Every week 10 Silly Seagulls get released for 10 weeks, totaling 100 seagulls by the end. Try and collect them all before they disappear, because once they're gone, "they're gone". After 10 weeks have passed, any Silly Seagulls that go un-sold will be kept and possibly later auctioned off - so if the opportunity is missed before, that's the time to try and complete the collection!