UV reflectance as a form of photography has been around for some time. However owing to the expense and technical challenges involved it has been primarily the domain of specialist photographers and researchers, until now. I came up with a simple and cost-effective solution which I researched, experimented with and promoted through the blog over a series of postings last summer. My postings on UV reflectance (eg. HERE) have been among the most popular on this blog to date. Last summer I failed to get any images of Blue Tit (Cyanistes caeruleus) so I decided to wait until this spring, when breeding plumage would be at it's best, to revisit this species.
There has been quite a lot of UV reflectance work carried out with Blue Tits over the years. Spectrophotometry is invariably the method used for scientific research, requiring capture of birds and careful measurement of UV reflectance from the blue crown feathers using a spectrophotometer (for an example of this in action see HERE). In one study (HERE) Anderson et al found that male Blue Tits have on average a higher UV reflectance than females, indicating sexual dichromatism in the UV spectrum. So, while humans have great difficulty visually separating the sexes, it is thought that Blue Tits can easily tell the difference as they can see in UV. The males would appear to have a brighter cap than the females. Of course, just how UV light appears to birds we can only speculate.
Whatever about being able to tell the sexes apart based on UV reflectance I was curious to see just how bright and reflective the crown would appear using my UV camera setup. The peak UV reflectance per Anderson et al lies at around 370nm, which should be well within the range of my camera. During earlier testing of my rig (HERE) I was able to obtain a bright UV image from a Peedar UV lamp, which the manufacturer states has a peak brightness at wavelength 365nm, roughly in line with the peak UV reflectance for the crown feathers of the Blue Tit.
This morning I obtained the images shown in this posting. The colour images were taken with my Canon 70D and 300mm lens, while the UV reflectance images were taken with my Sony DCR-TRV270E, modified only by a Baader-U filter in front of the lens as depicted HERE.
I am quite certain that I filmed both males and females as I encountered a number of pairs this morning. The results kind of speak for themselves. Once again I have found UV reflectance in birds to be extremely subtle. I had hoped that UV reflectance in this species would be a bit more dramatic, but sadly not so. I wouldn't rule out the possibility that if I had male and female Blue Tits together in the hand it might be possible to detect a subtle difference in UV reflectance, but it is certainly not obvious from the images I obtained in the field.
The evidence from studying flower nectar guides and butterflies (most notably the Common Blue Polyommatus icarus) has shown that this method works and produces some really intriguing results. But so far I have to say the results on the avian front have not been very exciting. I have no doubt that there is a lot to discover about UV in birds, as research using the far more exacting spectrophotometric method has already revealed. But UV reflectance photography may not be about to offer the kind of insights I had been hoping for.
Marsh Marigold (Calta palustris) demonstrates a striking nectar guide. Each petal has a UV reflective tip and a UV absorbing centre. The centre of the flower is also UV absorbing.
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