Radio telemetry has been an invaluable technique to study waterfowl ecology, but impacts of radio packages on ducks have not been experimentally assessed during the non-breeding season. We tested the hypothesis that backpack-style radios with 2 body harness loops influenced the body mass dynamics of 8-week-old captive-reared mallards (Anas platyrhynchos) released in Maryland in August. We attached mock radio packages and visual markers to 477 experimental ducks and visual markers only to 582 control birds. Half of experimental and control birds were released on a tidal marsh and received no additional care. Mean body mass of both experimental and control birds released at the tidal marsh decreased over a 3-week interval after release, but mass loss of experimental birds (10.2 g/day) was greater than controls (4.5 g/day). We released the remaining birds on a private Regulated Shooting Area (RSA) where grain was provided for several weeks after release. Among the latter birds, experimental mallards gained less mass (7.7 g/day) than mallards with only visual markers (10.5 g/day). This is the first study of free-ranging, but captive-reared, mallards during the non-breeding season that used an experimental protocol to test effects of radio packages on ducks. Our results parallel studies of free-ranging wild birds during the breeding season in showing that backpack harness radios were detrimental to mallards. We believe there is substantial risk that backpack harness radios influence attributes that biologists are trying to assess using telemetry.