Monday, 21 May 2012

A solid lining

I can see the nest ahead of me, wedged between the flat panel of a fence and a horizontal limb of blackthorn. The nest itself is a melon-sized construction of moss, grass and dead leaves, the work of a blackbird or song thrush. It is only when I get to peer inside that I will be able to tell which of the two species is responsible for its construction. The problem for me, however, is that there is a thick stand of blackthorn between me and the nest, the branches characterised by long thorns which are quick to impale, the tips breaking off and leaving a painful reminder buried in your skin.

Having weighed up the situation, and keen to add this nest to the list of those thrush nests being monitored locally this year, I work out that I can crawl ‘commando style’ under the lowest branches and, with a good deal of care, get to my feet within a metre of so of the nest. Armed with a video borescope, of the sort used by plumbers to check the inside of narrow pipework, I should be able to get a view inside the nest.

In addition to being able to record the nest contents, I will be able to make a firm identification of the species involved. The eggs of blackbird and song thrush are different but equally useful is the difference in nest construction. In blackbird, the outer foundation of moss and grass is followed by a lining of mud to form a cup, inside which is a final layer of finer material. In song thrush, the foundation of moss is followed by a layer of rotten wood that has been smoothed and bonded with saliva. There is no further lining, the song thrush laying its eggs directly onto the avian ‘chipboard’ that it has created. It is thought that the solid lining may reduce the impact of nest parasites but if this is the case, why isn’t the behaviour more widespread? Surely, there must be some additional cost of dispensing with a soft lining.

Once in position, I extend the snake-like cable of the borescope, camera at its tip, and inch it through the thorns to a position just above the nest. The small video screen reveals four bright blue eggs on a smooth and solid lining; it is a song thrush after all and a valuable record for the BTO’s Nest Record Scheme. I reverse the process and slowly inch my way back out from under the tangle of vegetation, before making a note of the nest, its location and its contents. I’ll be back out this way in a week, when another check on the nest is due.

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