Wind plays an important role in many of the biological processes that imply movements, such as plant dispersal or migration of flying species. Birds are known to select favorable wind conditions – usually tailwinds –and this selection is sometimes even indispensable to fly across long barriers, as the wind largely impacts the cost of flight. Therefore, it can be expected that wind also has an influence on the movement patterns of birds during their dispersal period, especially on raptors, whose large size and weight require them to be as energetically-efficient as possible when flying . Here we assess the effects of wind on juvenile dispersal movements in a long-lived large-sized bird of prey, the Spanish Imperial Eagle. In particular, we use high spatio-temporal resolution information from GPS-GSM tags in order to determine the effects of wind speed and direction on: (1) the first flight at independence (i.e., when juveniles definitely leave the natal territory), (2) the wandering dispersal flights, and (3) the flight speed of the species during the juvenile dispersal period. Our findings show that the direction and intensity of the wind influenced the onset of independence and the patterns of subsequent long-distance movements during juvenile dispersal in the endangered Spanish Imperial Eagle. The observed effects varied according to movement parameters and among natal subpopulations with different geographic and landscape features. Interestingly, low- to moderate winds (i.e., 2-5 m/s) were also found to be responsible for longer distances and a faster flight during the daily dispersal movements. This wind-related influence on the timing and movements of dispersals can have important consequences on individual fitness and population dynamics.

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.