Hawkman learns the language of birds in Flash Comics 23 (Nov. 41).
Fox and Moldoff have some thieves as the villains in this story, who release smoke bombs to shield themselves as they rob their prey. Shiera Saunders becomes one of their victims.
Hawkman pursues the criminals, but they capture him, knock him out, and try to kill him by dropping him out of an airplane. Hawkman survives the fall, and winds up in a hidden valley of intelligent birds. The leader, One-Eye, teaches Hawkman their language. Presumably this is the core bird language, as he will be shown to be able to converse with any birds in later stories, not just ones from this valley.
One-Eye accompanies Hawkman as he hunts down the thieves, and even sacrifices himself to save Hawkman’s life.
The rest of the birds then elect Hawkman as their new leader. Presumably this is why, from this point on, Hawkman is able to enlist any birds as helpers and spies in later cases.