Testing counts from vantage points as a surrogate for density estimates to monitor populations of Grey Parrot (Psittacus erithacus)

Gruppo di Ricerca: 
Filippo Marolla, Simon Valle, Luigi Boitani, Stuart J. Marsden

In ornithology there is unanimous agreement on identifying Psittaciformes (parrots and cockatoos,
commonly “parrots”) as the most endangered avian taxonomic group in the world. Most of parrots
species inhabit tropical dense forest habitats and are primarily threatened by habitat loss due to
human activity. Moreover, parrots are common pet species and therefore they are object of a
multimillionaire mostly illegal trade business which takes tens of thousands of individuals from the
wild. Grey Parrot Psittacus erithacus is one of the most popular avian pets due to its longevity and
mimicry ability; it is currently spread all over central and western African countries, but most of the
populations are decreasing because of the huge and constant harvest of individuals for pet trade.
The decline is so clear that IUCN has recently listed the species as Vulnerable. Grey Parrot
conservation – as well as other avian species conservation – strongly needs reliable methods for
estimating threatened populations abundance; highly accurate population size estimates are crucial
information to implement effective conservation programmes. In this work we examined accuracy
of the vantage points counts method – a relatively easy and inexpensive method, frequently used for
assessing parrots abundance and density in tropical forest habitats, that had never been reliably
validated. We performed counts of Grey Parrots from several vantage points in the island of
Príncipe, a small tropical island in the Gulf of Guinea which hosts a large population of Grey
Parrot, showing the highest density across the entire range of the species. Results of vantage points
counts were statistically compared with local densities obtained by distance sampling line transects
– to date probably the most trustworthy and employed method for estimation of birds density since
it accounts for detection probability (detectability). We found out that the two methods provide
results which are significantly correlated: this suggests that the vantage points method is virtually
capable of providing a good index of local Grey Parrots abundance. However, the selected models
showed that parrots detectability is strongly influenced by the time of the day at which the count is
performed and by the size of the surveyed area. The effect of these variables has to be taken into
account by statistically modelling it or by improving the sampling design, in order to avoid biased
counts. Moreover, it is possible that other environmental sources of variability affect parrots
detectability, as the models do not explain the whole variability of the data, and therefore they are
not capable to provide an accurate regression coefficient between counts and estimated densities.
Anyway, the small sample size urges caution on this hypothesis. Overall, our results foster to spend
energy and time for refining the vantage points counts method, in order to provide an easy and
reliable tool for monitoring Grey Parrot populations in Príncipe and elsewhere in the species range.

Anno del Convegno: 

© Università degli Studi di Roma "La Sapienza" - Piazzale Aldo Moro 5, 00185 Roma