Have retractable perches that are only usable after a coin or token has been placed in a proper receptacle. Make it so that the perch retracts after a user has let go of it.

For expecting mothers, the elderly and the disabled, you may have structures that more closely resemble nests. Again, have a coin slot activate its usability. These might have a conversible cover that stays closed until payment is made, ans remain open until a weight sensor lets it know the user has gone away.