Mad_Jock - to answer one of your questions (I guess you probably intended them to be rhetorical but there is an input which might help):
As a bird controller there are 4 major reasons I would ask ATC to add a "Increased bird activity in the vicinity/on the airfield" message, 2 of which are as a warning allowing you to be aware and expect to possibly take action as required.
Firstly is that I may be spending more time on the runway than usual and as such you may have to wait for a late landing clearance or you may be held from taking off whilst I clear the birds if they are settling on the airfield
Second is that I may have to ask ATC for a late Go Around or Take-off cancellation in the event that I either cannot clear the birds in time or they are transitting (e.g. Gulls or Racing Pigeons)
Third is to cover my ass as it indicates a much greater than usual birdstrike risk as the birds have risen above the level which I can properly control therefore I want you to be aware that there is an increased risk of damage to your airframe. It means that I have warned you of increased risk and passes more responsibility to you in the event of a birdstrike as you have been warned and elected to continue.
Lastly, the CAA tells us that we need to warn you if we have an increase in bird activity significantly higher than the normal levels. They also ask us on the Birdstrike Reporting Form whether you had been specifically warned of any bird activity.
However, I am very much aware that if we keep it there for too long pilots will start to 'tune-out' and its presence will no longer have any meaning. I therefore will ask for it to be removed once the 'threat' passes which may mean the message is only there for a couple of hours or it may be there for a couple of days. This is why I recently asked for a bird activity message to be removed from our ATIS as I noticed that our bird level was not actually much higher than the seasonal norm and the message had been on the ATIS for nearly a week and a half.
The only time we normally leave a message on that long is just after a major event at a nearby farm which, once a year, leads to 6-800 Crows/Rooks in the 1/4-1/2 mile undershoot as that hazard is there for that long even though we do devote resources to it constantly.
That said, from reading this thread it would seem that the airport I work at is one of the better ones for ATIS content (until we go into LVPs when we add an ATIS message about only entering and vacating at the ends of the runway for some reason) as the normal content is only Runway in use, Wx (Temps, Precipitation and Clouds <A60), QNH and runway state (if its not dryx3). 1 or 2 supervisors will include taxiway closures (all include runway exit closures) but most just tell the ATCOs to not direct aircraft into a closure. Anything that is NOTAMed is normally left OFF the ATIS unless more than a few pilots indicate that they werent aware or hadnt digested the NOTAM.