Avian breeding success generally declines with later laying because of seasonal reductions in food supply, late laying by less capable pairs, or both. To understand the direct fitness consequences of breeding time requires distinguishing between these two possibilities. We used egg removal and re-laying experiments to evaluate how date and parental quality affect breeding success in a zooplanktivorous seabird, Cassin's Auklet (Ptychoramphus aleuticus). Egg laying began at the same time in all 5 years of study at Triangle Island, British Columbia, but, compared with a cold-water year, the population laid later and less synchronously in 4 warm-water years in which prey populations peaked earlier. As a result, Cassin's Auklets were less successful in years in which they laid later. Within seasons, early-laying females whose breeding attempt we delayed did not follow the population-wide seasonal declines in hatching success. This indicates a strong role for parental quality at the egg stage, probably because early, high-quality birds maintained more constant incubation. By contrast, the experimental females followed the population-wide seasonal declines in nestling survival and fledging mass. This indicates a strong role for date at the offspring-provisioning stage, which accords well with a previous study that found that success while raising nestlings is largely determined by the degree of temporal (mis)matching with the copepod Neocalanus cristatus. Our results offer novel insight into the causes of seasonal declines in avian breeding success, indicating that date and parental effects can be differentially involved, depending on the stage of breeding.