We have a nest box.Bluetits are regular visitors.Why are they hammering their beaks against the wood like a woodpecker?

Trevor,

This is reported quite frequently and there are really two popular theories about why they do this. The first is that they are shaping the entrance hole to their exact specifications and the second is that they are testing the strength of the wood. When nesting in the wild, many hole dwelling birds will choose natural features, through broken branches etc. The broken branch will eventually rot and may form a hole that can be utilised by the tit. However, if the rot is too great, the security of the nest may be compromised such that predators of the chicks or eggs can break through the rotten wood. By tapping on the wood, the blue tit may be assessing the security of the box to protect it's young.

A colleague of mine who studies blue tits and other small passerines has suggested a third possibility:

"I think it could be because the beak of a blue tit has indefinite growing and they need to keep it at an intermediate length. They also at the same time sharpen the sides sliding it on the wood or other material. I know that when you keep a blue tit in an aviary with no material available for them to wear out the end of the beaks (e.g. only wire fence and no tree), they become too long, so my guess is that this hammering is in this use, but it's only an opinion!"

Another colleague - keen to identify himself as an expert on great tits rather than blue tits  - has sent me a further hypothesis for this phenomenon...

" as a expert of great rather than blue tits I have a boring (and probably true) answer, and a more exciting (and probably wrong) answer:

1) Like Dave Warburton wrote, they may simply try to make the nest hole a bit bigger. The hole is usually only just big enough, and as they are going in and out many times a day, this causes quite a lot of feather wear and tear. However, I don't think they are usually very successful at this (and a too big hole would of course increase predation risk), but then I guess they are hole nesters and not nestbox nesters, so this behaviour may still be quite useful when breeding in a rotten tree.

2) Another reason why they may do it is to keep their kids quiet by mimicing a woodpecker. Especially older nestling can be pretty jumpy, which makes it difficult to get them out of the box (or to keep them in) when you want to weigh or ring them. Usually the chicks shut up immidiately when you tap with your finger on the side of box. Maybe the parents are sometimes just a bit fed up with their constant begging for more and more food. Or maybe it is a way for them to find who is really hungry and is willing to take the risk of being eaten..."

The mimicing of a woodpecker is very interesting. I was watching (well, hearing!) a male great tit the other day knocking on the inside of nesting box, although there were no eggs laid (as yet, the nest had been finished though). I assumed that as the box had a metal surround on the front, the great tit may have been altering the shape of the hole from the inside...

This alternative answer has been sent in by
Coilin MacLochlainn, Editor, Wings magazine, BirdWatch Ireland:

A Blue Tit has taken up residence in one of several Schwegler nestboxes I 
have on my house and has been going in and out for about two weeks now, from 
April 7th 2015. It has been banging away on the inside of the box the whole 
time, like a tiny woodpecker. It took me a while to figure out that it was 
the Blue Tit making this strange noise. At first I thought it was trying to 
fashion the inside of the box for its nest, as a tit in a tree-hole might do, 
or as woodpeckers do. But Schwegler boxes are not made of wood, and Blue Tits 
are not able to tear small strips off them or make any impression on the 
inside surface.

Every day, the Blue Tit bangs away in the box, goes away for a while, sings 
loudly from a tree and then returns to continue banging; this goes on all 
morning. It struck me that what it was up to was trying to attract a female 
to pair with him and occupy the nest. His rhythmic banging not only drew 
attention to the nestbox and to his territory, but also to his strength and 
fitness.