(Tulsa, Ok)--It has been happening every summer since the 1980's. Purple Martins swarm in downtown Tulsa. However there are more of the birds this year then ever before. Purple Martin expert, Dick Sherry, estimates as many as 200,000 decend on the trees near the convention center every evening. He says the annual roosting in Tulsa takes place starting in May and can last until early September. As the birds dive from high in the sky into the trees the sound of thousands of Purple Martins chirping and flapping their wings takes on a loud hissing noise. Sherry hopes people do not look upon the annual visitors as pests. He says they eat millions of insects, plus the droppings are easily washed with water. Sherry says the roosting period could become a tourist attraction for Tulsa. Each night this summer bird watchers gather on the top level of the Double Tree hotel parking garage for a bird's eye view. In a little over a month the Martins will leave Tulsa as they migrate to Brazil.