Bully Baby

I just got 5, 3 day old chicks today 2 americaunas and 3 Rhode Island reds. One of the americaunas chicks is bullying everyone else. I just caught her pulling one of the legs of the other americauna. Why is she doing this? What can I do; I don't want anyone to get hurt?

Welcome to BYC and congrats on your new additions.
How are you brooding your chicks - amount of space, temperature, type of brooder, number of feed/water stations, etc? Behavior such as this is not all that unusual in chicks, and in milder forms is not all that concerning but it can become an issue if it escalates and there are several environmental factors that can cause that escalation to take place - among them overcrowding, overheating, a strong personality bird and only one feed/water station, etc. Increasing the amount of space available, adding distractions to the brooder, adding another feed/water station (to prevent the more aggressive bird from controlling all resources) and verifying that everyone is feeling as comfortable as possible (not to hot or cold) can help to alleviate the aggressive behavior a bit. That being said, chicken society is not always pleasant when viewed through emotional human eyes and often what seems horrible to someone unfamiliar with the ways of things can be normal interaction between birds forming a flock dynamic (yes, even at this young age). Until/unless serious injury is taking place my intervention amounts to the above.