I couldn't find a birdbath large enough for my two macaws, so I made this one based on a decorative "butterfly" bath I saw an artist from SF do on the DIY channel.
For bath base: find 2-3 terra cotta pots that stack nicely (and securely). Glue in place using a permanent adhesive like Liquid Nails.
So that the basin will hold water, you'll need to patch the drain hole in "bowl" using a permanent concrete patching compound.
A self-adhesive fiber wall repair tape comes in handy (holds the patching compound in while it dries).
Wipe the basin surface clean and dry (I use rubbing alcohol), read all directions for patching compound, apply repair tape and a thin coat of patching compound to bottom of basin.
Allow to dry (about 10-15 minutes).
Fill drain hole from inside basin with patching compound.
Wipe away any excess.
Allow patching compound to dry completely and cure for 24 hours to a week (depending on weather and type/brand of compound used) before filling with water.
I chose NOT to glue the top onto my base, to allow for easier cleaning and disinfecting of the basin.
You could also leave the drain hole open and insert a pump for water circulation (my birds would eat it...).
Add water, bird and enjoy!