Why are the roosters called "packing peanuts"?

[chicksgalore]

Okay, I've been checking out mail-order chicks and have read several places now including on here about the hatcheries sticking roosters in for --what do they put them in for--??

 

[bluey]

...heat as I understand it, and to take up some space to prevent shifting just like the "packing peanut".

 

[Farmer Kitty]

They use them to keep the temps up. Most people order pullets so there is an excess of roos and that is why they use the roos as packing peanuts.