Posted by Simon Valle & Stuart Marsden
At roughly the halfway stage of his PhD, Simon is about to leave for a second field season studying Grey Parrots Psittacus erithacus on the island of Príncipe. At a time when most grey parrot populations appear to be struggling at best, the population on this small island in the Gulf of Guinea is large and thriving, and could well be increasing. In this post, we ask why is this so and what makes Príncipe different from the many places where the species has disappeared or has become so rare? We argue that the parrot’s abundance on the island has as much to do with geography and history as it has biology.
|Grey Parrots fly from feeding sites in the north of Principe to roost.|
Príncipe (136 sq km) along with the larger São Tomé sit around 250 km off the coast of Gabon in the Gulf of Guinea. Being isolated islands, they exhibit low species diversity as compared to adjacent mainland sites, but of course, a high proportion of species which are there are endemic (Jones & Tye 2006). São Tomé has around 20 endemic bird species and Príncipe has around ten.
Some of these birds are rare (the Critically Endangered São Tomé Grosbeak Neospiza concolor and São Tomé Fiscal Lanius newtoni are examples) but several are very common – these
include the two Speirops spp, and the
Príncipe Weaver Ploceus princeps. We might expect birds in general on these islands to be common.
The island’s isolation has also meant that nest predators and nest competitors
may be few-and-far between. Perhaps particularly important is the absence of
hornbills. To put this absence into perspective, Timneh Parrots Psittacus timneh in the Gola Forest park (see Liberia Grey Parrot workshop and Gola Forest) have potential competition for nest cavities from 8-9 species of hornbills, many of which occur at high densities.
|The São Tomé endemic Giant Sunbird.|
Príncipe is a rugged island. While the north of the island is relatively accessible, almost sheer peaks dominate the centre and south of the island. There are virtually no roads in the south. This is hugely important for persistence of primary forest generally and for protection of parrot nests in particular. It has been absolutely key to the parrot’s welfare on Príncipe that there are many nests that local trappers do not know about and/or cannot reach. These nests have good chances of producing young each year – this recruitment is so very important in helping parrots to withstand harvest pressure.
|Above: cocoa farm workers and Below: present day Sundy estate with regenerating forest.|
The Portuguese arrived in São Tomé and Príncipe around 1500. First, they, or rather the many slaves they brought to the islands, produced sugar for export. But it is the cultivation of coffee, oil palm, and especially cocoa that has done most to shape the island’s recent land use history. Large areas of land in the north of Príncipe were cleared and large plantations (roças) set up like the one at Sundy. By the beginning of the 20th century, São Tomé and Príncipe were the world’s largest producers of cocoa. These plantations, however, were broken up on Independence in 1975, and since then, forest is returning to the north of the island. There are large areas of secondary forest, rich in fruit eaten by the parrots and with much oil palm. Oil palm on Príncipe, as elsewhere in Africa, is an important food resource for grey parrots. So, the parrots feed in the food-rich north, and many nest in the primary forests of the south, an easy commute away. This use of different habitats for different parts of their life is something found in parrots elsewhere (Marsden & Pilgrim 2003).
|Principe's extensive secondary forests are good feeding grounds for parrots.|
Our preliminary density estimates for grey parrots on Príncipe are 74 ± 10 per sq km in the north and 34 ± 9 per sq km in the south of the island – giving an island-wide population estimate of some 6,500 individuals. These are the highest abundance estimates for grey parrots anywhere (Marsden et al. in prep). There is evidence that the population is increasing, possibly as a function of recent reduced capture for the pet trade. In the 1990s, perhaps up to 1,500 parrots were caught on the island each year, mainly by taking chicks from nests (Juste 1996). Illegal trade still occurs but at a low level. Whether taking chicks is a somewhat ‘better’ way to harvest wild parrots than catching adults is debatable although we would expect natural mortality rates of these nestlings to be higher than older birds. In fact, one of things that we will be researching this year is rates of mortality of first year birds as compared to ‘older’ individuals. Other strands of research include examination of nesting success, and testing methods for low-cost monitoring of the population.
Jones, P. and A. Tye. 2006. The birds of São Tomé & Príncipe, with Annobón, islands of the Gulf of Guinea: an annotated checklist. British Ornithologists' Union.
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