Wednesday, 23 May 2012

Living below ground

When you think of blue tits and great tits you tend to think of familiar garden visitors, species attracted to bird tables and wooden nest boxes placed on the trunk of some garden shrub or small tree. Of course, many more of these birds make a living in the wider countryside, feeding on defoliating caterpillars in woodland and nesting in natural cavities.

Within woodland, natural cavities occur in many shapes and sizes, and only some of these will prove suitable for hole-nesting birds like the tits. There can be competition for natural cavities and it is perhaps unsurprising to discover that great tits will oust blue tits from suitable cavities and that, in turn, blue tits will oust coal tits, such is the dominance hierarchy that exists. The size of the entrance hole and the location of the cavity influence the degree of competition, largely because of the size differences between the three species. Being the smallest of the three, the coal tit can use cavities with a small entrance hole, cavities into which the other two species cannot squeeze. In particular, coal tits will use those cavities that occur low down, often no more than a narrow slit opening up into something a little larger within. Great tits require the largest entrance hole and blue tits fall in the middle.

The other day we were out in the forest, leading a group of volunteers learning the art of nest recording, when we came across two good examples of natural cavities used by nesting tits. The first was a coal tit nest, placed deep into a ‘snag’ line, the row of conifer stumps that is pulled together when a block of commercial plantation is harvested. The parents were busy feeding the chicks, taking in food and carry off faecal sacks. These sacks keep the nest clean and remove tell-tale droppings that might give the nest location away to a predator.

The great tit nest was more unusual (at least for a great tit), the birds having found a cavity in an old tree stump that disappeared down below ground level. We only stumbled across the nest when the sitting female was flushed by our approach – great tits usually sit fairly tight when on the nest. In her absence we were able to use a boroscope – essentially a tiny camera on a long and flexible cable of the type used by plumbers – to peer into the darkness. There, on the small video screen, was the image of six eggs in a neat nest cup. This nest would be fairly secure from most predators, though it might be at risk from a weasel or from flooding but with luck it should survive.

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