The afternoon sun was low in a blue sky with just a few wisps of white cloud. Sunbeams slanted through the trees and bushes, turning the tops of the reeds, heavy with seeds, golden yellow. It was the last weekend in September and with the final early start at our reed bed site the following morning we made a spur of the moment decision to camp out on site.
Soon the sun disappeared behind the trees leaving a trail of pinks and gold in the wispy clouds above. Darkness descended and stars appeared in the clear sky, a twinkling canvas stretching across the heavens. From the blackness of the woods behind us comes the call of a tawny owl and with the evening stretching out before us it was too good an opportunity to miss. With the little one sleeping, tucked up in layers of clothes and blankets, it is Lee who disappears off into the darkness to attempt a catch leaving me to watch over the sleeping toddler. With the final wisps of smoke from the BBQ drifting up into the silky black from the mass of stars I pick out familiar constellations: The Great Bear, Cassiopeia, Cygnus, Pegasus, Draco. Lying low, the planet Mars glows an eerie red. The flashing light of a plane crosses the sky from right to left.
In no time at all there is the bob of a head torch returning through the darkness. He strides up to me, confident, happy, an old pillow case held in one hand. Success. He passes me the bag and from the other hand reveals another owl! Double success!
Over the past couple of years we have caught a number of owls, not least through our project looking at tawny owls in Thetford Forest. Each time we have processed the birds, taken a number of different measurements and aged them based on the quality and patterning of its flight feathers.
During the time we have also have learned of a new technique.
Did you know that Ultra Violet light causes porphyrin pigments in the feathers of some birds to fluoresce? This means that when you shine a UV light on the underside of the wing the newer feathers will glow bright pink! Over time this organic compound degrades and the intensity of the colour fades. This means that older feathers in the wing appear greyer. So by shining a UV light on the wing one can easily see different aged feathers. Unlike smaller songbirds many owls do not replace all their flight feathers in one go, but will moult just a couple each year. Depending which feathers, and how many have been replaced can then help with the ageing process.
Aside from that it’s just really cool!
The technique is used a lot in North America, particularly with the ageing of Saw-whet Owls where there is little difference in patterning between adult and juvenile feathers and so using the UV light is the main method of ageing.
For us, it is most effective at this time of year when the adult owls are just completing their moult and the intensity of the fluorescence is at its strongest. When we have tried the technique later in the winter (as in into the New Year) the effect was not as striking as the porphyrin had faded.
This time though the effect was awesome! Both birds were adults having just completed their moult and under the UV light both wings shone brilliant pink! Not only that some of the flight feathers were pink and others were more washed out, clearly showing the different aged feathers (although it proved a little more tricky to show this in a photograph!).
And so with both birds ringed, aged and measured it is time for them to return to the dark wood. We turn our torches off, and wait, letting the birds eyes adjust back to night vision. Then with a flap of silent wings they are off, a brief shadow against the night sky before their black silhouettes disappear into the blackness of the wood.