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Update - 14 July 2009
A conclusion regarding the cause of this blue colouration has been posted on the blue birds index page.
(Click to enlarge)
In researching the blue sparrow and little corellas, I found a single reference to an Australian White Ibis (Threskiornis molucca) having blue feathers.
The full quote was made in May 2009 on a bird-related mailing list and is presented in full here:
"People might remember the blue Australian White Ibis seen last year, very similar to the blue colour of the sparrow. A couple of months ago the blue Ibis was at Freshwater Creek Wetlands at Chullora and it was in moult with the blue feathers being replaced by white feathers. I would suggest that there is a connection here with the blue sparrow. "
Following the recent articles here on blue birds, I was contacted by someone who in fact photographed a blue Australian White Ibis in Chullora, Sydney, in November 2008.
He writes that he saw a photograph of a blue Australian White Ibis published in Strathfield Scene in June 2008. He later sighted the bird at Freshwater Creek Wetlands, Chullora.
On 9 November 2008 he again sighted a blue ibis at the wetlands. Although it was possibly the same bird, its blue feathers were being replaced by white ones during its moult.
He took some photos, two of which are presented here.
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In a real breakthrough, he collected a blue feather which he is certain came from this bird.
Since the first reports of unusual blue birds came out I have learned much about blue colouration in bird plumage. There is no known bird species with naturally occuring blue pigmentation in its feathers. The appearance of blue colouration is caused by the reflection and diffraction of light due to physical properties of the structure of the feathers themselves.
Perhaps with a feather in hand we may now start getting some concrete answers on the cause for this unusual colouration - which has now been seen in 3 separate species over the past 12 months.
I am waiting on further advice from others with more bird knowledge than me.