I've heard this said about the Taino who greeted Columbus, though never heard it applied to Africa. Really though, this sounds like the kind of infantilizing othering that is unfortunately rife in discussions of American and African history, much like the idea that the Taino couldn't even see the ships at first, because of "perceptual blindness."

This is nonsense though. Maritime and, particularly, riverine trade was common in West Africa, including ships with sails. Applying this idea to East Africa is risible, given that the history of that region is a history of maritime trading states crisscrossing the Indian Ocean.

It seems equally ridiculous to suggest that the caravels of Columbus would be so foreign to the Taino that they would think the ships were birds. Given how often I've seen this idea repeated, I would not be surprised if there is a document somewhere out there wherein there is some sort of metaphorical/poetical reference to the sails as birds, but I am exceedingly doubtful that this was meant literally. Although given how limited the insight of the European writers had into the actual mindset of the Americans, I would be even less surprised if the source for this idea was either a European writing putting his own words into the mouths of the Americans, or a later invention of a more colorful depiction of the initial contact events by a writer I expect was being poetical.

We need to keep in mind that while there is no strong evidence to suggest that sails were used in North America and the Caribbean (sails are found in South America) (Epstein 1990), it's not like the Caribbean peoples walked there. Though they used single-hulled, indeed single-tree, canoes, these could be almost as long as the Santa Maria, if not as broad in the beam. These would often be topped with cloth canopies which actually led to some confusion with sails, and the skill of the Caribbean peoples into maneuvering and navigating these craft was remarked upon by early Europeans (Fitzpatrick 2013). These were not people who were ignorant of ships and sailing, even if they themselves did not have sails.

If we look further to the West, we can see just how easily the idea of concept of European style sailing ships was integrated into the conceptual world of Mesoamerica. The Nahuatl word for a boat/ship is acalli, which literally just means "water house." This term was readily and easily applied to the ships that arrived off the coast of Mexico. We can see this in Sahagún's Historia General..., Book 12 which covers the arrival of the Spanish. After a melodramatic scene setting first chapter, the second chapter is titled:

Uncan mitoa in quenin acico, in achto acalli oalla: in iuh quitoa ca çan centetl in acalli

Where it is said how the first boat which came arrived; they say that there was only one boat

(Anderson & Dibble 1975 trans.)

No need for any odious speculation about awe-struck natives, quivering in shock at the arrival gigantic birds.