I think they fill the line from the very start so you may start out on any of the 4 animals. In the IHMSA version of the game everyone starts on chickens.
When I started IHMSA, I used to think that was a silly rule. Now, I'm a big fan of it. Use the chickens to get elevation, use the pigs to get windage. Use the turkeys to just get through the silly things and then have fun on the rams.
J,
I don't know if NRA has thier rules on line, but it might not be a bad idea to print off and take with you the IHMSA rules. Yeah, some need some work, but for the most part, it'll problem solve for you when and if things happen.
Most importantly, if it's a non-sanctioned match, try your best to run it right, but more importantly, run it safe and run it fun.