The wildlife choose this really but, here's how it works - maybe there are ways to divert them :)
Your wildlife will typically choose to drop items near other items of the same type.
If there is no space nearby those items then they will choose the nearest open space.
If you tap the wildlife whilst they're carrying something, it will immediately be dropped in the nearest space.
Lastly, be aware; if your wildlife are carrying items and you leave the Garden, those items being carried will be lost.