I'd like to have my eagle-heartbeast Bjornaer create a spell (Feathers of Brass and Steel or somesuch) that gives her protection roughly equivalent to a full load of metal scale armor, without interfering with her ability to fly. Unfortunately, I'm not sure what the best way to go about doing this is.
My first thought is a MuAn(Te) spell, with a base level of 15 (a major unnatural change). That seems straightforward enough to give her metal feathers, but that would probably make flight impossible. So, her feathers would need to remain as light and relatively flexible as, well, feathers. How would I fold that into the spell? Would it just be extra magnitudes for complexity? I'm really not sure.
Or, would some version of Doublet of Impenetrable Silk work? Feathers are reasonably stiff, so they might not be limited to just +3 soak. Again, I'm not sure.
Advice and/or examples of similar spells would be greatly appreciated.
I personally wouldn't worry about the weight, as that implies that a bid flies due to the upward lifting force of pressure differences and airflow over their wings. I'm pretty sure birds fly in Ars because it's within their nature to fly, or something like that.
Even if that's not the case, you could pretty easily make the argument that you're only Muto-ing the property of Terram that grants durability, not weight. As such, it's actually LESS difficult to do, as you're not importing the entire aspect of brass - instead, your just taking one. (ie, in terms of MuTe effects, "change one property" is base 1, +2 for metal.)
EDIT - but yes, base 15 seems to be fine. I'd personally mark it down to 10, as the base guideline example grants numerous changes, and you're only doing one. However, by RAW - yeah, base 15.
I have some issues with the "without interfering with her ability to fly" part, so let me think about that as I write: armor's load penalty isn't just a matter of weight, but about how armor's bulk interfers with the movement of the wearer (see the load stats for staffs, that 2 isn't because of weight, as a staff weights less than a sword, is just that it's bigger and more complicated to wave around). So in one hand I would see perfect to have the metal armor soak bonus (but not that one! more on this in two paragraphs), but also some of the Load issues.
Not all of them, after all, as the bird already haves it's feathers and he's used to move them around, but having them stiffened would probably complicate flying a bit. But it could be argued that there was some complication before, if only because a human wearing a fur armor get's a Load penalty that an animal with that fur doesn't get.
So I would see it as an upgrade to an armor equivalent to Fur (Prot. 1, Load 2) to actually a Full Chain Mail instead of a leather scale (there are many layered feathers, after all, instead of some feathers side to side, so I'd go for the Full Chain Mail stats, Prot. 9, Load 6), and then consider that half of the Load comes from weight and the other by bulk complicating movements, so I think that spell should bring you +8 to Soak and +2 to Load.
I think the most straightforward way to "price" what is effectively a boost to soak is to look at the corresponding guidelines for Corpus (base +1 Soak/magnitude, up to +5) keeping in mind that according to ArM5, p.118, "Muto Corpus spells may be examined to get additional ideas and guidelines for Muto Animal spells".
Thank you very much to everyone who's responded. You've given me plenty to go on.
Alas, I doubt my troupe will go with the medieval paradigm and just let a metal-feathered bird fly because that's what birds do. Granting a select few of the properties of metal to her feathers will likely be their choice, and they'll almost certainly wind up pointing to the corpus guidelines for that. I can happily live with making the metallic feel and appearance of her feathers essentially a side effect of the spell. The ultimate goal, after all, is increased survivability combined with coolness.
As for spells granting additional Qualities to an animal, I really like that idea. I'll have to bounce it off the others, then hit the books if they're amenable. Given the highly limited time and energy I have for research during the school year, I very well might be back to ask about how such a spell might be worked out.
I'd base it on Doublet. Bump it a magnitude or two, for higher Soak (I'd argue that "feathers" are notably "soft as a feather," so (despite stiffness in one dimension), not inherently "stronger" -- that's what the magnitudes of magic are for!!!).
Just declare a "metalic sheen" (if desired) as minor side-effect, included for coolness' sake but without mechanical effect.