The long‐term effect of fitting back‐mounted radio tags to juvenile tawny owls Strix aluco

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Abstract
We investigated the long-term effect of fitting back-mounted radio tags to nestling tawny owls Strix aluco in northern England by comparing apparent survival rates of radio-tagged and non radio-tagged nestlings from two cohorts using capture-mark-recapture methodology. If radio tags had no effect, apparent survival rates should not have differed between the two groups of birds. Overall, juvenile survival was much lower than adult survival and yearling breeding probability much lower than adult breeding probability. Radios had no significant effect on yearling breeding probability, but they did have a significant negative effect on juvenile survival. Overall, just one bird (1.8%) was recruited from the radio-tagged group of 55 birds, whereas 14 (16%) were recruited from the control group of 89 birds. There was no significant difference in the distribution of sexes between groups that could account for these differences in apparent survival rates. Moreover, food supply and breeding performance of broods in the two groups did not differ significantly, indicating that differences in rearing conditions were not responsible for the disparity in survival rates. Based on our results, and those from other studies of tawny and spotted owls Strix occidentalis, we recommend that back-mounted radio tags should not be used on nestling or recently fledged Strix species.