Telemetry devices used on seabirds have been shown to affect their behavior, reproduction, and survival. However, studies have primarily been limited to larger species, and results are occasionally equivocal. We quantified tag retention time and effects of attaching subcutaneous anchor radiotransmitters on behavior, provisioning, and reproduction in the Common Murre (Uria aalge), a medium-bodied seabird. Between 1999 and 2001, we tagged 48 Common Murres on Tatoosh Island, Washington. Subsequently, 46 birds were detected during colony observation, radiotracking, or both. Activity budgets and time spent at the nest site did not differ between tagged birds and their untagged mates. However, tagged birds made fewer but longer trips away from the nest and provisioned their chicks significantly less than their mates did (0.07 ± 0.02 fish h⁻¹ and 0.18 ± 0.02 fish h⁻¹, respectively). Prey size and energy content did not differ. Despite the disparity in provisioning rates at the individual level, tagged pairs and control pairs had equivalent energy-delivery rates and reproductive success. Tagging effects did not persist in the long term, because percentage of tagged birds returning to the colony the next breeding season was similar to percentage of birds without tags, and tagged birds' reproductive success was comparable to that of the rest of the colony. Subcutaneous anchor attachments compared favorably to glued tags and implanted transmitters in terms of retention time and survival, respectively. We recommend using subcutaneous anchor transmitters for medium-bodied seabirds that employ flexible foraging strategies. Possible exceptions are years of poor food availability, when the capacity to absorb tag effects may be lower.