Why does a Long-tailed Tit have a long tail ? We thought it might be for balance when feeding, but Blue Tits seem to be just as acrobatic with short tails?

Good question! I have consulted with a colleague who has been researching social behaviour in long-tailed tits for a number of years and his response is a rather unsatisfatory (but honest!) "we don't really know"!

A number of hypothese have been advanced. One, which you allude to, is that the long tail helps with balance when feeding upside down/at the ends of very narrow branches etc. However, the ‘true’ tits (e.g., blue tits, great tits ) also feed like this and don’t have particularly long tails. So if this feeding pattern has selected for long tails in one species, others must have "found" a different evolutionary solution to the problem of maintaining balance.

The other obvious suggestion would be some form of sexual selection which often leads to exaggerated ornaments to attract the opposite sex (and/or weapons to compete with same sex rival). However, this process tends to cause sexual dimorphism (e.g. male peacocks have much longer tails than females). ni long-tailed-tits this "signature" of sexual selction isn't apparent.

Sexual dimorphism doesn't always accompany sexual selection: mutual sexual selection is also common, with both males and females displaying the same exaggerated traits. See http://blogs.scientificamerican.com/tet … terosaurs/ for a helpful layman-level overview.

Agreed - and there are actually some good bird examples of "ornamentation" that could be explained by this (although the ones I can think of are in swans and seabirds rather than passerines).

So, to clarify -  dimorphism usually indicates sexual selection is important, it's absence  doesn't necessarily imply there has been no sexual selection. In long-tailed tits (mutual) sexual selection is probably a sensible hypothesis but has not (to my knowledge) been tested.