Crows Ice Fishing for Caddis Larvae: Tuul River, Mongolia

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Corvids – a group which includes crows, ravens, rooks, jays, magpies and nutcrackers – fascinate. On a frigid December afternoon, we found them ice-fishing for caddis larvae in Mongolia’s Tuul River in the heart of Ulaanbaatar. (Click any of the photos for a larger view.)

The most intelligent of all birds, and with a brain-to-body-mass ratio rivaling that of the great apes and cetaceans, crows and their allies often exhibit remarkable behavior. Members of this family have been observed using and to some degree engineering purpose-specific tools – the only nonhuman animal other than the great apes to do so. There’s little doubt that certain feeding behaviors adopted by crows have been learned by watching other birds and animals.

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Note the object in the beak of the crow on the left. It’s a caddis larva casing.

The Tuul River flows through Ulaanbaatar, Mongolia’s capital city. Despite urbanization, the river’s water quality remains high – good enough to support species of trout as well as sensitive aquatic insects such as mayflies, stoneflies and caddisflies – species which spend the early part of their lives as larvae on the bottoms of clean rivers and streams. The Tuul is fast-flowing. Consequently, despite weeks on end of sub-freezing temperatures, here and there pockets of water remain open over shallow, stony, silt-free bottom.

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We took a cue from watching the crows and began scanning the ice for caddis larvae casings. As our eyes tuned into the proper size and shape, the empty husks appeared everywhere. This casing, constructed of tiny sticks and pebbles, is about 1.5 inches long (3.8 cm) and belonged to a member of the Northern case maker caddisfly family. 

Perhaps the crows learned of the abundance of wintertime food in these areas of open water by watching dippers – a robin-sized bird that happily plunges into pools and riffles in all seasons in search of aquatic insects and small fish. In fact, on this day we saw a white-throated dipper submerge itself in one of the riffles where the crows were feeding and come up with a small fish.

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Here a carrion crow (Corvus corone) has his prize – a caddis larva pried from its pebble and stick casing.

Not adapted to plunge into water, the crows were fishing from the edge of the ice – thus gaining access to water not overly deep, but deep enough to provide habitat for slow-moving caddis larvae. Resourceful birds, are crows.