I usually reckon to hear my first chiffchaff (Phylloscopus collybita) on, or just after, the 15th March each year; they are suddenly there in the bare branches of the trees calling their repetitive "chiff-chaff-chiff-chaff "song. I am always pretty chuffed when I first hear them as they are one of the heralds of spring for me but after a while they are quickly taken for granted and I look for 'more interesting' things!
The chiffchaff is usually back here a good three weeks or so before its close cousin, the willow warbler. The two are virtually indistinguishable in appearance up in the tree tops and it is through their songs that one can confidently tell them apart. Often heard but not always seen, the chiffchaff can be difficult to track down and photograph as it is continually on the move amongst the branches. It is obviously easier to do it early in the spring before the leaves appear on the trees.
Despite generally being a migrant species wintering in Africa you may see a chiffchaff during the winter months and it was once thought that some birds just decided not to head south in the autumn but ringing has shown that our winter birds are generally migrants from colder parts of Europe who decide that Dorset is warm enough for them and heading even further south is not worth the effort.