Eyeing up the age structure of grey parrot populations

Posted by Filippo Marolla and Simon Valle

Filippo Marolla is a biology student from the Sapienza University of Rome. He's working with Simon to test some new methods for estimating densities and age ratio of Grey Parrots in Príncipe.

Among the several objectives we have set for our second fieldwork season in Príncipe, is a test of a non-invasive method for the estimation of the age ratio of the population of Grey Parrots. Determining the proportions of age classes in an animal population is crucial to the understanding of its dynamics and viability. However, this operation can be complicated for species which are hard to age in the field. To date, the only reliable method available to age Grey Parrots is the colour of the iris (Dändliker 1992). 

Downy chick with completely black iris (photo Filippo Marolla)

Captive fully fledged 'grey-eyed' juvenile grey parrot
 on Príncipe (Photo: Filippo Marolla)

When the chicks first open their eyes (about 7-10 days after hatching), the iris is black. During the first year, it slowly turns paler, passing through different shades of grey, and after the first year, it gradually turns bright yellow. Dändliker (1992) reckons that individuals can be aged up to four years of age, but, due to the variability among individuals, the only reliable separation is between first-year birds and others. Using this aging criterion can be quite easy for birds in captivity, but it can be a real nightmare in the wild, as parrots are either very difficult to approach when perched, or they typically fly high and fast in closed canopy forest.









Our objective is to test if photographs especially taken from strategic lookouts can be used as a method of estimating juvenile/non-juvenile ratio in Príncipe. We will take high-definition pictures of as many parrot individuals as we can, whether they are flying or foraging, using a Canon Eos 700D camera with a Zoom Lens EF 100-400mm (f/4.5-5.6). We are currently carrying out a training period in order to identify logistic problems and to improve our skills in taking good photos as quickly as needed. So far results are encouraging and, despite many ups and downs, the number of usable photos (i.e. where the iris colour is clearly recognizable) increases from session to session.

Three parrots flying over São Joaquim are readily identifiable as adults 

(Photo: Filippo Marolla)

Parrots often use preferential flyways to move daily in small flocks from foraging areas to roosting sites. We are focusing our efforts on some open areas located along these flight paths. Oque Daniel and El Miradouro are two elevated lookout points, located respectively in the North-West and in the East side of Príncipe, from where it's easy to spot parrots flying over, often right above your head. Other observation points, e.g. the surroundings of São Joaquim, are located in feeding sites such as flat areas with high densities of oil palms Elaeis guineensis, which give us the chance of taking pictures of perched birds. A typical working session consists in two hours of photo-catching during the time of the day in which parrots are most active, i.e. between 6 and 8 AM or between 4 PM and the sunset at 6 PM. 

View from El Miradouro, one of the strategic lookouts from which photos of flying grey parrots are taken in Príncipe (Photo: Simon Valle)

After some weeks of work, we are now able to provide some figures about how things are going.  In the best session in El Miradouro, we photo-catched 71 flying individual parrots on about 150 sighted, among which 25 have clearly recognizable iris colour, i.e. an aging success rate of 17.5%.

A grey parrot flying over shows the bright yellow eye typical of an adult
 (Photo: Filippo Marolla)

In the best session in São Joaquim, we photo-caught 22 perched individual parrots on about 43 sighted, with 10 individuals with clearly recognizable iris colour, i.e. an ageing success rate of 23%. Considering the practical difficulties associated with the method, we can be satisfied with the ageing rate we reached so far. 

 A perched adult feasting on the pollen from on Erythrina variegata  flowers 

 (Photo: Filippo Marolla) 

The first results are encouraging, suggesting that this kind of ‘photo-trapping’ might be useful for the study of grey parrot populations elsewhere in their vast range. Our next step is to see if we can detect changes in population structure across months using repeat photographic surveys. We expect the juvenile/non juvenile ratio to be at its maximum in March-April – when juveniles have just fledged – and at its minimum in August-September – when juvenile mortality has taken its toll. 

Two adults flying in a oil palm plantation in São Joaquim (Photo: Filippo Marolla)

This distant bird flying away is still readily identifiable as an adult 
(Photo: Stuart Marsden)

References

Dändliker, G. (1992). The Grey Parrot in Ghana: A population survey, a contribution to the biology of the species, a study of its commercial exploitation and management recommendations. A report on CITES Project S-30. Unpublished report to CITES.


The research on the Grey Parrots in Príncipe is funded by 



and supported by