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We compared foraging behavior and productivity of harlequin ducks (Histrionicus histrionicus) in Newfoundland in 1997 and 1998, and central Labrador in 1997 to 2002, and tested predictions that productivity of this species is limited by available food. Females without broods were observed early in the nesting cycle (>70% of the local population), and were ascertained to be failed nesters. These adult females without broods were indicative of what previous researchers had defined as "nonbreeders." Productivity of harlequin ducks breeding in central Labrador ranged from 10.0% to 88.9% of females producing broods with 1.00 to 3.38 young per female present, annually. Females spent relatively low proportions of time feeding (mean ± SD: 0.385 ± 0.014 of ∼47 h of daylight), and there was low variability across time and space. Therefore we inferred that foraging behavior was not tracking a variable food resource, and birds could have budgeted considerably more time to feeding if this had been necessary to meet their nutritional requirements. Physical evidence of nesting, including anatomical and radiotelemetry data, indicated that all adult female harlequin ducks in our study area attempted breeding each year. We found no support for the paradigm that females were constrained by lack of sufficient food on their breeding habitat and deferred breeding. We suggest that hypotheses such as food limiting breeding productivity can be tested using behavior that can be readily quantified, and with greater rigor than attempting to measure aquatic epibenthos in fast moving rivers. [ABSTRACT FROM AUTHOR]