The Little Dipper
Part of my job this summer as an interpreter in a national park involves doing “point duties.” This means I set up information displays about specific topics at popular spots in the park and answer people’s questions.
This week, I was set up in front of a popular waterfall. Standing on the rocks in the creek the waterfall flowed into were two small, noisy grey birds. They were bopping up and down while making very loud beeping noises. Another grey bird would would eventually arrive and feed them.
These birds were hard not to notice and of course people wanted to know what type of birds they were. What puzzled people is that these weren’t the typical sorts of birds that are usually seen by water. They weren’t ducks or herons but looked more like songbirds.
They were dippers! More precisely, American Dippers (Cinclus mexicanus). Dippers are in the order that contains the songbirds (Passeriformes) but unlike other songbirds they are largely aquatic and inhabit fast-moving streams. Dippers are in the family Cinclidae, which contains five species. Only one species is found in North America, and the rest occur in South America, Europe, Asia, and the Atlas Mountains of Morocco. In Europe, they are called “Water Ouzels.”
Dippers are the only type of songbird that dive and walk underwater. Despite being aquatic birds, they do not have webbed feet and instead have typical songbird feet, which have three toes pointing forward and one (the “hallux”) pointing backward. The term for that configuration is “anisodactyl.”
Despite the lack of webbed feet, dippers do have several adaptations that make them well suited for an aquatic lifestyle. Their preen glands (“uropygial glands”), which are situated above their tails, are extremely large for their size. In fact, they are about ten times as large as the glands seen in other similar-sized songbirds. Preen glands secrete an oily substance that contains a complex mixture of waxes and other fats. When a dipper preens, it will rub its beak on the preen gland and will proceed to rub the oil on its feathers. The oil helps waterproof the feathers and prevents the barbules from breaking.
Dippers also have flaps in their nostrils that prevent water from entering their noses. While foraging, dippers often repeatedly dip their heads into streams. This bobbing or dipping motion gives them their name.
Perhaps the most interesting facet of dipper biology is the fact that they often stay by mountain streams all year, even in areas where winter temperatures drop well below zero. I’ve seen dippers out foraging in -20 C weather. All they need is open water, and mountain streams in Canada may contain unfrozen parts in winter if they flow fast enough. Many dippers overwinter in northwest Canada and even Alaska, although they will move if their streams freeze over. Their highly waterproof feathers and ultra-thick coats of down contribute to their ability to tolerate some very harsh conditions.
Dippers need to be able to go long periods of time without breathing while they forage underwater. They can indeed go longer without oxygen (30 seconds) than other song bird species can. Thirty seconds doesn’t sound like long but it is for such a small bird. Dippers have higher concentrations of hemoglobin in their blood than do other songbirds. Hemoglobin is the pigment in red blood cells that carries oxygen and delivers it to tissues. Since dippers can carry a great deal of oxygen in their blood, they can spend a lot of time underwater. They feed primarily on aquatic invertebrates and must often submerge themselves to find them.
To forage underwater, dippers must be able to see underwater. Their irises actually have better developed sphincter muscles associated with them than do the irises of related songbirds. When a dipper goes underwater, the muscle will contract and allow the bird to focus its eyes while underwater. The iridial sphincter muscle is actually strong enough to alter the shape of the lens.
Many aquatic birds (such as ducks and geese) go through a molt at the end of summer that leaves them temporarily flightless. This is because they replace their flight feathers all at once. Although dippers are very distantly related to the Anseridae (family that contains ducks and geese) they do this as well. In late summer, dippers molt out their flight and tail feathers and become incapable of flight at this time. They can fly during the rest of the year, although I rarely see them flying far from streams. Their wings are quite short and stubby, and therefore function well as flippers.
So, if you’re ever hiking by a mountain stream in western North America, keep your eye out for dippers. They may look plain, but are quite tough and fascinating birds!