Researchers often attach radio transmitters and other devices to free-living birds without a clear understanding of the possible consequences for their study organisms or their data. Although transmitters may affect parental investment (nest defense and offspring provisioning), this possibility has received little attention. We tested this hypothesis by placing mock radio transmitters on male Northern Cardinals (Cardinalis cardinalis) and comparing their behavior to that of uncaptured birds and procedural controls. Birds with transmitters defended their nests less vigorously than did uncaptured birds but did not modify their provisioning effort. This behavioral modification appears to have ultimately influenced predation rates, as nests of birds with transmitters had lower daily survival rates and were less likely to fledge offspring. Control birds that were captured, handled, and bled had intermediate levels of nest defense and productivity that were statistically indistinguishable from those of birds receiving other treatments, suggesting that capture, restraint, and blood collection may affect birds in ways that are independent of transmitters' effects. Interestingly, we also found limited evidence that females mated to males with transmitters increased their provisioning effort, possibly in compensation for a perceived reduction in their mate's care. Because attachment of a transmitter (and potentially blood sampling) directly affected the behavior and reproduction of birds with transmitters and may have indirectly affected the behavior of their mates, we suggest researchers cautiously balance the benefits of such methods against potential data biases and impairment of reproduction.