So, you have a ship that you can move around and it leaves a line behind it, and you have to enclose some birds inside the line you leave behind? Is that it?
If so, are the lines you leave linear, or are they curved? If they are linear, then you can do this without too much trouble. You'll have to record all the vertices (using a
flexible array (or see part 3 of
this tutorial) of a
record. Then do a bit of math to determine if a given point is contained within your polygon. Work the math out on paper. This is a good problem. Give it a shot, and let us know what you come up with, so we can offer advice.