This is a traditional Catalan Christmas carol that often gets pigeonholed as a children's carol. It's a pity though, as a number of quite complex SATB and a cappella arrangements exist. This is the version that I sang as part of my high school's Madrigal chamber chorus - I found out that there are several other versions, some quite cheesy indeed. I wasn't able to remember the traditional Spanish lyrics, but I did find them on a number of websites. Those lyrics, a good translation, and their source are given below the version that I remember.

On December five and twenty,
Fum, fum, fum!
On December five and twenty,
Fum, fum, fum!
For the love of man is given
The holy infant, son of heaven,
Of the holy virgin springing,
Peace on earth and good will bringing,
Fum, fum, fum!

Birds that live within the forest,
Fum, fum, fum!
Birds that live within the forest,
Fum, fum, fum!
All your fledgling leave behind,
And seek the infant savior kind.
Build, as well as you are able,
Downy nest to be his cradle,
Fum, fum, fum!

Little stars up in the heavens,
Fum, fum, fum!
Little stars up in the heavens,
Fum, fum, fum!
If you see the baby cry,
O do not answer with a sigh!
Form the skies your radiance beaming,
Pierce the darkness with your gleaming,
Fum, fum, fum!



Spanish:

Veintecinco de diciembre,
Fum, fum, fum!
Veinticinco de diciembre,
Fum, fum, fum!
Nacido ha por nuestro amor,
El Nino Dios,
Hoy de la virgen Maria
En esta noche tan fria,
Fum, fum, fum!

Pajaritos del los bosques,
Fum, fum, fum!
Pajaritos de los bosques,
Fum, fum, fum!
Vuestros hijos de coral
Abandonad, abandonad,
Y formad un muelle nido
A Jesus recien nacido,
Fum, fum, fum!

Estrellitas de los cielos,
Fum, fum, fum!
Estrellitas de los cielos,
Fum, fum, fum!
Que a Jesu mirais llorar
Y no llordis, y no llordis,
Alumbrad la noche ascura
Con vuestra luz clara y pura,
Fum, fum, fum!

English Translation:

On December five and twenty,
Foom, foom, foom!
On December five and twenty,
Foom, foom, foom!
For the love of us is giv'n The holy Infant,
Son of Heav'n,
Of the Virgin, Joseph's bride,
To all the earth good will be tiding,
Foom, foom, foom!

Little birds from out the forest,
Foom, foom, foom!
Little birds from out the forest,
Foom, foom, foom!
All your fledglings leave behind,
And seek the infant Savior kind.
Come, and build a downy nest
To warm the lovely Baby blessed,
Foom, foom, foom!

Little stars up in the heavens,
Foom, foom, foom!
Little stars up in the heavens,
Foom, foom, foom!
If you see the Baby cry,
O, do not answer with a sigh!
Rather, lighten up the sky
With Heav'ns beams of radiant brightness,
Foom, foom, foom!


Source for Spanish version and its translation:
http://www.angelfire.com/me/music17/fum.html