Technically, I think that's the blood eagle, and spread eagle probably
comes from being tied up for flogging and, well, the easiest way to draw
an eagle as a heraldic device to make sure everyone knows it's an eagle
and not, say, a starling.

But the blood eagle was my first thought too, so I looked it up. I found
one site that claimed it might simply have been a literary device (the
description is found in a saga), although with the vikings, well, you
never know, it does sound kind of like their sort of thing. (I'm
paraphrasing).

I suppose the author of the site, like me, was taught and read a lot of
history which, naturally enough given my origins, tended to be of the
'From the fury of the Northmen, good Lord deliver us' variety. Now, I
understand, the vikings are said to have been much misunderstood (mostly)
peaceful traders who hardly every looted and tortured. I suspect the truth
is somewhere between the extremes.