So today I've been Googling around ... still haven't figured out where I know it from ... but I've learned "Alla fåglar kommit ren" (all the birds come clean) is the Swedish translation of a German children's song Alle Vögel sind schon da (all the birds are pretty). It's widely known in northern and central Europe, and in addition to Sweden, it shows up in Norway as "Alle fugler små de er" (all the birds are small). The different lyrics in the first line appear to be for the sake of rhyme and meter in different languages - the gist of it is that it's spring and the birds have come back.
According to Wikipedia, Alle Vögel sind schon da is one of Germany's best-known children's songs celebrating spring. Lyrics in a poem by Hoffmann von Fallersleben (1798–1874), published in 1835. Since 1884 it has been sung to a traditional melody that has been traced to the 1500s. Judging by what's available on YouTube, the Germans consider it primarily a children's song.
But in Sweden, it is also associated with with May Day, or a spring celebration the night of April 30 and morning of May 1 called Walpurgisnacht in German and Valborgsmässoafton in Swedish. In much of northern Europe, this celebration derives from a very old spring ritual, and it is marked by bonfires, dancing and a lot of partying. Very similar to the traditional May Day celebrations in English-speaking countries. In Sweden, it also coincides with school graduation - an occasion for more partying as well as choral singing in university towns. There are a couple of lovely choral arrangements on YouTube.
Stämningsfull Valborgseld med Körsång i Hagaberg (a Walpurgis' fire with choral singing at Hagaberg). Stämningsfull, according to the Babylon.com translator, can mean atmospheric or well-tuned in English, and this clip qualifies on both counts.
Some more YouTube clips:
- Spring Concert, Dal, N.Sweden, 3 of 5. Sung by a "choir based on half-a-dozen families in the parish of Kirkdale (Kyrkdal) in a forested part of northern Sweden." The song begins at 3:39 (followed by Imse vimse spindeln [itsy bitsy spider], complete with hand motions, at 10:14.
- Sånghälsning till våren (song to greet the spring), as sung by a Swedish couple after a May Day demonstration, posted to YouTube by ToraJohanna.
- Piano arrangement by Torgny (Lundmark?) at www.torgnypianist.se (but the link appears to be dead).
- Preview of Alle Vöglein sind schon da on an Austrian CD Traditionelle Kinderlieder, mit Harfe und Geige, für Groß und Klein (traditional children's songs for harp and violin, for big and little).
- A Norwegian recording of Alle Fugler Små De Er by Marit Larsen und Marion Raven in 1996.
- An arrangement by a German pop orchestra with a big band sound directed by James (Hans) Last.