On further research we learned that Fred Gibson, born on Rotuma in 1886 was the eldest son of the seven children born to Alexander Gibson (b. Whangaparaoa, New Zealand, 1862) and Agnes Fagmanuia (b. Rotuma). At the time of writing, Fred, then married to Sefrosa, seems to be a local administrator for the Government. The letter largely focuses on different bird species, their scarcity, Rotuman names and uses made of their feathers in traditional adornment. He also includes sketches of a head adornment and of several birds.
A specimen of one bird he discusses, the Jea (Rotuman name) or Lalage maculosa rotumae Latin name), is held in the Museum collection (LB 7901) which you can read more about here.