From: Natalie Millar, MONGABAY.COM, More from this Affiliate
Published September 4, 2013 03:06 PM

World's biggest owl depends on large old trees

The Blakiston fish owl (Bubo Blakistoni) is the world's largest — and one of the rarest — owl species, with an impressive 6 foot (2 meter) wingspan. The giant owl, found exclusively in northeast Asia, shares its habitat with a menagerie of endangered and impressive animals, including Amur tigers, Amur leopards, Asiatic black bears and wild boars. Now, a recent study in Oryx, led by the Wildlife Conservation Society (WCS) has discovered that these owls rely on threatened old trees for nesting and foraging sites.

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In Primorye, Russia, a research team headed by Jonathan Slaght with the WCS and the University of Minnesota, looked at 20,213 square kilometers of forest and found that owl nesting sites are usually within old, large trees — such as Japanese poplars, chosenia and cork bark elms. The nesting sites are all located along streams — which are also the habitat for the owl's primary food source, salmon. This means Blakiston's fish owl is entirely dependent on riparian forests, which refers to the banks along a water source or the interface between water and land.

The nesting trees have to be large enough to accommodate the huge size of these owls, with the diameters at breast height ranging between 74 centimeters to 150 centimeters. The size of the tree is an important factor in determining a nest site, but the owls also seem to favor trees that show some signs of deterioration — particularly ones that may be dying or already dead. These trees, known as "snags" are ideal for nesting as they usually have cavities forming within the bole that can accommodate an owl nest. Although nests were found in the side cavities of trees, the owls appear to prefer broken top cavities, which better complement their bulk.

An intact riparian forest doesn't just provide prime nesting site, but also enhances the owl's prey when large, old trees fall into the water. Researchers found that the presence of tree trunks in the water was strongly correlated with owl hunting. The scientists further suspected that this was related to salmon abundance as the added obstacles affect the "stream-channel complexity" - combining areas of slow-moving backwaters and fast moving channels to suit different life stages of the salmon. Some streams also had warm springs, which attract salmon during the winter months, providing a food source for the fish owls when it may otherwise be scarce.

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Blakiston's Fish Owl image via Shutterstock.

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