The Common Tern (Sterna hirundo) rarely produces more than one brood per breeding cycle. In 1996, 18 of 365 pairs (5%) of Common Terns nesting at a Lake Ontario colony laid second clutches while caring for first-brood nestlings; no second clutches were recorded during four previous breeding seasons at this location. Second clutches were smaller than first clutches of the same pairs (2 versus 3 eggs), and were initiated when first broods contained 1-3 nestlings that ranged in age from 12-22 d. Success of second clutches was low. Only two nestlings, from one second clutch, survived for >18 d; all remaining second clutches were abandoned or predated prior to (N = 16) or just after (N = 1) hatching. Three proximate factors were examined, within and among years, related to the production of second clutches: 1) earlier initiation of first broods, 2) reduced parental effort toward first broods, and 3) foraging conditions during rearing of first broods. Second clutches were not related to earlier breeding or reduced parental effort: clutch initiation dates, clutch sizes, and egg volumes of first clutches were similar among years, and first-clutch size and initiation date did not differ between pairs producing one versus two clutches. Conversely, parental care patterns in 1996 had several unusual characteristics compared to other years, including: (1) shorter provisioning trips, (2) higher provisioning rates, and (3) reduced rates of kleptoparasitism. These characteristics suggest that an abundant food supply close to the colony in 1996 was the likely proximate explanation for second-clutching.