Population viability and conservation of grey parrots psittacus erithacus on the island of Príncipe, Gulf of Guinea

Valle, Simon (2015) Population viability and conservation of grey parrots psittacus erithacus on the island of Príncipe, Gulf of Guinea. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Parrots are among the most endangered birds in the world, owing to the international pet trade, and habitat degradation. Grey Parrot Psittacus erithacus, the most traded wild-caught species, has suffered a steep decline across its vast African range. This PhD investigates Grey Parrot’s ecology to understand the mechanisms by which harvest and habitat loss affect populations. The island of Príncipe was chosen as a study area owing to its healthy Grey Parrot population, its tradition of parrot harvest and the disturbed-to-pristine range of habitats. Parrot densities were exceptionally high (mean±SE: 53±3 parrots km-1), and their distribution within the island was shown to change significantly between pre- and post-breeding season. The presence of nest tree species was the best predictor of densities in the former (F=2.99, p=0.07), while slope and food tree species were in the latter (F=3.0, p<0.05 and F=8.04, p<0.01). Variation in habitat use across seasons highlights the importance of opportunely timed surveys, and the need of preserving a matrix of habitats. Three simple and inexpensive methods were tested, which may be useful where more precise methods cannot be used. Simple encounter rates were shown to be a workable proxy for actual densities: a relationship was found between the two (R2=0.8, df=9, p=0.01). Long watches proved to be far less accurate and suitable only in limited cases (i.e. enough vantage points overlooking small areas and numerous encounters). Counts along flyways were suggested to be a good monitoring method, although further testing is required. Nest densities were found to be very high (mean±SE: 16.8±7.9 and 72.4±26.2 nests km-2 in secondary and primary forest respectively) compared to those elsewhere. The habitat characteristics measured didn’t affect productivity, suggesting that cavity characteristics may be more important. The best demographic data available on the species, were used to build a Population Viability Model to investigate its population dynamics in the face of harvest and habitat loss. In Príncipe, Grey Parrots were shown to have increased rapidly after a trade ban was put in place, highlighting the recovery potential of the species. Harvest alone was shown to be harmless only when small quotas (e.g. 600 chicks year-1) are yielded from large populations, while habitat loss lowered the predicted maximum population size. The interaction of these threats can be much more unpredictable. The effects of harvest are worsened if poorly regulated or techniques that include the harvesting of adults are used. Extinctions were predicted to occur with significant delay (≥40 years), and the conservation implications were discussed. Finally, the contribution of this thesis to the understanding of Grey Parrots ecology and conservation is highlighted and research priorities identified.