Just what the title says. I was looking at the Synthesist, as my DM has cleared the class and seems fine with how powerful it is, suggesting even that I might need to be that strong for what he has planned. I hadn't seen that there was an Avian form previously. You have to have an eidolon form that's at least the same size as your current one, and it comes as a small to start. But you can spend two points from the evolution pool to make it medium. Does it's initially small size stop you from taking the ability in the first place?
Also, if it works and you were to do it, what might you want to do with it?
For those who wonder about this base form from Cohorts and Companions : See Archives of Nethys.
It should work fine. As soon as you add this special evolution, you can 'equip' big birdy.
However, keep in mind that this base form has no arms by default - which can be devastating for casting, using wands etc..
A gnome could save you the 2 evolution points, provide +2 Cha and add HP to the eidolon as FCB. I guess you are into the half-elf's FCB, but this one will need till level 12 to turn into a plus: 2 for size increase vs. 12 * 1/4 bonus. During this time, the gnome happily adds HP.
With a Small eidolon, you gain +1 AB, +1 AC, +2 Dex and +4 Stealth. The price is high, though: -4 Str, -2 Con (mostly neutralized by gnome FCB), -1 CMB & CMD, reduced damage dice and no access to Large and Huge size. Still you can go for many attacks with small dice, picking up Weapon Finesse and the Energy Attacks evolution early.