Dipper vs anchor ice

The anchor ice photographed yesterday covered significant portions of the creek bed. This ice forms in the same swiftly flowing streams that provide homes to dippers—birds that scour the stream bed for aquatic insects and fish eggs. Yet, anchor ice denies the dipper access to its food. A dipper seems to easily deal with the cold, but how does it handle the food problem?

Despite the fact that both dippers and anchor ice favour the same creeks, I could discover nothing that discussed how dippers might handle the situation. Consequently, these are my own speculations based upon anecdotal observations.

The dipper seems to have three tactics. It might:

• Seek a calmer portion of the steam where anchor ice hasn’t formed;
• Switch from hunting things on the creek bed to hunting things that swim;
• Abandon the creek and hunt along the lakeshore, where there is no anchor ice.

What the dipper actually does probably depends up the extensiveness of the anchor ice. Each tactic is illustrated.

Bird books assure one that a dipper lives and hunts in fast-flowing streams. Yet, in the depth of winter, I have often seen it hunting along the lakeshore. I suspect that it does this in response to anchor ice in its favoured stream. This picture of a dipper hunting along the lakeshore was taken in January a couple of years ago.

It would probably take a particularly long cold spell for anchor ice to obliterate the bottom of local streams, or for surface ice to prevent all access to the water. At the same time as photographing the anchor ice yesterday, I was able to find gentler portions of the stream without anchor ice; the dipper found them also.

The final tactic is to hunt things that swim, even when they are somewhat more ungainly than the dipper’s normal pickings. I watched this dipper reach down from the ice-covered shore and grab the fry of a rainbow trout. The interesting thing is that it was one day short of a year since the last time I saw a dipper capture a fry at the same spot. This suggests that this may indeed be a seasonal tactic.

Dippers seem to have difficulty eating such a large object. The bird made short hops as it tried to manipulate the fry, during which time, the fish froze.

Finally the dipper kicked up water as it flew off with its freshly frozen fish stick. While I assume the bird managed to consume it, I did not see it happen.