We explored the breeding biology of the Northern Pygmy-Owl (Glaucidium gnoma) from 2007 to 2009 in a forested reserve in Portland, Oregon. Large body size is often assumed to give animals reproductive advantages, and we tested whether body size affected timing of breeding and examined variation in diet, breeding date, clutch size, and reproductive success to explore whether the presumed benefits of large body size are evident in this species. The average size of 13 clutches was 5.8, and nest success was high (92%); 22 successful nests fledged an average of 5.2 young. Dates of first laying varied over an 18- or 19-day period in each year and averaged 3 weeks earlier in 2008 than in 2009. Early breeders laid larger clutches. After the effect of breeding site was controlled for, small rather than large female pygmy-owls bred earliest. The same pattern of small size was evident in all years. Diet varied by year in regard to whether mammals (2008) or birds (2007 and 2009) were the dominant prey taken, and the owls bred earlier in 2008 occurred when mammals dominated their diet. Although small females tended to breed earlier in all years, it was only in 2009 when pairs bred very late and birds dominated their diet that small females also laid larger clutches and fledged more young. These data suggest that the merits of small or large body size may vary with stages of the breeding cycle and ecological conditions.