In recent years, placing indoor enclosures for elephants, monkeys, apes, hippo's, ... inside some sort of greenhouse with birds has become very popular. To name a few: Rimbula (Emmen), Hippodom (Köln), Pongoland (Leipzig), Africombo (Magdeburg),... Without doubt these enclosures are wonderful for the visitor, and they help using the limited space the most useful. This might sound really weird, but in a certain way this has always puzzled me: how do they keep those birds in. Do they lock the mammals out, so that the gate is only opened for a little while or do they use some sort of system that the mammals can pass through but the birds can not.? I do know one case, in Planckendael, where gibbons could acces their outdoor Island via a hatch covered by a rubber flap. Now for these small hatches I can see how this works, even for small songbirds. With larger mammals like the elephants in Emmen, I however fail to see how such flaps can keep covering the entire gap consistently after an elephant would pass through without leaving a single gap large enough for some small songbirds like bulbuls. In Blijdorp, I know that the outdoor enclosures of the Okapi's are inside an aviary, eliminating the possibility of bird escapes through there. This is however often not an option for an elephant enclosure for example. As stated above, doors that are sealed off most of the time would work, but this would mean that mammals are either put inside or outside instead of having acces to both. Seen as in a lot of northern zoos, there are many days when it's hot enough to give the animals acces to their outside enclosure but too cold to not give them acces to the indoors. In those case, they just let the doors open for the animals to choose. If this system however would mean that open-door is no longer an option, wouldn't that mean that these mammals are forced to spent a lot more time indoors? Maybe someone who has some more inside-knowledge of this can shed some light on this, and off-course feel free to give your opinion about the pros and cons off this system.