by Scott Leslie
The species is found on the small equatorial island of São Tomé, which lies in the Atlantic Ocean about 300 kilometres off the coast of Gabon, in West Africa. With an area of only about 1,000 square kilometres, the country of São Tomé and Principe, Africa’s smallest, was formed by now extinct volcanoes. Despite its limited size, the nation is home to 28 species of birds found nowhere else in the world. São Tomé is a crucible of evolution, much like its Pacific counterparts the Galapagos and the Hawaiian Archipelagos.
The São Tomé fiscal, with a population estimated to be under 50, not only is among the rarest birds on the islands, it’s one of the most endangered songbirds on earth. In fact, for six decades it was thought to be extinct, until rediscovered in 1991, when a single bird was observed during an extensive survey.
Slightly smaller than an American robin, the fiscal has a striking appearance with its black back and mask, a creamy belly, a long tail, and a typically heavy shrike bill with a hawk-like hooked tip. It is found only in a small remnant of primary or old-growth lowland and mid-altitude forest. Very little of this original habitat is left on the island after centuries of clearing for sugar cane (São Tomé was the world’s largest producer of sugar in the 16th century), cacao, and coffee plantations. Although little is known of its habits, the fiscal appears to prefer a habitat with a closed canopy of trees above a bare, rocky forest floor; this would presumably suit its style of swoop-down hunting. It also feeds on seeds and fruit in the understorey. Nest predation by introduced black rats, civet cats, and weasels may have also had an impact on the species’ ability to successfully reproduce.
Although direct development of the birds’ remaining habitat has largely stopped (some of which is protected in a small, newly established national park), newly built roads in the area are nevertheless improving access for local people to illegally cut wood, harvest plants, and hunt in the forest. It’s also making it easier for the introduced species that prey on nests to penetrate deep into the fiscal’s habitat. With such a tiny population, the species can ill afford the loss of even one individual or to suffer greater disturbance of its forest habitat. Several other threatened endemic birds of São Tomé, including the São Tomé grosbeak and the dwarf olive ibis, also depend on this primary forest. With an extremely dense human population of 170 people per square kilometre (about 140,000 people live on the island) and scant natural habitat remaining, the future of the fiscal shrike and many of the island’s species, such as the São Tomé grosbeak, are up in the air.
SÃO TOMÉ GROSBEAK
With a total population of fewer than 50, the critically endangered, endemic São Tomé grosbeak is one of the least-known birds in the world. Until it was rediscovered in 1991 (the same year as the island’s fiscal) in southwestern São Tomé, a single specimen collected in the 19th century was all anyone knew of the grosbeak. This large, 18-centimetre-long, rusty brown finch inhabits lowland rainforest, where it uses its massive conical bill to crush seeds. With its tiny population and a preference for spending time in the forest canopy, the bird is rarely seen. Despite the reputation finch species around the world have for their ebullient singing, the São Tomé grosbeak evidently rarely vocalizes, though it does occasionally whistle like a canary. This generally quiet nature might be one of the reasons the species is seldom noticed.
Centuries of widespread destruction of the island’s lowland rainforest for the development of sugar cane and cocoa plantations has left this and other species of native São Tomé birds with little viable habitat, and newly built roads are making it easier to exploit what little forest remains. Much of the species’ habitat is located within the small Obo National Park. Apparently a park in name only, very little conservation enforcement takes place there. Introduced black rats, feral pigs, stoats, and civets also live in the area. The grosbeak remains poorly studied, so it isn’t known what, if any, effects these invasive species may be having on its population. A paucity of ecological knowledge about the grosbeak also makes it difficult to assess which human-caused factors are most impacting the species.
With such a small population and so little known about it, the São Tomé grosbeak is in a precarious position. Unfortunately, the species isn’t even protected under São Tomé law; without such legal status, its future is completely a matter of chance. Legal protection is the number one goal of conservationists, but they must first learn more about the species so that recommendations on a recovery plan can be made to government. Research into its population size, its distribution on the island, and its ecology is underway. A program to train local residents in population monitoring and conservation has recently been completed. It is hoped that the community’s knowledge of rare species such as the São Tomé grosbeak and the São Tomé fiscal will engender an appreciation of them, thereby helping to secure their future. But, with just a few dozen grosbeaks left, there is little time to waste in protecting them and their habitat.
As much as habitat loss threatens the world’s island birds, it may not be the most important problem facing wildlife living on another tiny island, nearly halfway around the world, where the Mariana crow and other native species of Guam in the South Pacific are struggling with a more insidious threat.
MARIANA CROW
Crows are one of the most successful groups of large birds on earth. About 40 species are found around the world, and some, like the American crow, are so cagey and have such a huge population that they’ve become pests. It’s no fluke that many crow species have been able to adapt so well to the modern world. They are highly intelligent, claimed by some researchers to be the smartest non-human species. Maybe being called a birdbrain isn’t such an insult after all.
But sometimes no matter how intelligent you are, circumstances get the better of you. That’s exactly what’s happened to the Mariana crow, a species exclusive to Guam and Rota Island of the Northern Marianas Islands, in the Pacific Ocean, about halfway between Japan and Australia.
Looking all the world like an everyday, typical crow, the Mariana species is dubiously distinguished as the current record holder for rarest of its kind on earth. This isn’t surprising when you consider the three-metre-long menace it has had to contend with: the brown tree snake, an accidentally introduced species normally native to Australia and New Guinea. This nocturnal, tree-climbing snake has devastated the island’s ecosystem since arriving on Guam after World War II. With concentrations of up to 5,000 per square kilometre, legions of them slithered their way over the island, eating practically every small wild animal in sight, killing thousands of domestic pets, and causing frequent power outages as they weighed down and shorted-out electrical lines.
Because the native birds of Guam evolved in the absence of predatory snakes, they were unable to adapt to this efficient predator, which caused the extinction of several small endemic species by the 1980s. The Mariana crow, perhaps owing to its larger size or to an intelligence that allowed it to adapt somewhat to a novel threat, managed to survive the serpent’s onslaught a little longer. But not much: in 1981, only 250 crows were left on Guam.
The crow was listed as endangered on the heels of the extinction in the wild of the Guam rail, Guam flycatcher, and rufous fantail in the 1990s. Soon after, biologists captured and placed 10 of them in US zoos (Guam and the Northern Marianas are territories of the United States) to stock a captive breeding program. Although six captive-bred birds were later released on the island, in 1997, and more than 20 since, only two (both males) survive on Guam today, neither of which are native-born, making the species functionally extinct there. Even the use of electrical tree barriers has failed to protect the crow’s eggs and chicks from tree snakes. There’s little hope of re-establishing the Mariana crow on Guam until the snake has been eradicated from the island. So far that hasn’t been possible.
Fortunately, Mariana crows are also found on the tiny island of Rota, 60 kilometres north of Guam. Until recently, the population was thought to be relatively safe there, with a tally of over 1,300 birds i
n 1982. Since then, however, that number has crashed to only about 100, a decline of over 90 percent in less than three decades. Although the brown tree snake hasn’t been found on the small island yet (its potential arrival is a serious worry that keeps conservationists up at night), many other factors have been involved in the crow’s decline there. Typhoons have destroyed much of the bird’s forest habitat, some of which has also been cleared to make way for golf courses and resorts, housing developments, and farming. On top of all that, rats and monitor lizards kill crow nestlings, and locals shoot the adults. A final insult is the black drongo, an introduced bird, which now competes with the crows for nesting sites.
Although the Mariana crow has been listed as a critically endangered species for decades now, its present predicament is not for a lack of effort. Some of its habitat has been protected on both Rota and Guam, birds have been translocated from the former to boost numbers in the latter (unsuccessfully), attempts have been made to protect it against the brown tree snake, and even a small captive breeding program was started. Although the dual forces of invasive species and habitat destruction aligned against it seem almost overwhelming, hope holds that the Mariana crow will survive.
TAHITI MONARCH
Perfect weather, swaying palms, turquoise lagoons, pristine black sand beaches, mist-shrouded mountains, and sunsets, don’t forget the sunsets. What could be better than living in Tahiti, one of the most beautiful places in the South Pacific? Who wouldn’t jump at the chance? As we just learned about Guam, however, life on a tropical Pacific island isn’t always what it seems to be. Tahiti is no different. Eighty percent of the 29 terrestrial bird species here are endangered, and nearly one-fifth are classified as critically endangered. The king of scarcity is the Tahiti monarch, a 15-centimetre-long, sparrow-sized flycatcher. Despite its grandiose name, this little songbird looks anything but regal in its plain, shiny black plumage and pale blue bill and legs. It is rare like any monarch: only 40 to 45 birds are left.
The Tahiti monarch is the rarest member of the Pomarea genus, a highly threatened group of birds found throughout Polynesia. Two of the species are critically endangered, two are endangered, and one is considered vulnerable. Moreover, another four species of Pomarea flycatchers have already gone extinct.
Occupying a total range of just 28 square kilometres of the island, the monarch lives in forest habitats of giant ferns, mara trees, and flowering hibiscus that are located in four of the island’s deep valleys at elevations between 80 and 400 metres. Highly territorial birds, they announce their presence with a rich and complex flute-like song. Like many flycatcher species around the world, monarchs feed on insects in both the forest canopy and its undergrowth.
Black rats, introduced to Polynesia centuries ago, have long been a menace for the beleaguered members of the Pomarea genus. Though nobody knows with certainty, they were probably instrumental in the decline of the Tahiti monarch. Rats are known to climb to the nests of the endangered bird and eat its egg or young, thus wiping out the entire breeding season for the parent birds. And because Tahiti monarchs lay only a single egg—very unusual for a songbird—they are slow to rebound from losses. Happily, an ongoing rat eradication program in the four valleys where the monarch is found has had a real positive effect on the endangered bird’s prospects for survival.
But there’s more than rats to worry about. Red-vented bulbuls and common mynas, both introduced birds, have also made recovery difficult for the monarch. The myna is thought to prey on eggs and nestlings, and the bulbul may compete for nesting sites. So far they appear to be less of a threat than the rats, and no eradication programs have been carried out on them. Even invasive plants such as the African tulip tree are at odds with the little songbird, dominating its forest habitat by threatening to squeeze out the mara tree in which the monarch builds its nest. Removal of tulip trees from its habitat is thus an important part of the recovery plan. As a contingency in case of a disastrous nesting season, a hurricane, or some other factor decimating the population, plans are in place to take some birds into captivity for a breeding program if it becomes absolutely essential or to translocate some to other islands. Still, this is only a contingency.
Barely 40 of these little musical songbirds hang on. This may be a small, small number, but it’s better than it was. Less than a decade ago, there were only 20 Tahiti monarchs struggling against invasive organisms, the scourge of many a threatened ecosystem, including the famed Galapagos Archipelago, 5,000 kilometres across the South Pacific.
FLOREANA MOCKINGBIRD
One of the biggest myths in all of science is the role that the so-called Darwin’s finches of the Galapagos played in the great naturalist’s formulation of the theory of evolution. Truth be told, he didn’t even specifically mention finches in The Origin of Species, his masterwork published in 1859. But what he did mention, both in The Origin and in journals he wrote while on the HMS Beagle soon after leaving the Galapagos in 1836, were mockingbirds. Mockingbirds, not finches, were instrumental in his idea that species evolve through adaptation to their surrounding environments and come from common ancestors—as opposed to the long-held view that species never change.3
One bird in particular, the Floreana mockingbird, got him thinking when he noticed how different it was from the other three mockingbirds of the archipelago. Darwin asked, did one of those other three species somehow end up on Floreana Island, only to evolve novel traits after generations as it adapted to the peculiarities of its new environment? The answer, of course, was yes.
Floreana mockingbirds are large songbirds of about 25 centimetres in length that spend much of their lives on or near the ground among cactus and other desert shrubbery. They are also noisy birds. In fact, it’s hard to miss the boisterous vocalizations of any one of the 35 members of the mockingbird family found throughout the Americas. All of them, including the four on the Galapagos, are intelligent, musical birds, with complex songs and raucous calls.
With a dark back, light belly, a long tail, and a curved bill, the Floreana bird resembles the northern mockingbird of the United States and southern Canada. The Galapagos bird’s diet is broad and includes such delicacies as insects, little crabs, lizard and bird eggs, fruit, nectar, seeds, centipedes, and ticks plucked from the skin of living iguanas (an unintentional favour for which the reptiles must be grateful). Living in small groups, the rearing of the young mockingbirds is a cooperative affair, with non-breeding adult members of the family helping to feed nestlings.
This famous mockingbird is at risk of disappearing forever. It no longer lives on its namesake island, where it was wiped out by introduced black rats, cats, goats, and human occupation in the late 1800s. Who knows, maybe rodents that escaped from Darwin’s HMS Beagle decades earlier even contributed to the bird’s disappearance from Floreana. Fortunately, a few still survive on two nearby satellite islets, Champion and Gardner-by-Floreana—mere rocks in the sea with a combined area of about one square kilometre. Although the population fluctuates, as few as 100 or even fewer adult birds cling to existence there.
Steeped as Floreana mockingbird is in the history of science, it’s not surprising that a lot of effort is being made to ensure its survival. The major goal is to eventually reintroduce birds from the two satellite populations back onto Floreana. Oddly enough, after nearly two centuries, the hand of Darwin may have a small part to play in the survival of the species. Two of the mockingbirds he collected there in 1835, currently housed in the Natural History Museum in London, are being used in the effort to conserve the bird. In 2009, scientists extracted DNA from these old specimens to use as a benchmark to learn how much the current birds’ DNA had changed (i.e., evolved). The analysis revealed that the living Champion and Gardner-by-Floreana populations have only recently begun to evolve apart from one another, though they are not yet separate species. This suggests that the now extinct population on Floreana Island acted as a bridge between them, enabling the mixing of genes across all three islands in the pas
t. So today, now too far apart physically to interbreed, the mockingbirds have begun to “go their own way” in an evolutionary sense.
The upside is that the slight evolution that has occurred in these two populations means the overall genetic variation within the species is greater. This should improve the chances of success for any introduction of the Champion and Gardner-by-Floreana birds to Floreana: the more diverse a population, the more resilient it is and the better its chance of survival. On top of all this, the rapid genetic change in the two populations confirms what we have known ever since Darwin: new species are evolved from older ones by adapting over generations to their specific environments—exactly what is happening to the birds on Champion and Gardner-by-Floreana.