Saint Lucia

How the Island Got Its Birds 
Now in the hibernal depths, I cannot help but contemplate distant lands under a tropical sun. Last summer I was lucky enough to go to such a place, the Caribbean island of Saint Lucia. This rocky island thrusts from the sea, a volcanic testament to the grappling tectonic plates below. It is, of course, not alone but one of dozens of volcanic islands that form the Lesser Islands, set at the eastern end of the Caribbean. For me, islands in general are great holiday destinations because they are discrete units and thus ‘doable’ in the allotted time. Add to this tropical rainforests, mountains and coral reefs and you have a potent mix of exploratory fun. Yet as a birder, one could be disappointed. If long bird lists are your game then islands are not the best destinations as their complement of birds tend to be small. Saint Lucia has some 70 regular species compared to the hundreds in the forests of Central and Southern America. But why might this be?

Les Pitons

How natural communities assemble on islands has been debated for decades, ever since Robert MacArthur and E. O. Wilson explored island biogeography in the ‘60s. Early theories of island biogeography were based on the equilibrium balance between immigration and extinction on islands.  These islands were imagined to be blank slates to begin with, new pieces of rock, rising from the ocean and providing new opportunities for life. The inital limitation on island species richness is individuals getting there in the first place. The chance of immigration will depend on the degree of isolation from other land (particularly the mainland). Islands close to shore will accrue species much faster than more distant islands. The second limiting factor is the size of island. It is a general rule in ecology that the greater the area, the more species. Larger islands can have larger areas of habitat allowing larger populations and minimising extinction risk. Larger habitats also allow larger species to exist which require more sizeable territories. Larger islands also are more likely to have a variety of habitats. Complexity of shape, topography and geology can all increase with an island’s size and all of these factors favour habitat heterogeneity. The more complex the habitat, the greater the number of niches, the more species can coexist.

Purple-throated carib

Already we have multiple ideas that could shape the number of species on Saint Lucia and other islands. One thing Classic Island Biogeography does not account for, however, is the creation of new species. There are two possible routes by which islands can create new species. The first involves the immigration of a species which then evolves on the isolated island. This process of anagenesis is responsible for many species that are only found on (endemic to) one or a few islands. Saint Lucia has five endemic bird species: a parrot (Amazona versicolor), a warbler (Leiothlypis luciae), an oriole (Icterus laudabilis), a black finch (Melanospiza richardsoni) and a pewee (Contopus latirostris), all of which I managed to see. Some of these species have sister endemics on other islands and they represent a historical process of immigration and speciation as a lineage has spread throughout the archipelago. Taking Amazona parrots as an example, there is an endemic species on Hispaniola, Puerto Rico, Saint Lucia and Saint Vincent as well as two each on Jamaica and Dominica. There are many lineages like this, related species strung out over island chains and I remember encountering them during my Tring work. Generally these species would have similar niches wherever they were. Thus every island would have its own parakeet, its own fruit dove, its own white-eye or its own flightless rail. These lineages are radiations of geographic opportunity, not specialisation.

Saint Lucia warbler

This brings me to the other form of island speciation: cladogenesis. On islands of sufficient size or habitat differentiation, species can form from the reproductive isolation of different populations. This could happen under the classic model of allopatric speciation where species on different mountains, rivers or habitats speciate. On very large islands such as New Guinea, this form of speciation is not very different from that described about as species could share niches but remain geographically separated. On smaller islands, however, where coexistence is necessary, species must be sufficiently different in order to coexist. Cladogenesis is often unlikely when the immigration of new species is much faster than the evolution of new species in situ. Thus it is only the most isolated islands with the greatest ecological opportunity which produce within-island radiations. Hawai’i comes immediately to mind. The Hawaiian honeycreepers are an unusual lineage descended from South American finches which have diversified in form and function. Some species have short, thick bills and specialise in crushing seeds, others have longer, thinner bills, equally adapt at seizing insects and probing flowers for nectar. The most extreme bills are very long and curved, indicating high levels of specialisation. These birds represent a true adaptive radiation as different species have evolved to exploit different niches in lieu of the lack of competition. This would not happen of the islands were regularly flooded with hummingbirds, finches and warblers which fill the niches on the mainland. Alas, many species of honeycreeper are now extinct.

Mangrove cuckoo

The shadow or extinction is especially dark over the islands of the world. In blissful isolation, unique island fauna have evolved, becoming specialised to their insular ecosystems (often flightless and defenseless). They are not adept at dealing with sudden change. As humans have spread over the world they have caused the extinction of countless organisms. I have spoken in the past of hunters eliminating New Zealand moas yet more destructive has been the suite of introduced species which have had cataclysmic effects on island wildlife. Rats and pigs eat eggs, cats catch birds and mammals, goats strip vegetation and extinction ensues. We have lost many bird species on islands, from the charismatic dodo to numerous elusive and flightless rails. Now the largest threat of all, climate change, threatens what remains as the narrow tolerances of island species offer little leeway when temperatures change and climate events increase in frequency and severity.

Lesser Antillean bullfinch

So given this melee of theories and ideas, what can be made of Saint Lucia’s birds? The island’s habitats are broadly divided into a humid west side and a dry eastern side. Scrub-favouring birds scurry through the eastern brush and cling to the coastline of the west whilst in the montane areas, the humid rainforest produces a more three-dimensional habitat for a higher diversity of species. It is clear that representatives of many species have reached the island and yet there are curious gaps as well, such as the lack of owls, woodpeckers and tanagers, and these could simply reflect historical accident as they are often on adjacent islands. The Lesser Antilles are a chain of relatively close islands (I could see Saint Vincent off the southern edge of Saint Lucia) which means that many species will be able to pass along the archipelago uninhibited and there are species such as seabirds common to many islands of the West Indies and even the mainland.

Grey trembler

The few endemics necessarily represent sedentary species with low dispersal ability, species that got there by accident and didn’t move once they arrived. There is very little opportunity for cladogenesis on such an island where competitors can appear more easily from elsewhere so sister species from different islands tend to differ only in plumage. Finally anthropogenic extinction has probably played a small part in shaping Saint Lucia’s avifauna. The endemic Semper’s warbler (Leucospeza semperi) is likely extinct with the last reliable sighting in 1961. It probably suffered at the paws of the introduced Indian mongoose yet suitable habitat still exists and there remains the possibility of its persistence. Invasive species and loss of rainforest are, and will continue to be, an issue on Saint Lucia but thankfully I am unaware of the loss of any other bird species at present and the once endangered Saint Lucia parrot is on the increase.

Thus Saint Lucia may not have the longest bird list in the world but the island’s species tell an interesting tale of colonisation and survival and they include some beautiful and charismatic endemics.

Saint Lucia parrot