Saturday, 8 December 2012

This is mine!

The forest echoes with the calls of Tawny Owls in the dark before dawn. These are territorial birds, proclaiming their rights with a vocal display that carries through the otherwise still air. The ‘hoo-hu-huhuhuhooo’ of the male is answered by another more distant. More often than not his mate will also answer with the ‘keewik’ contact call, forming a duet that reinforces the message that this is an occupied territory. Female Tawny Owls may sometimes give a similar territorial hoot to that of the male, though it is higher in pitch, less well phrased and somewhat scratchy in tone.

The chances are that this pair will have bred on this territory earlier in the year, for the Tawny Owl is a sedentary species not prone to wandering. The sedentary habit and maintenance of a year-round territory provide the Tawny Owl with the familiarity needed to move around the territory safely – although good, even Tawny Owl vision cannot see in complete darkness and birds may occasionally collide with unfamiliar trees or branches – and to find sufficient food. Knowledge of the territory supports success and a successful pair will seek to maintain ownership with their vocalisations.

A fair amount of work has been done, looking into the structure of Tawny Owl calls and this has revealed some interesting things. It seems, for example, that the structure of each territorial call is particular to the individual allowing other owls to recognise each bird as an individual. Territory-holding owls use this information to identify neighbouring birds and to discriminate these from strangers who might be more likely to intrude into the territory. Calling is thought to carry a cost, perhaps because it takes up time that could be used for some other activity (such as feeding) or perhaps because it exposes the caller to an increased risk of predation. Because of this, you might predict that calling Tawny Owls would direct their efforts towards strangers and reduce the amount of calling that they direct at known neighbours, which is precisely what they do.

Tawny Owl calls also appear to carry some information about the bird making the call. The pitch and duration of the call appear to reflect the size of the bird calling and it has also been found that the calls of birds carrying high levels of blood parasites – common parasites in owls and other birds – produce less diverse calls than healthy birds. This suggests that the calls might provide an honest signal of the ‘quality’ of the bird making the call, something that could be used by a potential mate to determine the suitability of a potential partner. Whatever being imparted to other owls, to our ears the calls are simple, conjuring up the spirit of the forest at night.

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