# Effects of Flight Restraint and Housing Conditions on Feather Corticosterone in White Storks Under Human Care

## TL;DR

This study found that flight restraint in white storks does not significantly affect their stress hormone levels, with housing conditions being a more important factor.

## Contribution

The study introduces a less invasive feather sampling method and provides new evidence on the welfare effects of flight restraint in white storks.

## Key findings

- Flight restraint did not significantly affect corticosterone levels in white storks.
- Housing location was the main factor influencing stress hormone levels.
- Behavioral observations supported the lack of significant stress differences.

## Abstract

Flight restraint in zoo birds raises welfare concerns, yet scientific data on its impact remains limited for many species. Our study examined whether flight restraint affected welfare in white storks by determining stress hormones (corticosterone) in feathers and observing behaviour. We compared fifty-three flight-restrained zoo birds, eleven hand-raised nestlings from German zoos, and seventy wild storks in rehabilitation. Using a minimally invasive sampling method that only required cutting feathers close to the skin, we tested whether flying storks would show different stress hormone levels compared to flight-restrained individuals. Surprisingly, we found no significant differences in hormone levels related to flight ability or restraint method. Instead, the location where storks were kept emerged as the main factor influencing stress hormone levels, with significant variations between different sites. Our behavioural observations supported these findings. This study provides valuable evidence that flight restraint may not directly impact stress levels in white storks, contributing important information for zoo management practices and avian welfare assessment in settings under human care.

Flight is part of the natural behaviours of most bird species, and as a consequence, flight restraint in zoos, even for those species that are primarily ground-dwelling, encounters increasing animal welfare concerns. While previous studies on greater flamingos (Phoenicopterus roseus) and white pelicans (Pelecanus onocrotalus) have found no significant effects of flight restraint on welfare, scientific data on other species remains limited. This study investigated the welfare implications of flight restraint in white storks (Ciconia ciconia) by assessing feather corticosterone concentrations (CORTf) alongside behavioural observations. We compared CORTf values of deflighted zoo birds (n = 53) and hand-reared abandoned nestlings (n = 11) from eleven different zoos in Germany and storks in rehabilitation. These birds were wild individuals, found injured, and therefore provided an opportunity to collect feather samples (n = 70). In line with the 3R principle proposed by Russell and Burch, we employed a recently validated, less invasive feather sampling method that involves cutting feathers close to the skin. We hypothesised that CORTf would differ significantly between the wild, airworthy storks and the deflighted individuals under human care. However, we found no significant difference in CORTf regarding the ability to fly, nor did the method of flight restraint show a significant influence on CORTf. Housing facilities with significant site-specific variations emerged as the predominant factor influencing feather corticosterone concentrations. These results suggest that flight restraint does not have a noticeable direct impact on the corticosterone levels of white storks, and the behavioural observations support the reliability of these findings. This study enhances our understanding of the effects of deflighting procedures on the welfare of white storks in zoos.