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 transmitter 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 relative transmitter weight (between 5 and 8% bodyweight). When we radio-tagged both parents during nestling provisioning (transmitter weight between 6 and 8%), tagged parents were more likely than control parents to desert their brood in two out of 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 transmitter 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 transmitter weight, but that decisions about radio-tagging both parents during nestling provisioning need to be made with exceptional care, taking both environmental context and transmitter weight into account. Reporting results from long-term radio-tracking studies comprising several environmentally variable years is crucial to understand and predict potential transmitter effects and maximise the tremendous potential of biotelemetry. Journal of Avian Biology
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