Wren (Troglodytes troglodytes)
Being such a small bird it can be easily over looked in winter when it is not singing and is busily looking for food, but come the spring, although one our smallest birds (only the Goldcrest and Firecrest are smaller) it has one of the loudest voices.
If you are familiar with its complex song full of crescendos and trills then you will often know there is a Wren around long before you see it, if you see it that is! In winter you might just catch a brief glimpse as it works its way around climbing plants in your garden looking for the occasional bug to eat.
One of the features of the Wren from a distance is that it frequently has its tail cocked up, sadly this one did not so I can't illustrate the point.
In spring, amongst the time spent singing its territorial song the male Wren is busy building four or five nests. He then shows his partner around them and she will choose which one, if any, she is prepared to raise her young in. If she doesn't like any of them he is out of luck as she will be off looking at another chaps efforts!