Rashi: It is a big bird. In the Mishnah it is called Bar Yuchni (Bechoros 57b).

Ibn Ezra: It is a species of bird. Perhaps it is called so (like Renanah, song) due to its pleasant voice.

Malbim: This is Bas ha'Ya'anah.

Ibn Ezra: It is spread, like "Nis'alsah ba'Ahavim."

Rashi: Every bird is called so, for it flies with [its Ever, i.e. wings].

Ibn Ezra: It is its wing, like "Yisa'ehu Al Evraso."

Ibn Ezra: Did you give wings to Renanim, or wings and feathers to Chasidah (a bird that comes to at a known time, like it says in Yirmeyah (8:7).

Malbim: Can Renanim fly via its wings to high places, like Chasidah does via its wings and feathers?! Rananim cannot fly!