In Karelian mythology, a soul-bird (sielulintu) was believed to serve as a guide and/or protector to a human soul, and sometimes amulets depicting one's sielulintu were used. Both in A Redtail's Dream and Stand Still, Stay Silent, story elements are loosely based on the sielulintu.
Finns and other Finnic people often held various waterfowl sacred, often assigning them a role in the creation of the world, e.g., asserting that it had its origin in a bird's egg or that it was created from a beakful of mud scooped up during a dive. The holiest bird was the swan, its neck allowing it to see all levels of existence including Tuonela, the realm of the dead.
The people in Karelia further believed in the existence of birds associated with human souls, hence called soul-birds or sielulintu. Their tasks were to bring a soul to newborns and take it away after death, as well as to protect it while the person slept. Regionally, people wore a depiction of their sielulintu, and sometimes this representation was mounted on top of the cross on the grave after its owner had died.
Besides birds, the bear and the elk played dominant roles as sacred animals.
Birds and the Birds' Path in aRTD
While the birds in aRTD don't seem to have a one-on-one assignment to humans and their souls, most of the aspects of sielulintu have been incorporated into the story.
This starts when, pretty much as the story begins, the villager's souls get swept into The Birds' Path (the Milky Way) by Puppy-Fox' mishap and the birds traveling along the path pick them up and carry them towards Tuonela, mistaking them for the souls of people that have recently died. One cannot say that the birds do much in terms of protecting them from harm, but then again, most of the birds shown lack the mental capacity required for such a task.Later, Hannu starts visiting the dreamworlds the souls have been trapped in, and as he enters each, a bird delivers a carved amulet to him that he must pass on to the "leader" of the group of villagers in said dreamworld. But since the amulets match the spirit animal that joined said leader to lend him support, only one actually shows a bird - the one given to Hannu as he enters Tuonela itself, with the Swan of Tuonela being the resident spirit. Besides the swan, Kokko the eagle assumes a major role in Hannus adventures, repeatedly aiding Hannu in his tasks and ultimately saving his life.
The Sielulintu in the SSSS Mages' Dreamworlds
When falling asleep, Lalli is able to enter a two-tiered system of dream worlds. The first tier contains the mages' personal safe place, called "haven" by the fandom. Havens are shown to be usually shielded against the remainder of the first tier, though some entities - in particular, Reynir - can break through more or less easily.
From his haven, Lalli is able to enter a second tier by opening a magical gate between the two. This second tier apparently allows him to attempt contacting other mages over considerable distance, hence being nicknamed "the mage chatroom" by the fans; in particular, we see him shout out to Onni in it at a time where Lalli was at the Öresund, and Onni probably still in Keuruu.
While a Finnish mage (noita) is in tier two, the position of the gate that leads back to his haven needs to be marked so as to allow him to find it again. (While at least Lalli seems to know their positions as seen from his haven by heart.) This marker is said to actually be the mage's sielulintu. This sielulintu is glowing electric blue or white, floating or flapping in the air at the position of the gate, and its size varies; it might be the gate, and thus somewhat larger than the mage himself, as it opens to allow return.
How the two tiers are experienced by Icelandic mages (seiður) like Reynir is still unclear. On one hand, Reynir apparently can easily travel long distance in tier one and enter mages' havens there at will, as he demonstrated with Onnis after joining the expedition. On the other hand, while it is unlikely that he is aware of gates in the first place, Lalli saw him in tier two in an earlier dreamscape episode.