Monk parakeets are native to southern South America but are established in Puerto Rico. AP Photo/Wilfredo Lee

My wife and I recently returned from a wonderful birding trip to Puerto Rico. Our priority was to find the 17 species of birds only found there.

We timed our trip to occur after the many species of wintering songbirds had departed for North America, so avoided having to sort through tons of overwintering songbirds for Puerto Rican specialties.

We hired Julio Salgado to be our guide. A friendly person, Julio has a vast knowledge of the birds of Puerto Rico. His hearing is acute, and he has the best eyes of anyone I have ever birded with.

Islands have played a major role in the growth of many scientific fields. The Galapagos Islands provided the inspiration for Darwin’s theory of natural selection. Other islands have been the focus of major studies that have advanced ecology, behavior, biogeography, geology, and paleontology. Biologists and geologists are fascinated by islands.

Puerto Rico is a relatively small island, measuring about 110 miles from east to west and 40 miles from north to south. It is barely larger than Rhode Island.

The number of species on an island is determined by the proximity of the nearest continent and the size of the island. The species diversity of Monhegan Island is high relative to Maine because it is so close to the mainland. Remote islands like Hawaii have much lower diversity.

Puerto Rico has 376 bird species of which 196 are vagrants, birds which appear out of range and are not seen regularly. Delaware has 420 species with 96 vagrant species. Rhode Island has 431 species with 109 vagrant species.

So, Puerto Rico has much lower diversity than Delaware or Rhode Island with 180 regularly occurring birds compared to the 324 in Delaware or 322 in Rhode Island, states of roughly equal size to Puerto Rico.

If we look at Florida, the closest state to Puerto Rico, we find it has 373 regularly occurring species.

The pattern of reduced diversity was evident to us by what we didn’t see. There are no crows or jays on the island. We did not see a single gull on our trip. Only three species of warblers nest in Puerto Rico.

A second feature of islands is that species introduced either intentionally or inadvertently by humans frequently become established. Particularly in disturbed habitats, introduced species can carve out a niche and thrive.

The first bird we saw in Puerto Rico was a monk parakeet in its massive nest built on an urban light pole. These birds are native to southern South America but are established in Puerto Rico.

We birded a disturbed grassland area near Manatí on the northwest portion of the island. We found many pin-tailed whydahs. These are birds of the African savanna. During the breeding season, males grow long tail feathers. They fly over the grassland attempting to attract females by lengths of their tail feathers.

In the same area, we found red bishops, orange-billed waxbills, scaly-breasted munias and bronzed manikins in the tall grasses. All are African birds, brought to Puerto Rico as cage birds. On a power line, we found an African collared dove.

Arriving at a park at Vega Baja, a flock of white-winged parakeets flew overhead. They are native to the Amazonian forests of South America.

We saw a small flock of bright yellow saffron finches perched on a chain-link fence surrounding a factory. These birds are found in northern South America.

While birding in a suburban area in Barceloneta, I caught a glimpse of a large orange and black bird. It looked like a Baltimore oriole on steroids. We got good looks at this Venezuelan troupial. It is in the oriole and blackbird family but, as the name implies, is a bird of northern South America.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu