“Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.”
(Victorian mathematician Augustus DeMorgan)
And so it seems true of islands as well … larger islands have little islands about them, ad infinitum.
We got a great view of the chain of islets that span the distance between Puerto Rico and its small island neighbors to the east as we flew at 1400 feet over the ocean expanse to our destination on Culebra.
Although birds are highly mobile and able to navigate the mileage between mainland and islands fairly easily, there is nevertheless an ever diminishing number of species on islands as the distance from the mainland increases and/or the size of the island decreases.
Birds could settle on Caribbean islands by migrating eastward from Central Mexico (1026 bird species) or southward from Florida (510 species). To illustrate the “island effect” of distance and size: Cuba has 368 bird species in its 43,000 square mile expanse, while nearby but much smaller Puerto Rico (3400 square miles) supports 349 bird species, and Culebra (12 square miles) has just 110 species, many of which are only passing through on migration to other sites (only 43 species actually nest on Culebra).
Over time island birds often develop unique characteristics that separate them from their mainland ancestors, becoming unique to that particular place (endemism), like the Puerto Rican Lizard Cuckoo and the diminuitive Puerto Rican Tody (from an earlier post).