Tuesday, 16 January 2018

The trapping of adult grey parrots is ruinous

Posted by Simon Valle & Stu

In a paper just published in Biological Conservation, Simon Valle et al. investigated how different capture methods and other aspects of parrot trade, other than just the actual volume of birds taken from the wild, can affect sustainability of harvest. This is of course important as the ‘currency’ which is generally used by CITES and government agencies to assess the impact of bird trade is usually solely the total number taken from the wild. This paper shows how crucial it is to also consider other facets of the trade – if adults or nestlings are taken, how may fluctuations in annual harvest affect sustainability, and what role may habitat loss or augmentation have in influencing population stability. 

The study was grounded in the building of a population viability model (PVA), a tool often used in conservation biology to see how populations might grow or decline in the future. These models use demographic parameters such as birth rates, mortality of juveniles and adults, longevity, and other life history traits to predict future population trajectories and to identify those aspects of the animals’ lives that are most crucial to safeguard. PVAs are data-hungry, and the lack of real data from wild populations means that, often, parameters have to be taken from other species, from captive birds, or are simply guessed. Our models used some data (e.g. on mortality) from other large parrot species, but Simon’s months of fieldwork on the grey parrot (Psittacus erithacus) population on Príncipe meant that we had a reasonable idea of most of the parameters needed for the running of the models.

Inquisitive subadult grey parrot kept as a pet on the island of Príncipe.This individual would have been originally caught as a chick from the nest, as is common  practice on the island .

Our first important finding was that there was a very fine line between a harvest being sustainable in the long term, and ruinous. A harvest of around 900 individuals per year allowed the population to thrive, but one of 1000 birds almost certainly resulted in extinction of grey parrots on the island. Hence, to keep trade sustainable, we would need to relying on a scientific balancing act, but this is the polar opposite of how grey parrot trade is currently managed – we have little or no confidence that quotas are strictly adhered to, mortality in trade is pitiful and unknown, and often we don’t even know from which country the parrots were even caught.     

Predicted population trajectories in response to no harvest (0% of the initial population); and annual harvests (SD) of 600 (100), i.e. 7.5% of the initial population, 900 (100), and 1200 (100) chicks, i.e. 15% of the initial population. Light grey lines =population trajectories resulting from each simulation; black solid lines = mean trajectory.

Even more disturbing is that the age of birds taken from the wild blows simple numbers of birds taken out of the water in terms of its importance. On Principe, there was a tradition of only taking chicks from nests, and leaving the adults to be. This is likely one of the keys to the species’ survival in large numbers on the island. The figure below shows how disastrous the taking of just a few adults can be. The taking of birds ‘randomly’ from the population, as might be done in places like Cameroon where birds are caught en mass at aggregations, is utterly ruinous.

Difference in predicted 50-year trends when the population is subject to a harvest of 900 ± 100 harvesting chicks only, nest raiding (i.e. one adult is collected with every two chicks) and indiscriminate trapping.

Another interesting finding of the study was that having a ‘steady’ (well-managed and well-policed) harvest of small numbers of birds every year is more likely to be sustainable in the long term than a harvest that is inconsistent across years. The message here is that booms and busts of parrot trapping are not a good thing.  

Predicted population trend and individual simulations in response to harvesting a fixed (left) or variable (right) quota of 900 (top) and of 1000 (bottom) chicks each year

But booms and busts, high and variable mortality and other real uncertainties are exactly the realities that have plagued the trade in grey parrots for the past decades. Effective trade management seems a million miles away from what is happening at present across the species’ range.

On Príncipe grey parrots are commonly raised together with other household pets.

The research on the Grey Parrots in Príncipe was funded by

and supported by

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