I was trying to figure out the characteristics of a Song Thrush taken as a Heartbeast.
The first issue is size. The Song Thrush weighs 50 to 100g. That would be size -9, bumped up to -8 because it's a bird. But! Applying the "noble example bla-bla" rule in HoH:MC, we can allow a PC to be a big thrush of size -7, and thus (barely!) large enough to be a heartbeast. Ok!
Next: Virtues, Flaws & Qualities.
These seem to be very similar to those given (perhaps less appropriately!) to the Swan, with two exceptions: I'd make it Carefree rather than Humble, and give it Mimicry rather than making it an Accomplished Flier. I am somewhat uncertain about allowing it Luck, but it does seem to be a symbol of Luck in a few myths (including Tolkien's Hobbit!) - at least, more than the swan.
So, summarizing: V&F: Luck, Inspirational, Puissant Music, Carefree. Qualities: Mimicry, Vocal x2, Timid.
The fat songbird that we find in HoH:MC (size -6, three quarters of a pound?!?) has Str -12, Qik +7. It seems to me that dropping Size down to -7 should bring Str down to -14 and Qik up to +8... Everything else is as per HoH:MV
So, summarizing: Str -14 Sta 0 Dex +3 Qik +8 Cun -1 Per +2 Pre -2 (-1,-1 from Timid) Com +2 (Mimicry+Vocal*2)
Since the book recommends allowing PCs to take larger sized animals by taking 'Large' or 'Giant Blood', it seems reasonable to extrapolate that you can also take undersized animals using 'Small Frame' or 'Dwarf'. Plus, IIRC, characters already exist who violate that rule having rats or scorpions as their respective heartbeasts.
I'll think about the other two and try and get back to you...
Sure, a character with a Heartbeast this small (-7) is supposed to have something like a "Small Frame" Flaw.
Allowing size -8 or less would be entirely at the troupe's discretion, and strictly require taking a "Dwarf" Flaw. Which is far trickier. A character with Small Frame is still "normal" -- could be either a short or gaunt magus, or a petite, elegant maga. A Dwarf is the size of a child, which disqualifies many more character concepts.
There's a rat in The Lion and the Lily, but that's still size -7, even without invoking the "particularly noble ... " option. Where did you see the scorpion?