@Mega_Spud
It may be worth mentioning my system specs which i suspect have something to do with this. The crappier the machine, the quicker this may happen.
Laptop: Toshiba Satellite P55W-C
Intel Core i5-5200U CPU @ 2.2 GHz (Turbo @ 2.7 GHz)
8GB RAM
64-bit Operating System, x64 processor
Touch support with 10 Touch Points
I have attached a world and set it up to simulate survival with a few circuits to make the process faster. The orange one spawns in a ton of chickens using spawn eggs. This is to give the survival feel.
You could always ignore that if you want and just fill the chests manually. The way i did the glitch in my video was filled the Adult holding cell and then fill up the hoppers and chests with eggs.
Then use the piston to move the hopper to activate the Hatcher mechanism. This autofires the eggs and all the baby chickens swim up and are sent to holding cell.
The whole process takes around 24 minutes when adults are spawned in Red chamber and the chests and hoppers are pre-filled with eggs.
This is not only a chicken problem but all entities in general. This can be done with villagers, pigs, ETC although it may be hard to get them to gather in a chamber quick enough. I know because it happened with adult villagers and broke my iron farm (Did not take nearly as many to do this)
Still broken in 1.1.5
The expected spawn rate i base my judgement on is Mumbo Jumbo's mob farm found here