Monday, 2 November 2015

Counting Grey Parrots made easier

Posted by Simon Valle

A new paper to be published in an upcoming issue of Oryx, and now available on their website, looks at encounter rates as a possible simple but effective way of understanding the magnitude of parrot populations without having to employ more time- and resources-consuming methods like distant sampling. The study is the result of a wide collaboration involvng African BirdLife partners surveying grey parrots (Grey Parrot Psittacus erithacus and Timneh Parrot Psittacus timneh) across its vast distribution range in Central and West Africa. The project was funded by CITES through BirdLife International.

Estimating population size and trends in abundance is an essential starting point for the conservation and management of any animal species. Nonetheless, this may often be easier said than done. Species can have huge distributions, be secretive, highly mobile, live in remote areas, or, like parrots, all of the above. Psittaciformes are among the most difficult bird species to survey, and as a result only 25% of species have a reliable population estimate (Marsden and Royle 2015). 

Grey Parrots (Psittacus erithacus), as most Psittaciformes, are canopy dwelling birds often difficult to detect in tropical forest. (Photo: F. Marolla)

Distance sampling is the most used and probably most accurate method for estimating population densities and size of most parrot populations, but the method requires a considerable amount of time, technical skills and economic resources - that often are simply not available. Hence, our goal of finding rapid, simple and economic methods for that can be used as an indication of population sizes for conservation or management purposes, when more precise methods cannot be implemented. 

The ranges of grey Psittacus erithacus and Timneh parrots Psittacus timneh in West and Central Africa.
We carried out independent distance sampling and simple encounter rate surveys for grey parrots at ten sites in five countries (Cameroon, Côte D’Ivoire, DR Congo, Liberia and São Tomé & Príncipe). Whilst the distance sampling surveys strictly followed the rules of the method and accounted only for perched birds (Buckland et al. 2008), in the encounter rate method recorders could spend variable amounts of time walking or standing and records of flying parrots were included in calculations. We found that densities were extremely variable across the range ranging from 0-0.5 individuals per sq km in Côte d’Ivoire and the Central Democratic Republic of the Congo to c. 30 per sq km  in Cameroon and >70 per sq km on the island of Príncipe. Most importantly, we found a significant correlation between the densities estimated with distance sampling and the number of groups encountered per hour.

The relationship between estimates of grey parrot density (± SE) and encounter rates (± SE), with 95 % prediction region (shaded).

This is important link between anecdotal information and actual densities, which can have useful applications for grey parrot monitoring in Africa. Data can be collected by existing monitoring schemes or by anti-poaching patrols without any further investment of time or money, and this can be converted into actual, albeit imprecise, parrot densities. We propose a hierarchical approach to surveying grey parrots, where results from easy and cheap encounter rate surveys may give a first indication as to whether the population is large enough to justify further and more costly distance sampling surveys. Where the latter is recommended for a precise estimate, the former can still be a valid method to monitor effectively the population on a regular basis.

Navigating through tropical forests is an arduous task, which park rangers (here rangers in Gola Forest Transboundary N. P., Liberia) regularly undertake on their patrols. These policing trips could be good opportunities to gather information on grey parrot densities. (Photo: S.Valle)


Buckland, ST, Marsden, SJ & Green, RE (2008) Estimating bird abundance: making methods work. Bird Conservation International 18: S91-S108.

Marsden, SJ & Royle, K (2015) Abundance and abundance change in the world's parrots. Ibis 157: 219-229 DOI: 10.1111/ibi.12236.


Data from Liberia, Côte d’Ivoire, Cameroon and DRC were collected during the BirdLife International African Grey Parrot Pilot Project funded by the Secretariat of CITES.

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The work on the island of Príncipe was funded by Parrots International and HBD kindly provided logistical support. Nat Annorbah's work is funded by Loro Parque Fundacion.

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