Thorondor?
 

Just a question- does anyone know if Eagles are immortal or not? Because Thorondor lived for the whole third age, but there is no mention of his fate. It seems that he has died, because I read in LoTR that Gwaihir is his descendant, and it makes it sound like he's dead. Any answers?

Well, Thorondor lived in the first age,not the third. But in any case, I do not recall Tolkien mentioning anything about the lifespan of the great Eagles but I can only assume they lived for a very long time.
Perhaps,the greater loremasters know more?

well i am not a loremaster,but i have a opinion that the eagles were not immortal and i guess that gwaihir was a descendent ,and a winglord so i think thorondor must have died.

perhaps the moderators will have a bit to say in this?

Thorondor and his eagles of the circling mountains went back with the Valar to serve Manwe, i would assume that Thorondor died a peaceful death, and probaly some eagles stayed behind...