The Story of the San Benedicto Island Rock Wren

by Garth Spellman, PhD

The Museum has one of the richest ornithology collections in the country, with nearly 55,000 specimens from around the world, dating from 1842 to today. After joining the staff as curator of ornithology last summer, I began familiarizing myself with the collection. One set of specimens immediately caught my attention: the San Benedicto Island rock wren.

The story of how the Museum acquired these precious specimens begins in April 1940 on a wave-whipped 78-foot schooner 300 miles off the southern tip of Baja California. At the helm is Bill Pemberton, owner of the vessel, California oil tycoon, and dedicated naturalist. His passengers are Alfred M. Bailey, director of the then Denver Museum of Natural History, and Robert J. Neidrach, the Museum's curator of ornithology. They are two weeks into a month-long expedition to film, photograph, and collect seabirds on the desert islands off the Pacific coast of Mexico. They have just dropped anchor off the island of San Benedicto, easily identified by the dark cone of the Bárcena volcano and the tens of thousands of frigate birds, masked boobies, and shearwaters soaring above coastal rocks and crashing waves.

The naturalists spent two days filming and collecting on the island. Bailey's own account of the expedition in his 1941 National Geographic article “The Cruise of the Kinkajou” suggests San Benedicto was a rather unpleasant place to work. He wrote that the rock appeared to have been “deposited within the last few days” and the seeds of the low-lying bushes “penetrated socks, trousers, and shirts,” intimating the devil himself must have conjured them.

San Benedicto Island possessed a unique community of birds. Unlike most islands off the Pacific coast of Mexico, the bird community shared affinity with more distant Pacific islands, such as Hawaii, rather than Mexico. And similar to other distant oceanic islands, the seabirds were tame, meaning they had not encountered enough humans to learn to be wary of them. Bailey's and Neidrach's photos picture the naturalists sitting within arm's reach of nesting masked boobies. The birds appear to be looking quizzically at the intruders. Nesting seabird colonies are obnoxiously loud places, and the cacophony must have been deafening, so it is notable that Bailey remarked on a familiar song that could be heard throughout the island. It was the song of the sole land bird to be found on San Benedicto, the rock wren.

Rock wrens are small rather dull brown birds that nest in rock crevices across the arid lands of Mexico and the western United States. Their songs are beautiful and complex, and since they are perfectly camouflaged in rocky arid landscapes, these wrens are more often heard than seen. Rock wrens are not particularly strong fliers, so even though the rocky landscape of San Benedicto is perfect habitat for the rock wren it is surprising that they found their way there.

The first naturalists to have worked on San Benedicto suggested the bird possessed a slightly different song and bill shape than the species in Mexico and the United States. These differences, in addition to its isolation, led naturalists to call it a different type of rock wren (what we in the business call a different subspecies). Bailey and Neidrach understood the importance of this little critter, and while they did not photograph it, they did collect several specimens. As fate would have it, these turned out to be to last record of the San Benedicto Island rock wren.

In 1952, the Bárcena volcano erupted, spewing millions of tons of ash into the sky, blanketing the entire island and laying waste to all life. An account of the first visit after the volcano reads like a naturalist's account of the aftermath of Pompeii. Nearly 20,000 seabird skeletons were observed poking out of the ash.

While tragic and sad, the Bárcena eruption did not mark the end of San Benedicto's story. One year after the eruption, seabirds returned to nest. Today vegetation has reclaimed much of the island, and the number of nesting seabirds equals or in some estimates exceeds the numbers prior to the eruption. So although San Benedicto Island lives on as a paradise for seabirds, it lives on without its music. The rock wren was lost that day.

As a scientist, I have so many questions for this little brown bird. How different were you? How long had you lived in such a remote place? Fortunately for us, Bailey and Neidrach saw that the San Benedicto rock wren had stories to tell. These specimens are safely housed in the Avenir Collections Center. Using new technology, I am looking at DNA from the San Benedicto rock wren and comparing it to DNA from other rock wrens from Mexico and the United States to unlock its evolutionary secrets, giving the past a voice again.

To lend your support to this or other museum initiatives, please contact DMNS Advancement at development@dmns.org or 303.370.8262.