Hematocrits were measured in Common Terns (Sterna hirundo) (n = 75) that were exposed to oil from a spill in Buzzards Bay, Massachusetts, USA, in 2003, and that laid eggs 17–39 days following the spill. Comparative data were obtained in three pre- and two post-spill years and from three unoiled reference sites. In non-spill years, annual means of hematocrit varied in parallel with breeding performance. Hematocrits were lower (mean 45.3, n = 75) and more variable in the oil spill year than in non-spill years, and 20% of birds sampled were anemic (hematocrit < 41.7). However, hematocrits were almost as low (mean 46.4, n = 44) in 2002, a year with naturally adverse conditions. In 2002, hematocrits and body-masses were depressed following two storms and recovered in parallel following the storms; hematocrits were positively correlated with body-mass and ambient temperature and negatively correlated with wind speed. In contrast, in 2003 hematocrits were negatively correlated with body-mass and temperature and positively correlated with wind speed. Hematocrits were very low (mean 39.8, n = 15) when birds were first sampled 20–23 days after the spill and recovered toward normal levels by day 39 (mean 49.4, n = 14). Reduced hematocrits can be a useful indicator of sublethal oil intoxication, especially in females, provided that they are not associated with low body-masses or adverse weather. Measuring hematocrits could be useful for rapid screening of populations sublethally exposed to oil.