Biotelemetry devices provide unprecedented insights into the spatial behaviour and ecology of many animals. Quantifying the potential effects of attaching such devices to animals is essential, but certain effects may appear only in specific or particularly stressful contexts. Here we analyse the effects of radio tag attachment on great tits (Parus major) tagged over three environmentally dissimilar years, as part of a project studying social- and communication networks. When we radio-tagged birds before breeding, only those tagged in the coldest spring tended to be less likely to breed than control birds. Breeding probability was independent of the relative tag weight (between 5% and 8% bodyweight). When we radio-tagged both parents during nestling provisioning (tag weight between 6% and 8%), tagged parents were more likely than control parents to desert their brood in two of the three years, while in the other year no tagged parents deserted. Tagged parents provisioning larger broods were most likely to desert, especially during lower average temperatures. Video analyses did not reveal any tag effects on provisioning behaviour of parents in the year with no desertion. We conclude that radio tagging before breeding did not lead to negative effects, regardless of tag weight, but that decisions about radio-tagging parents during nestling provisioning need to be made with exceptional care, taking both environmental context and tag weight into account. Reporting results from long-term radio-tracking studies comprising several environmentally variable years is crucial to understand and predict potential tag effects and maximise the tremendous potential of biotelemetry.This article is protected by copyright. All rights reserved.