Great tits do not compensate over time for a radio-tag-induced reduction in escape-flight performance

The use of biologging and tracking devices is widespread in avian behavioural and ecological studies. Carrying these devices rarely has major behavioural or fitness effects in the wild, yet it may still impact animals in more subtle ways, such as during high power demanding escape manoeuvres. Here, we tested whether or not great tits (Parus major) carrying a backpack radio-tag changed their body-mass or flight behaviour over time to compensate for the detrimental effect of carrying a tag. We tested 18 great tits, randomly assigned to a control (untagged) or one of two different types of a radio-tag as used in previous studies in the wild (0.9 g or 1.2 g; ~5% or ~6–7% of body mass, respectively), and determined their upward escape-flight performance 1, 7, 14, and 28 days-after-tagging. In between experiments, birds were housed in large free-flight aviaries. For each escape-flight, we used high-speed 3D videography to determine flight paths, escape-flight-speed, wingbeat frequency and act...