Why didn't Morgoth track the Eagles to find the location of Gondolin?

The Eagles were a constant pain in Morgoth's evil neck; they rescued several people from his own dominion as well as attacked him personally. He must have known that the Eagles were in league with the Elves and thus surmised that the Eagles fly to and from the Elven kingdoms.

Wouldn't it have been easier for him to find Gondolin if he tracked the Eagles? If so, then why didn't he do it?