In England, if you put up a bird box with a small entrance hole, the most likely species to make use of it is the blue tit.
So how do you tell male from female? Chances are you won't be able to.
But blue tits don't seem to have a problem picking a mate of the opposite sex. How do they do that?
Its that time of year once again when thoughts turn to... (no, not Christmas!) ...next years nesting season!
We have maybe 4 months before bird boxes need to be taken into the garden and fixed to trees, pergolas or the side of a garden shed. So any new ideas for BirdBox 2016 need to be thought about and developed over the coming weeks.
vive la différence!
Some time ago I read that blue tits can tell the difference between male and female by the variation in colour. Our eyesight differs from these birds in that we can't see the range of colour that they can see.
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| Boy or Girl? (...OK its a girl because she is nest building) |
To us humans, the blue crest of a blue tit looks the same for male & female birds. But the reflectance of the crest is highest for lower wavelength (higher frequency) "light".
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| Crest reflectance (upper line: male, lower: female) |
The peak reflectance wavelength in this case is typically around 350nm. Our visual response to light tends to drop off quickly below about 400nm, so we don't really notice the difference.
An idea is born
If we could view the blue tit's crest under UV light, maybe we could tell male from female. So my plan is to fit a few cheap UV LEDs inside the bird box to provide suitable illumination. I then have to hope that the Raspberry Pi camera has a reasonable response to UV light.
The LEDs I will be using are Sodial 10mm type, and are currently on a slow boat from China. The have a dominant wavelength of 400nm, but I'm hoping this will extend down to 350nm and not too far above 400nm.
These will be added in addition to the three visible light LEDs and controlled from the channel I was using for dim light operation (i.e. one output will turn on the regular LED set, the second output will operate the UV LED set). Please refer to the original circuit here.
By switching on just the UV lights early in the morning (before daylight enters the box) I hope to grab some video of both the male and female in the bird box.
What are the chances
This is such a simple (and cheap) idea that I suspect it wont work. Because if the idea is "sound" then I would have thought someone would have already done it.
The first limitation is probably the LED. Ideally I need to find a type with a 350nm dominant wavelength that produces little or no visible light. The type on order produce visible blue light at a longer wavelength, and so may mask the differences between male & female crest.
Add to this the response of the Raspberry Pi camera, which is clearly designed to cover the range that we humans can see. However, it does at least extend a little beyond this, into the infrared and ultraviolet spectrum, although at reduced sensitivity.
The issue here is that the image brightness is set based upon the peak image reflectance. So any visible light will dominate the picture.
And would there be any advantage in using camera without iR filter? It seems unlikely that the iR filter would have any effect on UV, as iR & UV are at opposite ends of the visible light spectrum.
It is clearly going to be very difficult to do any bench testing on this system with regards to useful colour/frequency response, unless I chance upon a dead blue tit in good condition!
However, it will only cost me about 60pence for 6 UV LEDs (3 each for my 2 bird boxes) and a bit of time to fit the LEDs, and modify the wiring and software, ....so worth a go!
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