limit bond membership and regulate partners' interactions. Among Common Grackles, Quiscalus quiscula, two characteristic activities of partners, following and vocal answering, develop during group activities and promote the individual specificity of pair bonds. Pairs differ substantially in the durations and seasonal timing of the early stages of nesting activities and in the ♂'s fidelity to his mate through incubation. After the relatively abrupt start of intensive nest-building, the seasonal course of a pair's behavior becomes more regular, an indication that this transition in the ♀'s state is critical in pacing the pair's breeding activities. Aggression serves primarily to adjust the spatial relationships of individuals, both between partners and between unpaired birds. ♂♂ that deserted their mates during incubation had earlier performed precopulatory displays to other ♀♀.