Islas Partida and Rasa
We spent the day exploring several rocky, volcanic islands in the southern region of the Midriff Group. Soon after sunrise we found an anchorage off the southern side of Isla Partida, close to one of the most dramatic geological formations in all the Sea of Cortez. It is a radially displayed series of basaltic columns which have been exposed by some weird fracturing or erosion that has left a cross sectional view of the formation. This island is very interesting from a wildlife point of view because it is the breeding site of least and black petrels, both of which nest within the loose rocks up on the slopes. Some of us were able to hear their plaintive calls coming right out of the ground while hiking on the island. Even weirder is the fact that little fishing bats often bed down in the same holes with these petrels.
By mid-afternoon, we reached the amazing little, flat island known as Rasa. Although many of us had never heard of the island before this expedition, it is one of the great wildlife sites of the world, and it has been documented through many nature films. Covering only 250 acres and composed entirely of Miocene basalt, it is the breeding ground for virtually all the Heerman's gulls and elegant terns in the world! Iquita and Sarah, the two women working on Isla Rasa, came out to the ship to greet us when we arrived, and we organized a very interesting question and answer session with them. We learned that Iquita is the scientific researcher and warden and Sarah is making a documentary on the breeding interactions of the two bird species found here. The history of this remarkable island is sad indeed, because before the coming of humans, more than one million birds nested together on this tiny island every Spring. Guano harvesters appeared at the end of 19th century; rats were introduced on the island at this time; and in the early 20th century, egg collectors began working the island. By 1960, the bird population had been reduced to just 5,000 individuals.
Happily, the Mexican government declared the island a sanctuary in 1964 and in recent years the rats were successfully removed. The population has now grown back to over half a million birds. The present population includes some 300,000 Heermann's gulls, 200,000 elegant terns, and 20,000 royal terns. The events surrounding the nesting habits and interaction between the gulls and terns of Isla Rasa are quite bizarre and one-of-a-kind and have long been the focus of scientific study. Every year, at the beginning of March, the Heermann's gulls arrive and select their nest sites. The elegant 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 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 an equilibrium is reached and all the terns are settled in. By the first week in April, almost all the birds are on eggs. 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. It's a complicated system which has been developed over thousands of years, and seems to work very well…as long as humans don’t disrupt it.
We spent the day exploring several rocky, volcanic islands in the southern region of the Midriff Group. Soon after sunrise we found an anchorage off the southern side of Isla Partida, close to one of the most dramatic geological formations in all the Sea of Cortez. It is a radially displayed series of basaltic columns which have been exposed by some weird fracturing or erosion that has left a cross sectional view of the formation. This island is very interesting from a wildlife point of view because it is the breeding site of least and black petrels, both of which nest within the loose rocks up on the slopes. Some of us were able to hear their plaintive calls coming right out of the ground while hiking on the island. Even weirder is the fact that little fishing bats often bed down in the same holes with these petrels.
By mid-afternoon, we reached the amazing little, flat island known as Rasa. Although many of us had never heard of the island before this expedition, it is one of the great wildlife sites of the world, and it has been documented through many nature films. Covering only 250 acres and composed entirely of Miocene basalt, it is the breeding ground for virtually all the Heerman's gulls and elegant terns in the world! Iquita and Sarah, the two women working on Isla Rasa, came out to the ship to greet us when we arrived, and we organized a very interesting question and answer session with them. We learned that Iquita is the scientific researcher and warden and Sarah is making a documentary on the breeding interactions of the two bird species found here. The history of this remarkable island is sad indeed, because before the coming of humans, more than one million birds nested together on this tiny island every Spring. Guano harvesters appeared at the end of 19th century; rats were introduced on the island at this time; and in the early 20th century, egg collectors began working the island. By 1960, the bird population had been reduced to just 5,000 individuals.
Happily, the Mexican government declared the island a sanctuary in 1964 and in recent years the rats were successfully removed. The population has now grown back to over half a million birds. The present population includes some 300,000 Heermann's gulls, 200,000 elegant terns, and 20,000 royal terns. The events surrounding the nesting habits and interaction between the gulls and terns of Isla Rasa are quite bizarre and one-of-a-kind and have long been the focus of scientific study. Every year, at the beginning of March, the Heermann's gulls arrive and select their nest sites. The elegant 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 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 an equilibrium is reached and all the terns are settled in. By the first week in April, almost all the birds are on eggs. 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. It's a complicated system which has been developed over thousands of years, and seems to work very well…as long as humans don’t disrupt it.
