I am currently attempting to spread entities using the following command:

/spreadplayers -20 -9 2 3 false @e[type=Chicken]

With this distribution, the maximum number of entities that it should be able to spread is 16 (see image 1 - green spaces indicate valid positions under optimal spacing). However, that spreadplayers command will not work with more than 12 entities (I've run it ~200 times with 13 chickens and it has yet to succeed).

Steps to replicate:

1. Summon 13 chickens:

 /summon Chicken -7 56 6 {Invulnerable:1,Attributes:[{Name:generic.movementSpeed,Base:0}]}

2. Set up a spreadplayers command block using the command:

/spreadplayers -20 -9 2 3 false @e[type=Chicken]

3. Run the spreadplayers command block.