Intra-specific predictions of the male-costs and mutual-choice hypotheses to explain variation in the timing of pairing in waterfowl were tested in a longitudinal study of marked, known-aged Harlequin Ducks (Histrionicus histrionicus). Pairing chronologies and seasonal changes in time-activity budgets and rates of aggressive interactions were compared in relation to sex, age, and paired status. Results supported the mutual-choice and not the male-costs hypothesis. Paired females gained no immediate benefits relative to unpaired females from mate defense and vigilance, and there was no evidence that females preferred to pair as early as possible, as postulated by the male-costs hypothesis. Unpaired males spent much more time in courtship than paired males spent in mate defense and constraints on male time and energy budgets was not a plausible reason for delayed and protracted pairing of young and re-pairing females. As predicted by the mutual-choice hypothesis, decisions by females about how much time and energy to allocate to the pairing process appeared to be the main factor controlling the timing of pairing in Harlequin Ducks. Newly-pairing females invested an extended period of time in courtship and mate sampling before pairing, in spite of an abundance of courting males clearly energetically capable of maintaining a pair bond. Many young females decided to pair during mid-winter when time constraints to males should have been most severe. Time-budget trade-offs were apparent for young females and their date of pairing was related to the amount of time per day that they allocated to courtship. Pairing success of males was not related to their rate of courtship, nor was it related to the length of time they invested in courtship as they began courting while they were still immature and generally courted for several years before pairing.