Castle Espie's captive bird collection has fallen from around 500 birds to fewer than 100, driven by a combination of Brexit and avian influenza, according to Alex Lane.
The reserve serves as a safe haven for native and exotic birds, ducks and geese, including globally at-risk species.
Beginning in November, all captive birds at Castle Espie will be transferred to other WWT sites across the UK.
The closure of the site's captive bird programme and guided school tour programme forms part of wider WWT cost-cutting measures, alongside difficulties in transporting animals since Brexit.