zip ties are not really the best thing to use. The problem with zip ties is that they can easily get tighter but they will never get looser. If the bird, or one of his flock pulls or pushes on the right part, it will ratchet tighter. After a few ratchets it's in danger of cutting off circulation.
The numbered plastic leg band that they sell in Ranch Stores or the spiral leg bands are the way to go for maximum safety. These bands will allow the bird's leg to grow and expand when needed.
If you decide that you're going to use zip ties anyway, please be extra vigilant. It only takes a few hours of lost circulation to put a chick in real danger. If you're taking photos of these chicks, I assume they hold some value to you. If that's the case, think long and hard about zip ties. They might be fine, but it only takes one bad experience for you to wish you had used expanding leg bands.
Dan