Forgotten Songs

This sight in Angel Place was new to me.  I googled it this evening to find out if it was significant and learnt that empty cages was originally a temporary art installation, Forgotten Songs,  signifying the lost birds that once inhabited Sydney, pushed away by settlement and development.  
There are 180 cages and 10 speakers that play the songs of 50 bird species.  The sounds of the calls change from day to night with daytime birds' songs disappearing with the sun and nocturnal birds sounding late into the evening.The names of the birds that once populated the area are engraved on the paved stones beneath the cages.  
This morning, unaware of this information, I was trying to figure out where the birds were.  Also, I missed the engraving but will go back tomorrow with more appreciation.