What made some Africans on the Middle Passage believe that they were bound for the slaughterhouse to be eaten by cannibals?

This is the only reference I can find in the text referring to cannibals.

On that part of it where we first attempted to land there stood some very large birds, called flamingoes: these, from the reflection of the sun, appeared to us at a little distance as large as men; and, when they walked backwards and forwards, we could not conceive what they were: our captain swore they were cannibals. This created a great panic among us; and we held a consultation how to act. The captain wanted to go to a key that was within sight, but a great way off; but I was against it, as in so doing we should not be able to save all the people; 'And therefore,' said I, 'let us go on shore here, and perhaps these cannibals may take to the water.' Accordingly we steered towards them; and when we approached them, to our very great joy and no less wonder, they walked off one after the other very deliberately; and at last they took flight and relieved us entirely from our fears.