I was reading some texts yesterday about sacred trees and animistic beliefs about birds and deities. There is a passage in the Nibelungenlied where Hagen says "Are we to raise cuckoos?" and there is a lot of discussion about what that means with no real conclusions except that it means most likely bastards. However, the leaf that falls on Siegfried when he bathes in the dragon blood is a Linden which is a sacred tree and these elements are the relics of the pre-Christian story.
Most of the scholars agreed that the strange little line was left in by the last poet because it could not be left out. The Linden leaf and invulnerability part of Siegfried's story exists only in the Germanic tales. These don't occur in the Norse versions but made more complicated by the Linden being sacred to Freyja but there is no evidence for a Freyja in the Germanic pantheon.
I think the cuckoo comment is a dig at Siegfried's father, Sigmund, who in the Norse versions that remain, fathered a son on his own sister. Zeus seduced Hera in the guise of a cuckoo. But the cuckoo is connected to the birch tree and Donar, which hints at Siegfried being Thor in fact, especially since the Volsungs are sons of Odin. The Linden could be simply because the leaf is heart shaped but, since Siegfried dies beside a Linden Tree, this could mean that Freyja, who gets half of the dead who have fallen in battle (Odin takes the other half), collected Siegfried.