Monday, 13 August 2012

Look up!

Lift your gaze above the horizontal, directing it towards likely spots as you wander through town, and you will soon discover that there are nesting woodpigeons everywhere. Two of the houses on our street, for example, sport climbing plants that trail the outline of the front door and in each of these, wedged just above the frame, is a nesting pigeon. There are another three nests along the high street, two in trees and one on the top of a metal cage that protects some passageway lighting from the mindless attention of the local hooligans. There is even one in my garden, a bulbous platform of sticks, formed from the combined nests of three previous seasons and many previous nesting attempts. On top of each nest sits the plump and unmoving form of a woodpigeon.

Many of these nests go unnoticed; despite their bulk the sitting birds are usually silent and many of us wander around with our heads down, watching the pavement or avoiding the gaze of passers-by. This is good news for the woodpigeons, incubating their two eggs or brooding young chicks. Not all of the sitting birds will be on eggs or young, however, as the two members of the pair may take turns at sitting on the empty nest in the days leading up to when the first egg is laid.

It might seem odd that woodpigeons only lay two eggs, but it is a feature characteristic of pigeons and doves more widely. Both parents take turns at incubation, the female incubating for roughly 18 hours in every 24, the male the remainder. The birds switch over at or near the nest so the eggs are rarely left uncovered for long.

One curious aspect of woodpigeon reproduction is that the young pigeons are initially fed on crop-milk, something that is known from very few bird species around the globe. Crop-milk is a secretion that is produced within special glandular cells in the crop wall. These cells become filled with a cheese-like substance and, when full, are sloughed into the crop from where they are fed to the young by regurgitation. The sloughing does not happen continuously and there appears to be a trigger that causes the cells to be released en masse just prior to the time at which the young are due for a feed. Feeding visits to the young decrease with age, the diet also switching from crop-milk to other material, so that older chicks may only receive feeding visits twice a day. This may reduce the risk of an adult being followed back to the nest by a predator and less activity at the nest also makes them more easily overlooked by us.

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