|This brown leghorn is trying to occupy the nest box with a barred rock.|
I've watched this quite a few times and I don't think it's aggressive behavior. I actually think the brown leghorns like the company. Because if they both need to lay eggs at the same time, I'll find both brown leghorns happily occupying the same nest. But my other hens are not appreciative of this behavior. Usually a squabble takes place with the leghorn getting her way. The only hen that doesn't allow the leghorns to win is Hoppy, our partridge cochin. She loudly defends her turf. And, sometimes I actually have to remove everyone from the coop and let them start over.
For the meantime, our nest box bandits will keep up their stealth operations and enjoy their new found laying companions.