The problem is. Most birds put into outdoor aviaries have never experienced predators. Wild birds deal with the threat everyday and just see it as a part of how they live, If the colony in the aviary is left to breed, than as each generation is hatched, the problem with Predators becomes normal to them, To quote an example, Years ago a bloke who lived a couple of doors away had an aviary full of cockatiels, Cockatiels doing what they do best, chewed threw and escaped. around 20 of them, The sparrow hawks had a field day, Not because they stood out as unusual but because they were unused to pre