Thursday, 29 October 2009

Bobbing and Winking

Dippers have always held some sort of fascination for me. Partly, because living in the south-east there are few opportunities to watch them, but more because of their unique characters. Worldwide there are five species of dipper and they are all very similar in there life styles.

The Dipper we have in England is one of the twelve or so sub-species of the White-throated Dipper that that stretches from Britain to the Himalayas and China. They frequently bob up and down, and modern research indicates that this is a method of showing potential predators that they are fit and able to look after themselves.

They are not the only passerines (song-birds) that are found using streams and fast flowing rivers as their prime habitats, but they are the only ones that spend a considerable time under water hunting for their food. Their food is mostly aquatic insect larvae, although they do take fresh water shrimps and occasionally small fish such as Miller's Thumbs, or Bullheads.

Another signalling adaptation they have is the blinking of the upper white eye-lid. This can be seen from a surprising distance.

The Dipper, when actively feeding makes frequent sorties into the water and it has various adaptions to help it it cope with this exacting way of life. As soon as it enters the water the heart rate slows down and speeds up again when the bird leaves the water.

The blood has a higher concentration of haemoglobin so it can can carry more oxygen. The Dipper frequently dives in shallow water, normally less than 1 metre deep and can stay submerged for up to 20 seconds, though most dives are for 4 or 5 seconds.

A lot of debate about how Dippers stayed submerged in fast flowing torrents was settled by filming them in tanks. This showed they the used their strong wings to maintain their position in the water by rapid wing-beats, and this also allowed forward movement. The legs are also used in a running motion, to help propel the bird, or are used to cling to rocks on the bed.

I watched the two Dippers around the bridge at West Bradford (nowhere near and nothing like the Yorkshire Bradford). Once again one of them spend some time, near the bank, on a prominent rock singing. Mate fidelity between seasons is quite high in Dippers, although some males are bigamous or polygynous. The fact that this one was joined by the second bird, for a while, made me wonder it they were a pair, still maintaining a territory.

One of them flew up stream in a blur of wings and I caught up with it on a little weir a few hundred metres up stream. Unfortunately it was content with the opposite side and was it too far for more pictures. Dippers have linear territories, along the rivers, and they range from a few hundred metres to three Km in length, depending on the productivity of the river. When I left the bird, it was with a promise that this summer I would return and get some pictures of the family when they have one.

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