It's a little weird how several levels of the Chicken (Levels 2, 4, and 6) have no benefit compared to the prior level. I guess with a 0 mana monster with low stats, there isn't as much wiggle room though in theory they could have increasing speed at those levels which then goes back down on the odd levels.

In any case, this level arrangement does mean there will be a higher supply of Chickens than expected since if you are targeting a particular league level, there is no reason for you to spend extra copies to level it up to Levels 2, 4, or 6.