Birds nesting in loft

Hi people first post here, but I think a good place to start with my 
problem.

Over the last few years I have birds nesting in my loft, Is there anything
I 
can do to discourage this. I understand that I will have to wait till the 
end of the nesting season, but I don't want them back next year. They are 
making quite a mess in my loft. The design of the house doesn't lend its 
self to simply seal access points, so I was wondering if there is some
kind 
of aroma or sound admitting device that I could put up there to discourage

them

mike