There is a lot of information missing from this site regarding rewards, which is captured in fan sites on Facebook and "how to" guides in "most useful topics" above in the forums. I'm hoping that when the site is transferred off Facebook that missing information such as this is included in the new site.
To get the birdbath, you need to complete bottom dwellers in the pond section of the wildlife guide. This requires frog breeding as bottom dwellers eat frog spawn. As well as all this, you need to do the chicks as only chicks that have previously been spotted in your garden will visit to the birdbath.
Frog breeding guide is here:
https://www.fairylandgame.com/fbfairy/forums.php?tid=143572 Basic bottom dweller strategy is here:
https://www.fairylandgame.com/fbfairy/forums.php?tid=120291 
