But they don't all. The most clear-cut is Mimic the Christ's Miracle. Wine has very nearly the same density as water, so you end up with 1/00 of the volume or mass that you started with. Look at Conjure from the Mist, noting the Individuals (+2 size doesn't matter since it applies to both). Sure, stone has approximately 2000 times the density of air (varies, but decent estimate). Even so, you can end up with roughly 1/500000 the volume and 1/250 the mass.
Meanwhile, when turning non-flesh into animals we also have spells like Inmost Companion, where the caster loses no mass nor volume in the creation of the bird.
There doesn't seem to be a complete agreement about mass and /or volume conservation. AFAIK there's nothing explicit in the rule about this and we're back to saga's level of power and balance.
It's difficult to know whether the effect of the spell itself is more powerful : changing the soil into tens of thousands of birds (creating a lot of birds is powerful) or into a single bird (such a change of mass and volume is powerful too)
As for what you can achieve later using other spells :
-You can use the single bird's transformation to drop a significant mass on a target (sink aboat, break a roof...) and control it very easily being a single individual
-On the other hand while much harder to control with the group target and +2 size you end up with a birds army.
I would actually think that the real question is how your particular saga wants to deal with definition of Ind/Part/Group.
You are, effectively, turning PART of earth into a GROUP of birds. Is that something you want? Can I turn someone's left arm and right arm into a cobra and mongoose respectively?
Obviously, I chose an extreme example - I think just because of mystical thematics you should be allowed to do what you want here. I might argue since you're creating a group of birds, to add a magnitude for that (as if targeting a group) Otherwise, it would only turn a bird-sized section of ground into a bird.
I personally don't are about density or volume as much as what FEELS right for Mythic Europe. Turn the ground beneath your enemy into a murder of 50 ravens? Sweet, I like that spell. Much more than the idea of turning a patch of ground into tightly-packed brick of crow.
Just remember, it's a formulaic spell, so players can't easily modify the specifications on the fly, which means I often let the players be a bit more creative.
I think the volume (or mass) dirt to crow might be irrelevant. You are transforming about 100 cubic paces worth of dirt (equal to 10 individuals by the Terram guidelines, handled by '+1 Size') into crows. You will get 10 individuals worth (by the Animal guidelines, '+2 Group') of crows. I think if you change the spell description into 'transforms dirt into flocks of flying crows', then the base 5 Muto magic is already doing enough (there is nothing un-natural about flying crows, and the MuAn base 5 guideline alters an animal far more) to make the Rego requisite just a +0 cosmetic effect ... but you have no control over where those ten flocks of crows will be (and drop 10 cubic paces of dirt, each) in two minutes. If you figure 300 crows is the same mass as a pony, then those 300 crows are appearing in a volume of 3 x 3 x 30 ft; they are close together but not tightly packed. Given that the airspeed velocity of an unladen crow can be approximated to about 11m/s; and assuming the caster wants that velocity oriented upwards, the bottom-most crows will easily clear the top of the pit within one second (one sixth of a round, far shorter than the casting time).
I think you end up at:
MuTe 35
Telluric flock
The dirt in a circle 6 paces across transforms into a flock of flying crows, which flee upward, leaving a pit 10.5 feet deep. If the target area contains material other than dirt, target materiel is not affected.
(Base 5 (dirt to animal); +2 voice; +1 size; +2 Group; +1 Diam; +0 Rego (cosmetic: crows fly away))
If the caster wanted to control the resulting crows, I think I would be a little less generous with the size & group modifiers. On a botch I might have a flock decide to alight somewhere inconvenient.