Thursday, 4 March 2010

Nomads on the wing

The number of Short-eared Owls wintering in the county this year suggests an influx of birds from further north, possibly from breeding populations on the uplands of Northern Britain or from Fennoscandia and beyond. We have our own very small breeding population within East Anglia but this has never exceeded more than a few pairs. These day-hunting owls are often portrayed as opportunistic wanderers, nomads that follow the volatile breeding populations of their favoured small mammal prey, the voles. If you read back through the old ornithological literature, you will come across examples that illustrate the fluid nature of the owl’s breeding populations.

Voles breeding within the structurally simple landscapes of the northern uplands and arctic tundra show a strong multi-annual pattern, a population cycle that peaks in one year and falls to a trough soon after to produce a four-year cycle in abundance. This has a knock-on effect on small mammal predators and, for the Short-eared Owl, a good vole year means a good breeding season, but a subsequent decline in the number of voles will see the owls move elsewhere to seek other opportunities. The scale of such movements explains why 116 of these delightful birds were seen to roost at Halvergate in December 1972 but the following winter saw just three birds in the same area. These birds seem to show little in the way of site fidelity and will winter in different areas in different years, depending upon food availability.

Having a good number of individuals here this winter provides an opportunity to take a closer look at these birds. There were four in the air together at Chedgrave Marshes the other day and they appeared to be roosting on the steep bank protecting the lower lying land from the river. Superficially, they are similar to a Barn Owl, in that they have a light and rather buoyant flight on long wings. The similarities between the two species reflect their common habit of quartering open ground for food, a behaviour that is made more energetically efficient by having a low wing-loading. The short-eareds most closely resemble the related Long-eared Owl which, although normally nocturnal, may occasionally be seen hunting during daylight. Short-eared Owls are slightly longer-winged and show a white trailing edge to the upperside of the wing (not seen in Long-eared) and a pale belly (streaked in Long-eared).

Out on the marshes, the Short-eared Owls will be feeding on small mammals and birds, the latter proving to be an important component of the diet at this time of the year and including small waders, pipits and species that tend to roost communally. They will not be around for much longer so make the effort now to see them.

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