Ok, I've been trying to do this for a few days now and I can't figure out how to do it in the existing rules without basically throwing out some of the rules about the base forms. I'm trying to make it for a 10th level summoner.
To me, a bird has the following configuration :
Head
Torso
Two Wings
Two Legs
Tail
Now, the head is easy, everyone get's a head. Torso same thing. The tail on a bird is not the prehensile or striking tail that an Eidelon with a Tail evolution gets, so that's no issue.
The problem is, none of the base forms work for this.
Quadruped has four legs, not two, and you can't 'buy back' the form abilities, which means your penalizing the summon for having a bird form. Same with biped (it would be nice if there was a way to transform 'arms' to 'wings' when taking flight to make it cheaper to take). I almost went for serpentine, but then you've got tail slap, which doesn't fit either.
So, i don't know what base form to give it, nothing works right. What I have done so far is this. I took the 'quadruped' form and dropped the first set of legs and turned them into claws. The 'claws' I put on the remaining legs and defined as Talons, and reduced it's land speed to 10 feet. But, I kept it's fly speed based on the original base speed. Basically I'm going for something like a baby frost roc form here. So, here's what I have at 10th level :
Quadruped Form (exchange legs for claws and call them talons)
Bite (0 evo pt)
Improved Damage (Talons) (1 Evo Pt)
Pounce (1 evo Pt)
Skilled (Fly) (1 Evo Pt)
Energy Attack (Cold) (2 Evo pts)
Flight (40ft) (2 Evo Pts)
Large (4 Evo Pts)
Grab (Talons) (2 evo pts)
Magic Attacks ( 1 Evo Pt)
I'm giving it the following feats :
Flyby Attack, Hover, Wingover and Toughness
Anyone else have a better idea for how to simulate a bird with the 3 forms in the handout? I hope they pop some new base forms in before it goes to the printers. None of the 3 currently there work for bird. Vermin you could handle with legs (although putting 4 extra legs on for a spider would make it fast beyond belief, 60ft). I'd also love to see some base forms built on not being combat monsters (high mental stats in other words) but that's just me.
The closest you can get, with the current rules, is with the Serpentine form.
You can have a look at my 'Alpha-version Valefor' (from Final Fantasy X) here.
You could allow the Eidolon to 'drop' the Tail Slap and gain back 1 Evolution point as a house-rule, if the Tail Slap doesn't really fit with your idea of Eidolon (for Valefor, which has a really long serpentine tail, I decided to leave it in place).
Even for dragon-like Eidolons, the Serpentine form is the most suitable, IMHO (after all, they are WYRMS...) - unless you go for Bahamut (the Final Fantasy version, of course), which has a broader frame - either biped or quadruped is indicated for such a bulky dragon.
Just my 2c.
The closest you can get, with the current rules, is with the Serpentine form.
You can have a look at my 'Alpha-version Valefor' (from Final Fantasy X) here.
You could allow the Eidolon to 'drop' the Tail Slap and gain back 1 Evolution point as a house-rule, if the Tail Slap doesn't really fit with your idea of Eidolon (for Valefor, which has a really long serpentine tail, I decided to leave it in place).
Even for dragon-like Eidolons, the Serpentine form is the most suitable, IMHO (after all, they are WYRMS...) - unless you go for Bahamut (the Final Fantasy version, of course), which has a broader frame - either biped or quadruped is indicated for such a bulky dragon.
Just my 2c.
No, I agree, quadruped or serpentine work great for dragon types. They are, as you said, worms.
Where it breaks down is traditional bird forms (birds, bats, etc).
Any of the base forms would allow you to make a bird like creature. It's really only a matter of which stats you want to use as a base and how many legs you want it to have (0, 2, or 4).
Furthermore, there is absolutely no reason you have to describe your eidolon as having more legs than you want it to have. I take the extra legs trait for the extra land speed (and stability) all the time, and yet still describe my eidolon as having two legs.