Do let us know if you suspect birds are nesting. It is very important from a safety point of view to be forewarned, especially when working from ladders. This will not stop us from clearing a gutter, but as stated, it is better to know where a bird might fly into your face if spooked.
Nesting season is between March and August, some sites say February. It is actually an offence to impede/destroy nests in use during that time period.
We are more than happy to help with identifying entrance places either on our quotes with the inspection camera or on the gutter clearance job itself where we may be using ladders. Generally, however, the inspection camera is only useful for observing in and above the gutter but not so good under tiles and eves. Only on rare occasions will we add a product or attempt to seal entry points and of course this will need to occur out of nesting season.
Pigeon Control Advisory Service has lots of useful information.