My first playthrough I had trouble finding the birds. there were NONE visible from the coastline of my island anywhere. I ended up making a little boat and circling the island while slowly getting further and further from shore. I eventually found 1 bird, followed them to an island and the next one had a couple of birds I could see from it, etc. That first one was just a pain.
The render distance is sort of... bad. You can sail a circle around an island and never see it, look on the map and never see it, get 2 feet closer and bam, it pops up. To get an idea of how far you can see, load up a map and look at how big the circle that is revealed is, on a ship, the render distance is ~twice that from what I can tell. That will help you figure out where your blindspots are.