Isla Rasa, southern Midriff Islands
We were very fortunate this morning to have an opportunity to visit the island of Rasa (sometimes spelled Raza). This low, small island (covering only 250 acres) is truly one of the great wildlife sites of the world. Many of our guests were already aware of the place through their readings, but I think few were prepared for what they actually witnessed. Isla Rasa is the primary breeding site for both the Heermann's gull and the elegant tern. This fact is all the more astounding judging from the fact that these birds range over vast distances along the Pacific coastline from Washington to Peru. After waiting at anchorage an hour and a half, we had enough water from the rising tide to make a landing within a protected lagoon and safely get ashore. We were met by our friend Enriquetta Velarde, the head warden and researcher on the island, and her assistant Sergio, and were shown around part of the island.
The story of this place is remarkable. At the beginning of the 20th century, it was estimated that more than one million birds nested together on this tiny island every Spring, but guano harvesters, egg collectors, and introduced rats disrupted the breeding cycle so much that by the late 1950s a total of only 5,000 elegant terns and Heermann’s gulls survived. Various conservation organizations pushed for protection of the few birds remaining and the Mexican government declared the island a sanctuary in 1964. The predatory rats were exterminated in the 1990s and the bird population has responded with a tremendous population resurgence. Today, it is estimated that 250,000 Heermann's gulls, 200,000 elegant terns, and 15,000 royal terns nest on the island (as well as one pair of peregrine falcons, which we saw while cruising around the island in our Zodiacs).
We learned all about the dramatic and fascinating events surrounding the nesting habits and interaction between the gulls and terns of Isla Rasa. Every March, the Heermann's gulls arrive first and select their nest sites. The elegant and royal terns arrive a little later and begin to gather around the shoreline of the island. Then, upon some unknown signal, thousands of the terns suddenly flock into their pre?chosen nesting area on a flat, sandy area during the dark of night. The hapless gulls already in that particular area are forced out and their eggs are lost. The next day, the gulls start whittling away at the edge of the tern colony and shrink it down in size a bit. The following night, more terns join the colony and enlarge it to even bigger than it was before, and again, the gulls will beat it back during daylight. The process continues for several days until equilibrium is reached and all the terns are settled in (see photo). The strategy of the terns is to nest shoulder to shoulder so they can present a solid phalanx of upturned sharp beaks to any would-be gullish predators, although the terns closest to the edge are very much at risk to losing eggs and/or chicks to the larger and stronger gulls. It's a complicated system that has been developed over thousands of years, and seems to work very well now that they are once again left alone in a natural state.
We were very fortunate this morning to have an opportunity to visit the island of Rasa (sometimes spelled Raza). This low, small island (covering only 250 acres) is truly one of the great wildlife sites of the world. Many of our guests were already aware of the place through their readings, but I think few were prepared for what they actually witnessed. Isla Rasa is the primary breeding site for both the Heermann's gull and the elegant tern. This fact is all the more astounding judging from the fact that these birds range over vast distances along the Pacific coastline from Washington to Peru. After waiting at anchorage an hour and a half, we had enough water from the rising tide to make a landing within a protected lagoon and safely get ashore. We were met by our friend Enriquetta Velarde, the head warden and researcher on the island, and her assistant Sergio, and were shown around part of the island.
The story of this place is remarkable. At the beginning of the 20th century, it was estimated that more than one million birds nested together on this tiny island every Spring, but guano harvesters, egg collectors, and introduced rats disrupted the breeding cycle so much that by the late 1950s a total of only 5,000 elegant terns and Heermann’s gulls survived. Various conservation organizations pushed for protection of the few birds remaining and the Mexican government declared the island a sanctuary in 1964. The predatory rats were exterminated in the 1990s and the bird population has responded with a tremendous population resurgence. Today, it is estimated that 250,000 Heermann's gulls, 200,000 elegant terns, and 15,000 royal terns nest on the island (as well as one pair of peregrine falcons, which we saw while cruising around the island in our Zodiacs).
We learned all about the dramatic and fascinating events surrounding the nesting habits and interaction between the gulls and terns of Isla Rasa. Every March, the Heermann's gulls arrive first and select their nest sites. The elegant and royal terns arrive a little later and begin to gather around the shoreline of the island. Then, upon some unknown signal, thousands of the terns suddenly flock into their pre?chosen nesting area on a flat, sandy area during the dark of night. The hapless gulls already in that particular area are forced out and their eggs are lost. The next day, the gulls start whittling away at the edge of the tern colony and shrink it down in size a bit. The following night, more terns join the colony and enlarge it to even bigger than it was before, and again, the gulls will beat it back during daylight. The process continues for several days until equilibrium is reached and all the terns are settled in (see photo). The strategy of the terns is to nest shoulder to shoulder so they can present a solid phalanx of upturned sharp beaks to any would-be gullish predators, although the terns closest to the edge are very much at risk to losing eggs and/or chicks to the larger and stronger gulls. It's a complicated system that has been developed over thousands of years, and seems to work very well now that they are once again left alone in a natural state.
