But his excitement turned to horror when locals explained that they had killed it the previous night and eaten it that morning for breakfast.
All that remained of the Rufous Fishing Owl - Latin name Scotopelia ussheri - was the bird's head, some feathers and a few bones left in the cooking pot.
The story of the unfortunate owl - so rare that only about 30 sightings have ever been reported by ornithologists - comes in a faxed message to the African Bird Club in Britain.
Mr Hall, who emigrated to Nigeria from Derby 24 years ago and works for a conservation group, is on a four-week bird tour and could not be contacted this weekend.
But he said in his fax that after catching a glimpse of the bird, which is about twice the size of a British tawny owl, in a forest glade near the River Niger in Edo state in 1995, he returned this year to attempt another sighting.
In the formalised report of the ornithologist, Mr Hall explained: "Inquiries were made of local villagers to ascertain if they knew of the existence of the owl.
"It transpired immediately that not only were they familiar with the owl but one had been killed the previous evening and cooked and eaten that very morning."
Plans are now being drawn up for a conservation scheme to protect the fish-eating bird.
A spokesman for the British Trust for Ornithology said: "It must have been heartbreaking for Mr Hall to discover the owl had been killed and eaten.
"This is the rarest of three species of fishing owl in Africa and found only in a very small area of threatened forest."