Is this normal blue tit behaviour?

Several times recently I have seen a blue tit feeding another blue tit in our garden.  Below is a picture. From what I can see both blue tits are adults and I thought this behaviour was quite odd. Has anyone else seen this happen or knows why it might happen?

Thanks, 

Jan

Lovely photo! Courtship feeding is seen in many bird species. The female is testing the male's ability as a provider (as he will need to do plenty of providing when there are chicks to feed), and she also benefits from the extra food supplies when she is going through the energetically demanding process of forming eggs.

Thank you so much aiki for this this valuable information. I did wonder if the behaviour was something along the lines of your explanation and it is great to have my theory confirmed.