Erin Hunter said this quote

Bluestar blinked. There are cats who would argue that there should never have been a fifth Clan in the forest at all. Why are there four oaks at Fourtrees, if not to stand for the four Clans?Firestar gazed up at the massive oak trees, then back at Bluestar. Fury pure as a lighting flash rushed through his body. Are you mouse-brained? he snarled. Are you telling me SkyClan had to leave because there weren't enough trees?