As previously mentioned, a human has three souls according to Finnish mythology. One of these, henki (= breath, your life force), can occasionally take the form of a bird, most typically at the moment of death when it flies out of the body. The other occasion it does this is when you’re asleep and can sometimes travel long distances before it eventually returns for you to wake up. It can also help you navigate your way in the land of dreaming. In SSSS-verse Lalli’s sielulintu would probably be his Henki-soul, his lynx his Luonto-soul and his own dream being his Itse-soul… well, that’s my theory of it at least. :D
Karelian Finns often placed a carved wooden bird called “unilintu”, “sleep bird”, above a child’s cradle just because of this sleep traveling business. It was considered an extra help in keeping the child’s soul bird present, after all a child’s soul could easily get lost while asleep and if it didn’t return - well, remember how losing this soul means literally losing your life? Yup.
The dead were sometimes given similar wooden birds for their graves because one way they could return to visit was by taking the shape of a bird. This tradition still lives on in f.ex. one really popular (and depressing as heck) Christmas song about a little girl who gives food to a sparrow, that then tells her he’s actually her dead little brother come visit her! It’s a really sad song.