Post-fledging dependence and dispersal in hacked and wild Montagu's Harriers Circus pygargus

We compared the post-fledging dependence period, dispersal patterns and resighting rates of captive-raised and hacked Montagu's Harriers Circus pygargus with those of wild reared and naturally fledged Montagu's Harriers, in order to assess the validity of hacking as a release technique for this species. Among hacked birds, we found significant relationships between the length of the dependence period and both hatching date and body condition (birds hatched later in the season had shorter periods of dependence; birds in poorer condition had longer periods of dependence). The condition index increased with the amount of time spent in captivity. The length of the dependence period did not differ significantly between released and naturally fledged birds, when hatching dates were accounted for. Observation rates of wing-tagged fledglings in years after marking were higher for released birds (15.5-20.9%) than for naturally fledged birds (9.4-9.9%). Released birds also tended to be resighted closer to their released location than did the naturally fledged birds. We suggest that the differences in resighting data are related to the better condition of the released birds, compared with their wild counterparts. We conclude that hacking is apparently a good release technique for captive-reared Montagu's Harriers, and that comparisons of behaviour at the post-fledging period may be a valid method for evaluating the success of release techniques.