What is the purpose of Kalya Orev?

Rashi: It is to prevent ravens from resting on the Heichal.

Ha'Kosev: Tamei birds should not be on the Heichal - "Lo Yanu'ach Shevet ha'Rasha Al Goral ha'Tzadikim."

Tosfos: The Aruch says that there was no need for it in Bayis Rishon, due to its Kedushah. This is wrong. It is proven in Mo'ed Katan (9a) that it was in Bayis Rishon!

Rav Elyashiv: Tosfos (Menachos 106a) says that initially Shlomo made Kalya Orev, for he did not know whether or not the Shechinah will rest in the Bayis. When he saw that it did, he removed the Kalya Orev. Even though one may not add to Binyan Beis ha'Mikdash without a command - "ha'Kol bi'Chsav mi'Yad Hash-m Alai. Hiskil" (Chulin 83b), it is not part of the actual building. It is a tangential matter.

What is the form of Kalya Orev?

Rashi: They are iron plates with a sharp edges and sharp nails. They cover the roof of the Heichal with them.

Aruch: It is a scarecrow.