Why "île aux Oiseaux" and not "île d'Oiseaux"?

Question

In the Arcachon Bay, there is an island called "île aux Oiseaux". I was quite surprised that aux is used as a preposition instead of de or d'. Why is that?

Answer 1

✔ Although "l'Île aux Oiseaux" became a proper name (as the capital Î and O suggest), "aux" originally means "avec des" in this case.

~ The island with birds (on it).

See https://www.cnrtl.fr/definition/au III D. 3. b)

la chambre nue aux persiennes closes

les routes aux vieux calvaires

✔ "L'île des Oiseaux" is the contraction of [ île de les Oiseaux ]

~ island of the birds = these specific birds, definite

It can be a place name with the same meaning as "aux oiseaux". Similarly, there is an "île des Faisans".

See https://www.cnrtl.fr/definition/de

ferme de Rouge-Sel

domaine des Sept-Pendus

✘ "L'île d'oiseaux" is the contraction of [ île de oiseaux ] and would mean the island made of birds (indefinite) which can be a poetic thing but not the meaning we're looking for.

Au milieu du lac flottait une île d'oiseaux immobiles.

Answer 2

It could not have been l'ile d'Oiseaux because oiseaux is in the plural, if we were talking about a single bird then indeed it could have been l'ile de l'oiseau.

So your question is Why île aux Oiseaux and not île des Oiseaux?

Using de¹ would imply that the island belongs to the birds, obviously that's not the case. The island is a refuge for migratory birds, it's a place where there are lots of birds, hence its name.

About the use of de there's a good answer there.

---

^{1 Reminder: des stands for de les.}