Abstract
Birds of prey frequently feature in reintroductions and the hacking
technique is typically used. Hacking involves removing large nestlings
from donor populations, transferring them to captivity, feeding themad libitum . Potentially, via the hacking method, stress of
captivity and disruption of parental feeding may be detrimental.
Alternatively, provision of ad libitum food may be advantageous.
Although hacking has underpinned reintroduction project successes there
has been no research on how the method may affect the health and
nutritional status of translocated birds during captivity. We compared
blood chemistry data from 55 young White-tailed Eagles, translocated
from Norway as part of the species’ reintroduction to Scotland, from
sampling soon after arriving in captivity and again (≈ 42 d later)
before their release. Numerous significant differences between first and
second samples were found, but no significant interactions showed that
sexes responded similarly to captivity. According to hematological and
biochemical metrics, individuals showed several changes during
captivity, including in red blood cell parameters, plasma proteins and
white cellular parameters related to the immune system, that indicated
improved health status. Captivity with ad libitum food was
associated with decreased urea and uric acid values: high values can
indicate nutritional stress. Urea values became more normally
distributed before release, indicating that ad libitum food had
reduced nutritional differences between early nestlings in the season
and later ones. Despite plentiful food, both sexes lost body mass before
release, suggesting an inherent physiological mechanism to improve
flight performance in fledglings. We conclude that hacking improved the
health and nutritional status of released eagles which is likely to
enable birds to cope with greater costs of exploratory behavior which
they may require in reintroduction projects. In this context, we note
the absence of survival differences between hacked and wild raptors in
previous research.