What an exciting find for you.

Treat it well , it will most surely remain in your family for vast generations.
Nice story to accompany it as well.
Here is some gossip on the bird on your cover

THE FRIGATE BIRD POST
Some sixty years ago, tame Frigate birds were employed on the Ellice Islands in much
the same manner as carrier pigeons are used. The natives were encouraged to take the
young birds without injuring them. Comfortable sheltered perches were then erected
near the beach, the young birds were tied to these by the leg, and fed until they were
very tame.
When then let loose, the birds would go to sea for food and return to their perches
when fed and satisfied with flying. It was noticed that they would invariably come
back at once on the approach of a storm. The Frigate bird post, as it might be called,
was started by the missionaries, who sent and received letters rolled up in a quill that
was attached to the bird.
Reprinted from
Weekly Philatelic Gossip
of March 11th, 1939