White Leghorns seldom become broody, but maybe you have one that will? Leghorns were developed for LAYING EGGS, lots of them, for most of a year. Actually, you should be dating those eggs in the nest every day you find a warm one!!! Pencil is fine. She will not SET until she has finished a "clutch" and that may be 25 or 30 eggs or more). IF Mother Nature says "set", she will. Hens do not "decide to set". You cannot MAKE a hen set. When it is time, she would SET on an empty nest==for a while. It is a hormone thingy. She should only have about 8 eggs for a first setting. I gather all eggs every day, date them and label the hen's band number, keep a dozen at a time on the kitchen counter, in an egg carton. And then when a hen SETS, I choose the freshest ones from the hen or hens I want chicks from--and eat all the extras--they are not started incubating and are just fine to eat. (And maybe both Leghorn hens are laying in the same nest??) Hens love to crawl onto a warm nest egg that has been laid by another hen (just like we like to climb into a warm bed in winter!) The eggs do not show any visual incubation until the hen STAYS on the nest for a couple of days. You can tell this by candling an egg or 2--then leave her alone, but KEEP other hens away from laying in the nest!! After 21 days of setting, all eggs hatch at the same day-- and if you have the date due, written on your calendar, you will have chick feed and waterer for the babies and protection around them, so your other hens will not destroy them! GOOD LUCK--and don't waste any extra eggs.....