If you can find out which hatchery they are from and what they are called, that can help. Hatcheries sell two different types of sex links and I don’t mean red or black. One type is made from crossing two breeds, maybe a RIR or New Hampshire rooster over a Barred Rock hen for a BSL or a RIR or New Hampshire rooster over a White Rock, Delaware, Rhode Island White, or Silver Laced Wyandotte hen for a RSL. These are a lot like the parents, a good dual purpose bird. They normally lay really well since the hatchery birds that are their parents normally lay well. They have larger bodies which means they are not really great at converting feed to eggs. They have to use more of what they eat to maintain their larger bodies. Most people are very happy with the egg laying ability of this type or Sex Link. I’ve gotten males from this type to use for meat just to fill out a order and meet the minimum.
Some hatcheries sell Sex Links that are the commercial egg layers. These are egg laying machines. They are smaller than the other type of sex links, about the size of a Leghorn so they are very efficient at converting feed to eggs. These are a little more prone to medical problems from egg laying, especially if you feed them a higher protein diet because they lay large eggs and have that smaller body. The more protein they eat the larger the eggs they lay. Some of these are RSL’s, some are BSL’s.
If you can read the hatchery write-up on these specific BSL’s you should be able to tell which type they are. The adult weight is a good clue too. If the adult weight is similar to a Leghorn they are probably the commercial Sex Links. If the adult weight is closer to a dual purpose bird, they are probably made from two dual purpose breeds. Some hatcheries sell both types, dual purpose and commercial egg layers. That’s why you need to know what that specific hatchery calls them.