Miniature transmitters now permit ornithologists to track the annual cycle of smaller (< 150 g) marine birds. To assess annual movements of Common Terns (Sterna hirundo) and endangered Roseate Terns (S. dougallii), 2.5 g satellite transmitters were attached with backpack harnesses. Using the same capture techniques and investigator to fit harnesses, five Common Terns were tagged in Maine in 2017, and 10 Roseate Terns in New York, USA in 2018. Common Terns were tracked from 102 – 652 days to destinations as far as Brazil with no documented adverse impacts. Three of five Common Terns returned to Maine, USA one year after tagging, and non-functioning tags were recovered from two Common Terns in 2018 and 2019. In contrast, Roseate Terns were tracked for only 1 – 72 days before transmissions ceased, with only two birds tracked for > 7 days. We detected three Roseate Terns with their bill stuck in the harness material, with one documented mortality. Common Terns tolerated the tags and provided useful data on annual movements but were not a suitable surrogate for predicting the effects of backpack mounted transmitters on Roseate Terns. Researchers considering tracking Roseate Terns should avoid using backpack harnesses.