Hacking Techniques Improve Health and Nutritional Status of Nestling
White-tailed Eagles
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 them ad
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.