Interesting that you went in the Tropical House in May and there were birds inside. We visited the day after the member preview and were told they were having heat issues, we've been once a month since and the building was always closed. In August it was open but there were no animals inside at all, it was set up as a conservation learning exhibit. They must have removed all the species they had added due to the heat issues.
My son found this page today on the website
Bristol Zoo Project which made us think it was open but I think it was uncovered via a Google search and may not actually be "live" on the site since it talks about bringing the Mouse Deer in May and Beetles in early June both of which have now come and gone.