ABSTRACT: Seabirds consume forage fish, which are keystone species in many marine ecosystems. The Junk Food Hypothesis proposes that high-lipid prey should produce better reproductive performance by seabirds. In the Gulf of Maine, changes in the forage fish community followed rapid warming post-2005 and included a decline in high-lipid Atlantic herring Clupea harengus. We studied 3 species of alcid (Atlantic puffin Fratercula arctica, razorbill Alca torda, common murre Uria aalge) over 23 yr at 3 colonies to assess changes in chick diet and its relationships with reproductive success. Puffin and razorbill chick diet changed over time; puffin diet was highly variable taxonomically, whereas razorbill diets were more consistent, showing proportional changes within fewer taxa. For puffins and razorbills, herring was replaced by sand lance Ammodytes spp. and other taxa with lower energy density. Puffins did not require high-lipid fish to breed successfully, but diet-reproduction relationships became unpredictable following extremely warm winters (2013 and 2016). Razorbills and murres provisioning with low-lipid fish showed reduced chick condition and breeding success. We concluded that razorbills and murres need higher-quality diets than puffins, which more frequently exploited lower-lipid food during food shortages. However, puffin reproductive output was much more vulnerable to ocean warming owing to their longer breeding season and more varied diet. Different responses of closely-related species to changes in prey are driven by differences in chick-development strategies with clear implications for using seabirds as environmental indicators.