The small and flat Isla Rasa is a real jewel in the Sea of Cortez. The history is very interesting and it was recognized quite early as worthy of protection. Every spring hundred thousands of breeding sea birds use every square centimeter of the flat, sandy area for their nests. Since the 19th century tons of guano were mined and the eggs of the sea birds were collected commercially. So the population was drastically reduced to a few thousand birds in the middle of the 20st century. Therefor a few Mexican and US-American scientists decided to protect this island, which was realized in 1964. 1978 this small protected area was integrated into the much bigger “Protected Islands of the Gulf of California”. Already since 1979 ornithologists study the sea bird population on the island. Important was the eradication of the house mouse and the black rat. Since then the population of for example the elegant tern (Thalasseus elegans) recovered from only 30.000 birds in 1979 to 180.000 in the year 2004.
We anchor in the southwest of the island and are welcomed by a big group of Heermann’s Gulls (Larus heermanni) and Elegant Terns. The weather is calm and we really want to see the bird population close up. So we take the dinghy and try to find a landing spot. On the way we discover an osprey nest (Pandion haliaetus) on the high rocks close to shore. Finally we go ashore in one of the lagoons, which is only filled with water at high tide. We are welcomed by ornithologist Enriqueta Velarde, who stays on the island every year during the three months of breeding season. Some of her students are also on the island weekly to do their research work. Enriqueta explains that the birds are in a critical state at the moment. The chicks already hatched but are still small and weak. If they leave the surroundings of their territory or nesting area for any reason (for example a human walking by), they will be hacked to death by the other birds which see them as intruder. Therefor she asks us to stay on the track to the research house and its surroundings. This is more than enough since we have a great view of part of the huge Heermann’s Gull (Larus heermanni) breeding colony from above. And there are plenty of chicks and gulls around the research house. They mainly nest on the flat, sandy areas but also around the stone mounds which are leftovers of the guano mining. We can watch a Heermann’s Gull which defends her two chicks and later on feeds them. The two chicks have different sizes, which happens when one hatches before the other. Enriqueta tells us that every normal year up to 50 % of the chicks die. In El Nino years even more (up to 90 %), because in those years the sardine and anchovies swarms, which the sea birds feed on, fail to appear. Long-term the populations of the Heermann’s Gulls and Elegant Terns are declining since there are more and more El Ninos and the fishery of sardines and anchovies in the Gulf of California increases.
In the visitor book we see that we are the first visitors since the end of April, which means not a lot of people visit this area. The scientists do not mind if people come to visit during the nesting and breeding period, before the chicks hatch. During that time it is even possible to walk through this huge breeding colony. But we are happy to be able to see the little chicks and observe fascinated the thousands of birds.
The smaller breeding colony of the Elegant Terns (Thalasseus elegans) can not be seen from the research house because it is located in another one of those flat and sandy valleys. Even the scientists do not leave the area around the house at the moment to give the chicks the highest possible survival chance. After we leave with the dinghy we drive by one of the other valleys and can at least see parts of the Royal Tern (Thalasseus maxima) breeding colony.
At the moment there are about 260.000 Heermann’s Gulls (app. 95 % of the worldwide population), 180.000 Elegant Terns (90-97 % of the worldwide population) as well as an additional 15.000 Royal Terns breeding on this small island. Those three sea bird species are only on the island during breeding time and spread out between July and January from southern Canada to Mexico (heermann’s gulls) and the whole Southamerican Pacific (elegant terns) respectively. Only at the beginning of the breeding season they appear around Isla Rasa. Since the Heermann’s Gulls are very aggressive towards other birds and keep away birds of prey the Elegant Terns nest close to them to benefit from it.
The scientist who stay on the island during the breeding season are very important for the protection of the birds, the eggs and the chicks and mainly help against interference of humans. We really hope this special but vulnerable place will remain a sanctuary for sea birds and the over-fishing can be stopped.