Circumnavigation of Sumatra

On Location: Back from the Dead...The Curious Case of the Silvery Pigeon in the Batu Islands

Zegrahm Contributor|January 22, 2011|Blog Post

We’ve just made history. Not only have we successfully completed Zegrahm’s first-ever Circumnavigation of Sumatra but, a couple of days ago, we also became the first group ever to see a silvery pigeon, a bird considered extinct until Christmas Day last year.

The silvery pigeon is a bulky, pale pigeon first described from Pontianak, Borneo, around 1850. Apparently always a scarce bird, the next 80 years saw scattered records from the small islands around Sumatra and Borneo. The last confirmed record from the 20th century came from the Karimata islands, off Kalimantan, in 1931 . . . and then the bird seemingly vanished! For the next 80 years, all attempts to relocate the species came up empty-handed, though unverified claims of its continued existence trickled in from various sites around Sumatra, especially the remote southern province of Jambi, during the last decades of the 20th century. Birdlife International’s “Rule of Thumb” is to declare a bird extinct if it hasn’t been conclusively recorded, despite searches, for more than half a century.

Then a flight photograph taken on Masakut Island, south of the Mentawai group off Sumatra, in 2008 gave renewed hope for its survival, and on December 25, 2010, a British birder exploring the Simeulue Islands received the best possible Christmas present: prolonged, close studies, with incontrovertible photographic evidence, of three silvery pigeons (see photograph by James Eaton). The species was back on the map, at last!

Zegrahm’s inaugural Sumatra circumnavigation had always planned to visit some of the outer archipelagos south of the world’s 6th largest island, and I was naturally extremely enthusiastic about setting foot in these far-flung forests. On the afternoon of the January 13, barely a fortnight after the official “rediscovery” of the silvery pigeon, our band of birders located and photographed at least three individuals on the tiny islet of Bojo, off the southern coast of Tanahbala in the Batu Archipelago. The habitat was primary forest at the edge of a clearing that had been freshly felled for slash-and-burn agriculture. A number of the adjacent trees were in fruit, attracting healthy numbers of frugivores, of various species. Commonest were the widespread green imperial and pied imperial pigeons (see photographs by Jonathan Rossouw), and then came a tantalizing sighting of a single, somewhat smaller pigeon, pearly-hued, cruising across the clearing. Although I was certain that this was the almost mythical silvery pigeon, we needed a better sighting than this! Fortunately, the birding gods smiled on our cause and a few minutes later, another bird landed much closer, affording us good views through the telescope of its ruby-red eye, purplish eye wattles, dark-based pale bill, and sloping forehead, features that separate silvery pigeon from the very similar pied imperial pigeon. Finally, a very distant individual posed longer enough for a quick snapshot by one of our expedition members, Mr. Wei.

The fascinating thing about this species, from a biological standpoint, is the striking similarity in its outward appearance to the pied imperial pigeon, a bird from an entirely separate genus! The imperial pigeon is a gregarious animal, which breeds and feeds in large groups that makes predation by raptors tricky, and so this appears to be a classic example of convergent evolution and so-called Mullerian mimicry on the part of the silvery pigeon, in which its survival is improved by its appearing, and behaving, like a pied imperial pigeon.

In marked contrast with adjacent Sumatra, on which native bird populations have been dramatically affected by deforestation and trapping for the cage bird trade, the Batu Islands are sparsely populated, mostly forested, and with healthy populations of native birds, giving hope for the long term survival of the silvery pigeon.

Twitching Tip: This critically endangered bird, with a total global population estimated at fewer than 50 birds in the wild by Birdlife International, has been seen by only a handful of birders ever but this could change with our new discovery on the Batu Islands. Although they are incredibly remote, visiting the Batu group (and hopefully seeing the pigeon!) has just been made much easier, due to the establishment of a very comfortable, Australian-owned and operated, surfing camp called Telo Lodge (see www.teloislandlodge.com). It is a short flight from Padang or Medan to the local airport Lasondre on the island of Tanah Masa, followed by a boat ride across to the idyllic lodge. The owners, Matt and Jenny, know the islands intimately and can arrange a visit to the north-eastern tip of Bojo island, where the pigeons are a mere five minute walk into the forest.