Friday, 12 February 2016

Dippers take 2

The post name is exactly what happened. I had a short session today and on one net round there were two Dippers in the net, one bird was in the bottom shelf and the other in the top shelf.
Both birds were females, one bird was an adult and the other a juvenile based on the criteria I talked about yesterday.

Perfect timing for this photograph, more luck than judgement I hasten to add. Why do dippers have white eyelids and blink. I didn't know so I have researched this and other facts about this incredible little bird.

Dippers are small birds that have short muscular wings, short tails and very strong legs and are unique in passerines as they are aquatic. Their habitats are fast, clean oxygenated water and the river at the bottom of my garden is very turbulent with lots of white water and strong flows.

Their environment can be very cold especially at this time of year and because they feed in and under the water their feathers are waterproofed with lots of oil from their preen glands. When not walking on the river bed using their strong legs and feet to grip the bottom they can move about by swimming with their muscular wings. When under water the flaps on their nostrils close and their eyes are designed to cope with underwater vision by being protected by what is called a nictitating membrane.

To feed on the bottom it just walks underwater and turns stones and pebbles over with its beak to get the larvae of caddisfly, mayfly stonefly and other aquatic insects. Their wings can be used to help stabilise themselves and keep them on the bottom.

Being territorial it is believed that Dippers dip to ward off competitors when they become agitated in any territorial encounter, its a way of communication. Another way of communication is the blinking of the eye. The eye lid is completely feathered which are white and when the bird blinks it is very visible. Some people say that the blinking display can occur at the same time as it dip's and is again it's another way of communication. I also found out that some people do not really understand or do not know the reason why Dippers blink. So I think you have to have your own opinions and make up your own minds. If anyone has an opinion about this please make contact because I would like to fully understand why they do this.

Thursday, 11 February 2016

A Dipper at long last!

Since I lived in Wales I have ringed Dipper pulli in the spring but never made an attempt to catch adult birds until recently. My neighbour allows me to use his land where the river is far more accessible than at the bottom on my garden. After several failed attempts in the last couple of weeks I decided to  investigate further the potential of the river at home.

At the end of my garden the ground drops away to the river so the first thing was to find a ladder and position it safely so I could get down. Once down I cleared away the undergrowth and soon got a 4metre net spanning across the river. I don't know what other people do but my concern was having the bottom shelf too low and risking a bird flying into the bottom shelf and getting wet. The male is a bigger bird than the female and its weight range from the BTO Birdfacts is 62gms to 74gms so I got a a piece of a tree branch about 40mm in diameter and cut a length until it weighed 100gms. I then dropped this piece of wood into the bottom shelf of the net in different places along its length and adjusted the bottom shelf height so the wood was hanging about 3 inches from the water.

As I suspected the dipper I caught today was in the bottom shelf and was well clear of the water with a weight of 64.9gms

Using Svennson and again BTO Birdfacts the male bird plumage is the same as female plumage so the bird was unsexed because of this. However the male bird is larger than the female and the biometrics are as follows:

Male 62gms to 74gms
Female 52.5gms to 69gms.

The weight of my bird was 64.9gms and there is a clear cross over so still unable to sex the bird.

Wing length
Male 93.5mm to 101mm
Female 86mm to 92 mm

The wing length of my bird was 95mm so I concluded it was a male.

The age of the bird was a juvenile based on the white or greyish white tips of the PC's and GC's.