Saturday, September 28, 2013

Some comments on the changes in brightness of certain colors of the tail feathers of the Puerto Rican parrot as you look at them from different perspectives.

top view

Reverse view

Side angle view

front angle view

Of all the species in the psittacine genus Amazona, the puertorican parrot, Amazona vittata is the least colorful.¹  Aside from a limited amount of red in the forehead, and the white skin patches around the eye, the rest of the body is covered with green feathers.  On a perching PR parrot there is little color besides green to be seen, the turquoise and black of the wings, the blue, red and yellow areas on the tail are all kept well hidden.  The reason for such lack of color is probably the need to be as cryptic as possible to avoid attracting the attention of predators.

The PR parrot brightest colors are in the forehead and in the tail feathers.   The red band in the forehead varies widely in width, length and shape.  Generally males have a wider red band than females, but not always.  Perhaps the best example of the variability of the forehead color in the species is the pair of PR parrots currently on exhibition on the Juan A. Rivero Zoo at Mayaguez. The male has a single tiny red feather in his forehead, the female has a red band wider and larger than almost all females, so one on first sight could easily be fooled into thinking she is the male and vise versa.

Very few people are familiar with the color in the tail feathers of the PR parrots.  The reason is that because they can only be sighted when the parrots fan their tails, a normally brief occurrence, they are essentially invisible to the casual observer.     But even if an observer happened to be at the right place and time to see a bird with its tail fanned, from a distance the colors are barely visible and unremarkable when compared even with even modestly colorful species, such as the hispaniolan amazon (Amazona ventralis).

In the aviary the birds interact in many ways during the day.  Some of these interactions involve body language, on occasions this includes tail fanning which is normally accompanied by vocalizations and sometimes bowing and wing cupping.   The birds can do these displays as parts of being aggressive and also for other reasons which are not necessarily so easy to discern.   Except when we are selecting breeding pairs from the flock, or are concerned about a bird being bullied, we don’t pay much attention to these displays. 

About a decade ago, when I was standing next to her breeding cage, a female known as Ann, did a bowing and tail fanning display standing in front of me on the wire floor of the cage,  very close to my face, this is, to this day, an uncommon event (by the way, Ann and her mate, Pepe, used to be among the tamest and sweetest birds in the aviary, but as they grew older they became more and more intolerant of people around their breeding cage, now as they are getting close to the two decade mark in their lives, when breeding, they are hideously aggressive and among the most fierce of our pairs, but I digress).

When this event happened, it was morning and sunlight was coming from my back and hitting the parrot head on.  When Ann did her display, for a brief moment, her green color became brighter the way a mirror becomes brighter when the sun hits it at just the right angle.  I was intrigued by this but given that I had many other concerns at the time, filed it in the back of my mind for future reference.  Early this year I found some a shed tail feather and decided to photograph it to record how light reflected from it.

The feathers that are the last ones at the sides of the tail are different from the other tail feather and any other feather.  They are asymmetrical and one of their sides is blue.  I found that the blue of these feathers changes in brightness and you look at it from different perspectives.  If you look at the feather from an angle of 90 degrees from top looking down at the top side, the color is not particularly bright.  But as one changes the perspective, the blue becomes brighter and brighter as one approaches the horizontal plane.  If one looks at the feather in the orientation that it would have if a parrot was displaying standing in front of you the blue becomes almost mirror-like in its brightness.  You can see how the blue changes in the photos.  

What is the meaning of this change of color?  The birds have a different color perception system than us, with four receptor cell types.   Personally, I don’t know how the eyes the parrot’s perceives the changes in color that my mammal brain reports to me.   My own guess would be that if the feathers undergo changes in color and brightness as a result of the birds fanning and bowing, it may be that those changes play a role in courting displays, alternatively it also could be important in aggressive interactions.   It has been shown that budgerigar females favor males whose face feathers display fluorescence.²   For the moment, from a strictly scientific point of view, I can’t say with any certainty what the brightness changes means for the parrots.   For all I know it could be an artifact of perspective or of my own perception.   Nevertheless I find it an intriguing phenomenon. I don’t foresee the program exploring this matter as we have worked hard to keep our birds from getting imprinted with humans and breeding pairs are not fond of either people or photographic cameras.

¹Low, R. 1984. Endangered parrots

²Proc Biol Sci. 2001 Nov 7;268(1482):2273-9.
Ultraviolet vision, fluorescence and mate choice in a parrot, the budgerigar Melopsittacus undulatus.

No comments: