Where does a species belong? This question has a variety of interlinked answers. For instance, there is the geographic answer: the species’ natural habitat. Then there is the ecological answer: the role which a species plays in its ecosystem. There is also the issue of taxonomy: where the species fits into the evolutionary tree of life.

This is the tale of a species for which all these questions were long unanswered: the Liverpool pigeon. Despite its common name, this creature was never native to the north-western port city, nor indeed to any part of the British Isles.

The first known record of this pigeon dates from 1783, when English ornithologist John Latham described it, having seen two taxidermied birds in private collections. Latham gave these birds their original common name, the “spotted green pigeon”. In 1789, this apparently new species got its first binomial: Columba maculata. Decades later, Latham illustrated his descriptions, as shown below.

Spotted green pigeon, John Latham, 1823. Image public domain.

One of the specimens Latham saw belonged to a London-based military officer and sometime naturalist named Thomas Davies. After Davies’ death in 1812, his pigeon was purchased by the 13^th Earl of Derby, journeying with him to Merseyside. The fate of the other taxidermied pigeon which Latham saw is unknown. This lost specimen may have been more mature than the Earl’s pigeon, since Latham also drew a more brightly-coloured version of the bird. In 1851, the Earl’s collection was incorporated into the Derby Museum, later renamed the World Museum, Liverpool.

By the late nineteenth century, with no new specimens forthcoming, let alone any observations in the wild, the species faded into obscurity. In 1901, famed zoologist Walter Rothschild had this to say on the spotted green pigeon, lumping it together with the Nicobar pigeon in Part II of his Notes on Papuan Birds:

‘It is extraordinary that the home of this bird is not yet discovered, and we suggest the possibility – although there were two specimens – that it is an abnormity.’

For a century after Rothschild’s dismissal of the bird as an abnormal Nicobar pigeon, virtually no attention was paid it. The IUCN deemed it ‘Not Recognised’ and so it might have stayed if not for the attentions of natural history author and illustrator Errol Fuller. In 2001 Fuller suggested the reason for the bird’s fall into obscurity rested largely with Rothschild’s unwillingness to consider it a distinct species. Fuller also coined a new common name for the animal: “the Liverpool pigeon”, in recognition of the only known bird’s final resting place.

Soon after, the IUCN also recognised the contentious skin as representative of a species. The Liverpool pigeon thus made an ignominious move from “Not Recognised” to “Extinct” on the Red List. The basis for the decision to list the species as extinct lies in the lack of hard evidence for the pigeon’s continued existence beyond the early 1780s.

Whilst now acknowledged once again as a distinct species, many questions about where the Liverpool pigeon truly belonged remained unanswered.

By extracting and analysing DNA from two of the bird’s feathers, the species’ story became a little less opaque. The results indicated that the Liverpool pigeon is indeed a true species, sharing a genus with the Nicobar pigeon of the Indian Ocean and South Pacific. Consequently, it was given a new binomial: Caloenas maculata. Fittingly, the test results would also make it a fairly close relative of the likewise lost and enigmatic dodo.

A Nicobar pigeon, the Liverpool pigeon’s closest living relative. Image Wikipedia Creative Commons, by Tomfriedel

So, with the Liverpool pigeon’s (former) place in the tree of life now apparently resolved at last, can any light be shed on where this bird once made its home? Besides the dodo, the species has two other close extinct relatives: the peculiar solitaire of Rodrigues and the Kanaka pigeon of the South Pacific. The latter species is known only from subfossils, most likely being hunted to extinction by the ancestors of the Polynesians around 500BCE. Given all the Liverpool pigeon’s closest known relatives are island dwellers, it is reasonable to think it too favoured this way of life. Tahiti has been suggested as the bird’s possible home, with stories of a similar-looking bird being recorded there in 1928, but this is far from a definitive answer.

For millions of years, the Indian Ocean and South Pacific proved an excellent home for pigeons, thought to have island-hopped from Oceania or South-East Asia, often developing terrestrial habits or even flightlessness in the process. The Liverpool pigeon appears to have taken a different evolutionary path.

Based on analysis of the World Museum’s pigeon, some hypotheses have been advanced about its ecology. It has been suggested the bird was a canopy-dweller, foraging in trees for soft fruits to eat, and thus functioning as a seed disperser within its ecosystem. It has also been proposed that this species avoided flying across open water, staying put on its island home, which may well have been small and remote.

The precise cause of the pigeon’s extinction is also mysterious. The IUCN considers it a reasonable assumption that human hunting in addition to possible predation by introduced species is likely to have ended its existence. This fate has been shared by many island birds since humans began expanding across Earth’s oceans. It is also thought likely that by the time Europeans first encountered the creature, it was already on the brink of extinction.

The species’ story now comes to its sad end. In 1851, the Earl’s taxidermied pigeon was crudely refashioned into a study skin and now forms part of the World Museum’s cabinet collection. Once a living part of an island ecosystem and a member of a strikingly diverse grouping of birds, for the last 166 years the Liverpool pigeon has belonged only in a drawer.

 

 The only known Liverpool pigeon specimen. Photo by Clemency Fisher, World Museum, National Museums Liverpool; released under Creative Commons for BioMed Central.

‘If we and the rest of the back-boned animals were to disappear overnight, the rest of the world would get on pretty well. But if the invertebrates were to disappear, the world’s ecosystems would collapse.’ – Sir David Attenborough

From a human perspective, there is much about invertebrates which can be hard to relate to, or even to conceive of at all. As mammals, our instincts likely count against them. It is no great stretch for many of us to empathise with an orang utan or a wildcat – but an octopus, or a lobster? Even the word “invertebrate” implies a lesser form of life, defined by what it lacks – a backbone.

Historically, perhaps the most high-profile invertebrate grouping has been the gigantic order known as Lepidoptera (butterflies and moths). These creatures are arguably quite well-known from a human perspective, by virtue of their appeal to our well-developed sense of sight. Their frequently bold and distinctive colouration has drawn our interest for millennia. One striking result of this interest has been the identification of approximately one hundred and eighty thousand individual species of butterfly and moth. For context, the total number of all vertebrate species known is below seventy thousand.

Sadly, amidst the ranks of described species of all kinds there exists a growing body of organisms which will never be seen alive again. One such is Sloane’s urania, a large moth formerly native to Jamaica. Though moths are often thought of as less brightly coloured than butterflies, this species was a glorious exception.

The large size and dazzling colour of these animals seems to have bought them to the attention of science quite quickly. The species was first described in 1776, being named in honour of the recently deceased Sir Hans Sloane, whose collection formed the basis of the British Museum. Whilst most moths are active at night, Sloane’s urania was a day flier whose beautiful wings served notice to would-be diurnal predators that it was, in fact, toxic.

Unfortunately, this moth’s natural defences would prove useless in the face of human activity. Surviving relatives of Sloane’s urania migrate periodically, after population explosions amongst their caterpillars defoliate certain areas. Consequently, their populations are not only subject to significant natural fluctuations, but they require multiple large bodies of suitable plants on which to feed. Interestingly, most adult moths do not eat at all, doing all their feeding at the larval stage and living only to mate once they emerge from their cocoons.

It is suspected that, during the late nineteenth century, when Jamaica’s lowland rainforests were being cleared for agriculture, one of this species’ key larval foodplants was lost. Urania larvae in general are picky eaters, feeding solely on a single genus of rainforest vine. Without a food supply for the next generation, extinction beckoned. Sloane’s urania was last reported in 1895 and is believed to have vanished entirely by the early years of the twentieth century.

Although six members of the Urania genus, similar in appearance and habits to Sloane’s urania, are still known to exist, this does not lessen the significance of the species’ loss. Indeed, the mere fact that Sloane’s urania is known to have gone extinct at all renders it hugely significant. Whereas it can be reasonably assumed that most of Earth’s mammal species are known to science, the same is much less true for other classes of animals, perhaps none more so than insects. Estimates of the total number of insect species stand at between six and ten million, of which around one million have been described.

The upshot of our continuing profound ignorance of so much invertebrate life is that no-one has much idea of how many species humans have driven to extinction. A 2005 study estimated that around 44,000 insect species had been driven extinct by human activity since the fifteenth century. More worrying still, in 2014 entomologists concluded that the total number of insects on Earth has dropped by about forty-five per cent since the 1970s. However, the number of documented insect extinctions in that time period stands at around seventy. Sloane’s urania is a rare insect indeed in that its passing managed to draw the attention of its nemesis.

Recent dramatic declines in bee numbers appear to have begun to alert humanity to the stupefying risk inherent in gambling with the future of invertebrate life on our planet. We would do well to remember that even the asteroid strike which ended the Dinosaur Age did not manage to cause a mass extinction amongst insects.

Matt Stanfield