Monday, 23 December 2013

White-winged Choughs – expanding red eyes.

White-winged Choughs (Chough is pronounced chuff) are birds of dry open eucalypt forests and woodlands. Highly social birds with fascinating group behavior they spend much of the day fossicking in ground litter for food. At this time of year the young are out of their large mud bowl nests, following the adult birds of their extended family group about, begging incessantly to be fed.
The white patches on their wings are only visible when the wing is opened in flight or when displaying or grooming.
An adult White-winged Chough - note white wing panels in flight feathers.
Adult choughs have a red eyes, the iris is orange, while young birds have a brown iris.
Adult bird excavating hole in soil under ground litter to uncover insect larvae. Note the red eye.
This is a juvenile White-winged Chough note white eyebrow. The iris is brown however a small section of the eye ball visible in this photo shows the eye ball surrounding the iris is pink as in adult birds.
The choughs on our property are used to humans and are quite confiding, allowing close approach for observation and photos.
I have noticed in the past that the red eye of adults sometimes appears larger than normal and until now had not given it much thought or looked closely to see how this is achieved. Choughs are highly excitable birds sometimes responding to disturbances within or external to the group by spreading their wings, fanning their tail and also by enlarging their red eyes.
This is the adult bird feeding in photo above. It has moved on and found an insect and suddenly become excited by my presence. The red eyes have been enlarged, wings and body feathers spread and tail fanned. The display is meant to look threatening.
The following two enlarged shots show first a calm bird feeding with normal eye appearance and then an excited bird with an enlarged red eye.
Enlarged photo of adult bird from feeding shot above showing close up of head and the normal elliptical eyelid position of a calm bird.
A close up of the excited bird displaying in photo above. The wing, body and tail feathers have been contracted however the eye lids remain expanded showing an enlarged eye with an orange iris and pink surrounding eye ball. Its display has distracted it from eating the small beetle it caught before becoming alarmed by my presence. 
One field guide states that the outer eye ring flushes brighter red giving the impression that there is a colour change to the eye. The above photos show that rather than the eye changing colour, the bird achieves an enlarged red eye by uncovering the eye by expanding the surrounding eyelid. The eyelid is normally held in an elliptical shape and is opened out to a circular shape when the bird becomes excited. 
The field guides describe a red eye and from a distance the eye does appear to be red however on closer examination the iris is orange and the surrounding eye ball is pink.
If you have ever wondered about the Chough’s expanding red eye, now you know how it is achieved. For those readers who already knew this I hope you enjoy the photos.

1 comment:

  1. Another really interesting post. Hasn't digital photography changed some things about birding?