Feather colour is sensitive to diet (if you don't feed flamingos the particular plankton they're used to, they're white, not pink, frex), so perhaps the zoo (and possibly the ones you've got) doesn't provide the necessary variety of lizards to produce that robust chestnut colour.
The lack of huge trees is certainly a hardship to large tree nesting birds, but I don't think it'll overcome the general raptorial disdain for communal nesting, else osprey would never have taken up nesting on telephone poles.
Re: my guess?
Feather colour is sensitive to diet (if you don't feed flamingos the particular plankton they're used to, they're white, not pink, frex), so perhaps the zoo (and possibly the ones you've got) doesn't provide the necessary variety of lizards to produce that robust chestnut colour.
The lack of huge trees is certainly a hardship to large tree nesting birds, but I don't think it'll overcome the general raptorial disdain for communal nesting, else osprey would never have taken up nesting on telephone poles.
-- Graydon