Three Hawaiian solitaires. Top to bottom: Kāma'o (Myadestes myadestinus), Oloma'o (M. lanaiensis), and Ōma'o (M. obscurus). Illustrated by John Gerrard Keulemans between 1893 and 1900. Retrieved via Wikimedia Commons.
In terms of the world's most biodiverse regions, the Hawaiian islands of the central Pacific have had a spectacularly bad run of luck over the past ten centuries. As with many of the world's oceanic islands, the Hawaiian Archipelago, some 2,500 miles (4,000 km) away from the nearest continental landmass, is home to numerous endemic species, including plants, invertebrates, and birds. With hunting and the introduced Polynesian rat (Rattus exulans) arriving on the archipelago with the first human settlement (between about 500 and 1300 CE), a number of ground-dwelling birds with vulnerable nesting sites, such as the massive moa-nalo waterbirds, disappeared. Later European exploration, conquest, and settlement on the islands brought a host of introduced Old World and New World species alike, which visited further devastation still on Hawaii's endemic species. Mammals like mongooses, pigs, and cats (and the Toxoplasma bacteria carried by the latter, which alone has devastated many native species) have wreaked untold havoc, especially on the native bird life. Only six passerine lineages seem to have made it to Hawaii in its history. Of these, one (Mohoidae, the Hawaiian honeyeaters) is now entirely extinct, the only avian family to have become so in the modern era. Another, Hawaii's sole native corvid, the Hawaiian crow or 'alalā, is currently extinct in the wild and persists only in captivity. Of Hawaii's two sylviids ("Old World warblers") one is extinct and another critically endangered. Even out of the largest of the endemic passerine groups, the Hawaiian honeycreepers (fringillid finches of the subfamily Carduelinae), a great number of species are either extinct or in danger of extinction. Hawaii's three monarch flycatcher species (Monarchidae) seem to have fared the best, by contrast, but on the whole it wouldn't be too great an exaggeration to say that all of Hawaii's native songbirds are in dire straits indeed. But more on these other groups another day - the sixth lineage is comprised of Hawaii's five thrushes, which will be the focus of today's post.
For a long time, the Hawaiian thrushes were grouped together in the unique genus Phaeornis, but closer analysis has instead borne out the conclusion that they are nested within Myadestes, the solitaires. American readers might be familiar with this genus in the form of Townsend's solitaire (M. townsendi), or Mexican readers with the brown-backed solitaire (M. occidentalis). Other members of this clade are found in the Caribbean, Central America, and into South America as far south as Bolivia. The solitaires' closest relatives are the bluebirds (Sialia) and, more distantly, West African thrushes in the genus Neocossyphus. It seems likely that Neocossyphus thrushes disseminated across the Atlantic Ocean from within the ancestral Neocossyphus-Myadestes-Sialia clade, given that the Myadestes solitaires crossed the Pacific to Hawaii in a similar fashion. Thrushes at large seem to have emerged sometime in the late Miocene epoch (most likely between 11.6 - 7.5 Ma) when they diverged from the Old World flycatchers and kin, Muscicapidae. This seems to imply that thrushes are ancestrally an Old World lineage and, if so, that they invaded the Americas on three separate occasions - the solitaires and bluebirds, Catharus and allies, and several species of Turdus and relatives. (And, further, that if the assumption about Neocossyphus crossing the Atlantic is correct, that the thrushes emerged in Afro-Eurasia, migrated to the Americas... and then migrated to Afro-Eurasia again.)
Relationships between the solitaires of Mexico, Central America, the Caribbean, and Hawaii - two interpretations. After Klicka et al. (2005).
From whichever stock they arose, the solitaires of Myadestes disseminated rapidly across the Americas, if genetic data is any indication. The various species of Myadestes diverged from one another perhaps as much as 7 Ma, not very long after the origin of Turdidae itself. The Hawaiian solitaires in particular seem to be closest to the Mesoamerican and Caribbean species, and very possibly are closest to the brown-backed solitaire (M. occidentalis). They dispersed to the Hawaiian islands, apparently by wing, at least 4 Ma. Although no phylogenetic study has been made of these five Hawaiian species together, their broad similarities and the difficulty of transit between Mexico and Hawaii for medium-sized passerines like these probably means that they are one another's closest relatives and arrived as the result of a single dispersal event. Today, three of these species are probably extinct, but all of them persisted into the historical era, leaving behind enough information to draw some conclusions about the whole group.
The first of Hawaii's solitaires to vanish from the islands was a cryptic bird known as the 'āmaui, a shortening of the Hawaiian manu a Māui, "bird of Māui", after a Hawaiian folk hero. In fact, all the thrushes which ranged onto Oahu, Maui, Moloka'i, and Lāna'i (those latter three originally one island, 'Maui Nui', which was partially submerged in recent geological history) were originally known by this same name, but only this bird, M. woahensis, seems to have retained it. Known only from a single holotype, collected by Andrew Bloxam in 1825, as well as some subfossilized bones, what little we know about this species is owed to guesswork based on other solitaires. Abundant in the early 19th Century, it vanished without trace or record by about 1860, thus marking the disappearance of Myadestes from the island of Oahu.
The remaining species have had the fortune to persist for much longer, with two vanishing late in the 20th Century and two more remaining extant to the present day. Each of these shares a common predilection for fruit, feasting occasionally on insects instead during the breeding season, and can be recognized by their calls, which are said to resemble a policeman's whistle. The oloma'o (M. lanaiensis) of Maui, Moloka'i, and Lāna'i, though still officially listed as Critically Endangered, was last definitively spotted in 1980 and is feared to be extinct. A reclusive bird, it was often seen to sit motionless, hunched in the underbrush in between bursts of song.
Illustration of the kāma'o (Myadestes myadestinus) by Frederick William Frohawk from between 1890 and 1899. Retrieved via Wikimedia Commons.
The kāma'o (M. myadestinus) similarly disappeared during the course of the 1980s, last being seen in the Alaka'i Swamp in 1989. The largest of the Hawaiian solitaires at 20 cm (7.8 inches) in length, it was endemic to the island of Kaua'i. It was noted for its habit of suddenly bursting into the air from the underbrush and singing a few notes, and then falling back into cover, a behavior unknown in other solitaires. As early as the 1960s the kāma'o had become very uncommon, retreating into higher altitudes, and was often seen with lesions around its mouth and feet. These are a telltale sign of avian malaria, introduced by mosquitoes breeding in the sties of invasive pigs. This disease must have devastated the kāma'o's population, and has taken its toll on countless other endemic bird species as well. Alaka'i was hit by a succession of hurricanes in the 1980s and 1990s which must have had a terrible effect on the small range of these birds, With no sightings in the last 25 years, even in the face of detailed surveys of Alaka'i and its avian life, it has to be assumed that the kāma'o is extinct.
Puaiohi (Myadestes palmeri). Taken by Eike Wulfmayer at an avian conservation center in Volcano, HI, 2002. Retrieved via Wikimedia Commons.
Another Kaua'i solitaire, however, has happily avoided the kāma'o's fate. The puaiohi (M. palmeri) currently resides in craggy areas above 1,000 meters or so in altitude (3,280 feet) in Alaka'i Wilderness Preserve, in the same area as the kāma'o's last haunts. Unlike its only living relative amongst the Hawaiian solitaires, the 'ōma'o, the puaiohi typically nests in cliff faces. This is a bit odd for a thrush, since most species prefer to nest in branches, cavities, or in a few cases, on the ground. Its particular environmental needs doubtless put additional pressure on this species, which seems to number at most at only 200 individuals in the wild.
'Ōma'o (Myadestes obscurus) as portrayed by Joseph Smit, 1869. Retrieved via Wikimedia Commons.
The 'ōma'o (M. obscurus) of the big island, Hawai'i, seems to be doing somewhat better, with perhaps as many as 200,000 individuals living in the wild on the eastern and southern slopes of Mauna Kea. Efforts to maintain its shrinking habitat on the island are underway in the form of aggressive reduction of the range of feral pigs and other invasive mammals. The puaiohi is under the careful watch of conservationists as well, with captive breeding programs seeing considerable success in reintroducing birds to the wild. The future is brighter for these two birds than for their vanished kin, but introduced mammals which predate upon the solitaires, invasive birds like laughingthrushes which compete with them for food and space, and foreign plants which threaten to reshape what little habitat they have left, continue to endanger the 'ōma'o and puaiohi. In addition, climatic events such as El Niño imperil the species by shortening their breeding seasons with the droughts they often bring to the islands. As we enter another El Niño period in early 2016, it can only be hoped that man-made climate change doesn't doom this unique lineage of thrushes to permanent extinction like the Hawaiian honeyeaters before them.
For another extinct island-endemic bird:
Sources:
- BirdLife International (2016) Species factsheet: Myadestes lanaiensis. Downloaded from BirdLife on 25/01/2016.
- BirdLife International (2016) Species factsheet: Myadestes myadestinus. Downloaded from BirdLife on 25/01/2016.
- BirdLife International (2016) Species factsheet: Myadestes obscurus. Downloaded from BirdLife on 24/01/2016.
- BirdLife International (2016) Species factsheet: Myadestes palmeri. Downloaded from BirdLife on 25/01/2016.
- BirdLife International (2016) Species factsheet: Myadestes woahensis. Downloaded from BirdLife on 24/01/2016.
- Klicka, John, Gary Voelker, and Garth M. Spellman. "A molecular analysis of the 'true thrushes' (Aves: Turdidae)." Molecular Phylogenetics and Evolution 34. 2005.
- Miller, Matthew J., Eldredge Bermingham, and Robert E. Ricklefs. "Historical biogeography of the New World solitaires (Myadestes spp)." The Auk 124. 2007.
- United States Fish and Wildlife Service: "Draft Revised Recovery Plan For Hawaiian Forest Birds." 2003.
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