My big questions are:
1. Why are galdrar unique to herons and heron-descendants (eg. Apostles)?
2. What makes seid magic different from conventional magic, and why?
3. Do the mechanics of seid magic bear any resemblance to real music / musical theory?
4. What are the mechanics of the discipline?
5. How powerful can one singer or one galdr get?
6. What does "reading hearts" have to do with the rest of their talents?
Like always, I will use Lehran as my primary example, because he's the character who achieved the most significant feats with seid magic, and also the only heron who has lost his birthright, but the others will show up too - don't worry. It's not a total fangirl fest.
Since Lehran lost his ability to sing galdrar along with his ability to transform, the game tells us flat out that seid magic is a laguz trait of the same general caliber. He can use seid magic because he's a heron. No one outside the bloodline can do this. But why? It seems to use the same language conventional magic does, so there aren't any special linguistic differences; it has to be sung, but that isn't drastically different from any other type of chant. In Archanea I believe magic is bound to tomes, which is why mages have to use books to cast it, and the same might be true of Tellius - magic seems to come from spirits (hence spirit charmers being able to do fantastic things, supposedly), so perhaps the seid magic chants draw directly on the spirits, while beorc mages need that focus to channel it.
(I don't like that mechanic either, but I'll let it go for now.)
Problem is, even if the herons speak to the spirits directly - and there's no saying that's how the magic works - why is it so closely linked with their heritage? A human mage should theoretically be able to crack the code and learn how to sing the correct chants. Unless there's something literally different about their singing voices, versus their normal speaking voices - since nobody comments on those, at least - I don't see why there should be a difference. There is one moment in PoR that might give that some basis:
[Path of Radiance, Chapter 17: Day Breaks]
Mist: Did you...hear a sound just now?
Ike: Huh? No, I didn't.
Mist: Really? I guess it was just me then.
Lethe: That sound you heard, was it high-pitched, like the chiming of a bell?
Nasir: It is said that those of the heron clan all practice the art of seid magic. Perhaps this sound is related to that?
However, Tibarn and Ulki comment on the same sound, and claim they've never heard its like. This moment is probably Leanne's awakening, and the sound might have been the spell that kept her asleep breaking. You could speculate that Reyson just hasn't been singing galdrar since his rescue, perhaps out of mourning, but I find that unlikely.
So: I still find the linking of seid magic with the heron bloodline unreasonable. But a lot of things about them are unreasonable:
[From Chapter 16:]
Nasir: The heron clan possess no fighting skills. Their tribe survived by living a life of peace and piety within their forest. Unlike other laguz, they never focused on developing their strength for the purposes of combat. [...]
[From Chapter 17:]
Nasir It involves arcane songs known as galdrar. The effects of galdrar on the listener depend on the lyrics and melody. For example, a galdr can restore lost strength and vitality to those who hear it. And if the singer is of royal blood, the galdr may be powerful enough to move its listener to extraordinary feats. I've even heard tales of a galdr that could give one the speed to do the work of two men. The galdrar grant many powers.
Fire Emblem as a series likes to attribute special power to music and dance; from FE4 onward, we get units who either dance or play instruments that can refresh their comrades during battles and give them extra turns. Tellius is just giving us a new spin on that gameplay mechanic with herons. Unlike previous games, however, the world is claiming that ability is special, while the others make no attempt to tell us that Tethys or Ninian are especially mystical in character - though Ninian may be a bad example for other reasons. Neither Elphin nor Lalum display special power. I always assumed the explanation I was supposed to think of was along the lines of music bringing respite, and therefore reinvigorating your troops on a long march.
Galdrar are also music. Are you trying to tell me that music, as it exists in Tellius, is restricted to pretty, tweeting bird laguz? I don't think we see any instance in-game of musical performance. I think that would be ridiculous, but it's one possible explanation, I guess.
I speculated galdrar might've been a gift from the goddess, but in that case, Lehran's Mantle, which was discussed here (locked entry) and prevents him from taking any kind of physical damage, should have also passed on down the generations, and we know the younger herons can be hurt. It's most likely an ability they evolved with. Don't even get me started trying to speculate on what kind of environment would make singing a trait to select for, because I have no idea - unless you want to go with my stuck-up-heron-culture headcanon, in which they purposely select for beauty and musical pitch.
What Nasir says, coupled with Lehran's apparent ability to summon spirits (unless that was Ashera's doing, or unless he became a spirit charmer to achieve his goals), makes the "seid magic calls on spirits" seem most likely, but again: why would that be unique to them?
There's no answer to that, pretty much, except, "because the designers wanted it that way," or maybe, "to make them special." It's great as far as making Lehran's Medallion a pain in the ass. I considered heron abilities as a parallel to the beorc ability to use magic, as the laguz characters say at one point that in general, laguz just can't use magic; however, Lehran knee-caps that theory by being able to use both light and dark magic, as well as staves. He can kill stuff!
I remember giving Rafiel some item that's supposed to do damage according to one's magic stat, and it did, like, one HP damage. He also got 90+ experience for killing the soldier in question, who I primed especially for this purpose, so... I don't know why I'm including this anecdote, except to say that Rafiel, the oldest and presumably most powerful of the siblings, can't cast magic worth crap. Clearly it's not a matter of age or experience. (He gained four levels that way. It was awesome.) Normally I don't use game mechanics to determine things like this, but that's a big, fat, glaring difference between Lehran and everyone else, so. Light and dark have some things in common, though, so I don't know - maybe his ability has something to do with his proximity to the goddess.
My god this is getting long. One more thing: mechanics.
Galdrar can apparently make miracles, more or less. In light of that, the appalling weakness of the herons seems more like a balance; great power with a great flaw to balance it. But herons can only do these amazing things under special circumstances. In the original post, I talked about layering galdrar, i.e. getting more than one person to sing, makes a spell stronger. If galdrar work like music, that makes some sense; adding harmony to the melody creates a stronger, more melodic piece of music, etc. But like regular mages, they also need amplifiers:
[Path of Radiance: Reyson-Tormod supports C-A]
Tormod: That song of yours...does it work on everything?
Reyson: You mean the chant?
Tormod: Yes, whatever it was that made that drab forest bloom with color.
Reyson: Why don't you tell me more about what you have in mind?
Tormod: Well, I was hoping you would...you know... use your magic chant to transform all that sand into soil.
Reyson: Sand into soil?
Tormod: Exactly! Rich, fertile soil that will yield a bountiful harvest. We'll build our village there.
Reyson: That is...utterly absurd!
Tormod: No...I mean... It was just incredible how you forgave the apostle like that and breathed new life back into the forest.
Reyson: That was only possible under very special circumstances.
Tormod: Why? I don't get it.
Reyson: It was Serenes Forest. For my people, there is no more sacred a place. And the galdr I chanted was a part of an ancient clan ritual performed on a very holy altar. Most importantly, my seid magic succeeded because Leanne was by my side. That galdr holds little force when I chant it alone.
Tormod: Then all we need is Leanne!
Reyson: You're not very quick, are you? Even if both of us chanted the galdr until we collapsed from exhaustion, there's no way we could turn sand into soil. Even if the desert was a fertile valley eons ago, I don't have the power to restore it. Have I made myself clear?
In addition, during chapter seventeen, Reyson seems to think he can sing the Dirge of Ruin by himself as long as he has the altar. Herons have philosophical problems with this song, but apparently it exists - and as usual, destruction is easier than creation. I wonder of Lehran is responsible for that galdr; he would've had some use for it during the war.
What I'm thinking here, is... the only reason for seid magic, and herons specifically, to be so different from both laguz and beorc, is their favored standing with the goddess. Lehran is apparently immortal because Ashera loves him. I've always speculated the goddess could give his power back if she really wanted to, but I don't like that theory for a lot of reasons, so I don't push it in fic.
However, his galdrar had the power to affect the goddesses. Yune gave him permission to lock her in the medallion, but the fact remains that he confined a divine essence into a little bronze ornament with the power of his songs, and his songs also had the power to calm the chaos enough to prevent her awakening - hence affecting her, in a sense controlling her. Reyson can destroy an entire army (er, well, large fighting force) as long as he's in his own forest. That link is the only detail I can think of that might lead to an explanation. In an earlier quote, Nasir describes the herons as pious, so goddess worship might have played a large part in their daily lives. For all we know, galdrar are prayers. Maybe they're not singing to spirits, but to the goddess, who controls them.
And now the question I have to ask is: which goddess? Ashera, Yune, or Ashunera? My money is on the last.
I guess the "reading hearts" thing will have to wait for another time, but that's also pretty unusual - and it bears a resemblance to the goddess's abilities, so that's interesting, if I'm going to follow the speculation that seid magic is linked to the goddess.
I'll do that later.