Not sure about seasonal changes, but I can tell you what we saw a couple weeks ago. From the north shore road, we saw about 130 of them. From the south shore lookout at the creek, about 20. Bring binocs - they're a half mile away. We also went snorkeling with Alex (Anegada Adventures) at the east end, where there's usually a flock near the conch pile, but none were there - they were all on-island.