It comes from the fact that there have been a whole bunch of seagulls visiting these threads and doing their ‘business’ and pretending like that’s a worthwhile thing.
http://t1.gstatic.com/images?q=tbn:ANd9GcQa7Gz5EV9Cg0rfwKph9wJA6AtH8umQVLqfUOSVZBEPCxLI9lJW5GB05yU