The birds follow direct lines from ysland to ysland. It is possible you saw a bird flying half way between 2 islands and sailed right between them. Try to follow the birds and find out which direction they are going. If you sail more than 10 min in open water without any sign of birds or ylands you are probably not going to find anything. turn around and sail in a different direction.
For best results look for birds on the shore of your island or from top of a mountain. If you dont see any there, try to sail around your yland away from shore and look for birds. There should be at least one coming and going from your yland to another one. follow it and you should find new lands.
I have only a small boat but have found 2 different ylands using this method not far from my original spawn point. My second hop I found a desert yland that had a small patch of sulfur (the item I was looking for) on the surface.