My niece loves this series and is an avid reader. She loves books which have animals as the main characters (can also include humans). She is going to be 13 so she can handle "older" books as well (I would prefer no "adult" books but Young adult vs children/teen is fine). I read paranormal and historical fiction so I have no clue what to get her. Any help from anyone please?

I'm not familiar with Erin Hunter at all, but the description of the first Warrior book remided me of Tad William's Tailchaser's Song.  I don't think it's officially young adult, but I've seen it on several YA lists.  It's a wonderful book.

Other options might be the Redwall series or Watershipdown.

Thanks Melanti, I ordered the Tailchaser's Song book. It looked perfect!

Owly is a great graphic novel series by Andy Runton your niece will probably really enjoy since she loves animal stories.  Also, another suggestion from my daughter is "The Fire Within" and "Icefire" books by Chris D'Lacey.

Thank you for the suggestions bookfan, I appreciate it.

there is also the Rats of Nimh series, I liked those when I was young. :)

and the Redwall series is awesome, if she is a mature reader... the battle scenes can be a little graphic and the villians can be baaaad.

both series have anthropomorphic animals like Warrior

I work at a Barnes and Noble where I am in charge of the children's department. Kids who enjoy the Warriors series usually like "The Guardians of Ga'hoole" series by Kathryn Lasky. Same sort of thing, it's just owls instead of cats. Also, as someone else recommended, the Redwall books are great, I started reading those when I was about 12. You should also check out the book "Dragon Slippers" by Jessica Day George, it's not about animals necessarily, but dragons. It's one of the best juvie fiction books for girls, in my opinion, and every girl I've recommended it to has promptly come back in for the others in the series :) Another is "Wild Magic" by Tamora Pierce it's a series about a girl who talks to animals. I started reading her when I was about 11, and still re-read her as well, she's stayed one of my all-time favorite authors, and is great for teenage girls, her characters are all girls who are strong, and brave, and confident!