Winter Checks

It had been a foul night as Storm Deidre swept into the UK. For many it brought snow and very strong winds. For us in Norfolk it was a night of wind and rain. The morning however dawned calm and bright, with a beautiful blue sky. After the nights battering the forest was soggy. Droplets of water hung from bare branches and dark green pine needles. Little globules that sparkled in the golden sunlight filtering through the canopy. They hung like miniature Christmas decorations before falling to the brownish red carpet of leaves, twigs and bracken of the forest floor below.

Sunlight filtering through the forest

Our cars squelched through mud and splashed through puddles on the gravel tracks. The sky above was bright and blue. On either side of the track, the sun played hide and seek behind the dark branches and trunks. Consulting our map we gain our bearings and then continue on, searching for particular groups of trees. Not the blocks of commercial pine, but permanent stands of pine and deciduous trees that will not be cut down on rotation. At strategic points within these blocks we chose a tree and using a ladder climbed up and attached a large nest box. Five times we repeated this process and thus we had five new owl nesting boxes to go with the 31 boxes already up around the forest.

Mike finishing up the installation of another owl box

With the new boxes in place we continued our rounds to clean out the final few boxes that had been up a year or so. For any nest box put up for birds, regardless of size or species, it is recommended that they are cleaned out during the winter. This gets rid of old nesting material, parasites, food, eggs, poop etc… For our tawny owl boxes it means we remove the debris of old nests, especially where stock doves have nested as they have a tendency to fill boxes with material, adding nests on top of nests throughout the breeding season. There is however a time limit. Nest boxes can only be cleared out between 1st September and 31st January. After this it is illegal to do so.

As with many bird boxes our owl boxes provide not only nesting space but roosting habitat. So when clearing and checking the boxes we make sure we are ready to catch any birds that might be present. This data then also contributes to our project on understand the breeding success and movement of individual owls with the forest.

Today is a lucky day, as two of the boxes have owls in them. The first is a small, young male. His open wing clearly shows the feathers that have been replaced this year. The broken barring at the end of the primary feathers indicates the owl was born last year. The second bird is an adult male and he is already ringed. Again his wing clearly indicates his age, with the complete broad bars and the tip of the wing feather. Looking at his capture history shows he is faithful to this particular box and surrounds, having been caught within the box and its vicinity over the last year. All to be expected in a species that is highly sedentary.

Grey tawny owl

What is particularly striking about these two birds is their colour. They are both noticeably greyer and paler than the warm rufous brown of the other tawny owls we have recently caught in both Norfolk and Wales. There are in fact two (or three – depending on which authority you follow) colour morphs of tawny owls, one with more brown upperparts and the other more grey upperparts (the third type is a rufous colour morph). Brown forms tend to be more common in the warmer, humid climate of western Europe with the grey phase more common further east. In northern most areas owls are a cold-grey colour. Evidence indicates this change is clinal ‘a gradual change in an inherited characteristic across the geographic range of a species usually correlated with an environmental transition’. It seems likely that in areas with more snow a greyer/paler plumage provides better camouflage, while browner birds may do better in areas of denser woodland.

While different colour morphs predominate in particular regions of the owls range, it is not absolute and within areas there are mixes of all three colour morphs. As is the case in Britain where while the rufous or brown morph does prevail in areas, in others lighter morphs dominate. It also appears that in some areas, like Thetford Forest it seems, both grey and brown forms exist together.

And so with the mist starting to gather in the tracks and rides between the trees, we clear out the last box and head for home. The next time we check these boxes will be in the Spring, and hopefully we will have some more tawny owl nests to monitor.

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