Nice photo! I wonder if these are Torresian Imperial-Pigeons (Ducula spilorrhoa) and not Pied Imperials (Ducula bicolor)? I say that because of the bill color. The two species are extremely similar and I don't know that I can reliably tell them apart; however, the Pied Imperial-Pigeon has a silver- to gray-colored bill while the Torresian Imperial-Pigeon has a yellow bill like this one. Also, the Torresian is native to northern Australia and it might be easier for Australian zoos to obtain.
