Fitting items wont fall into hopper, if unfitting items lie on hopper

I have been trying to create a chicken slaughter mashine using a hopper that will only accept raw chickens and stubled upon the following issue:

The hopper has all 5 stacks filled with a single item (stackable to 64), one of which is a raw chicken, none of which is a feather or egg. Raw chickens should be (and testedly are) able to fall into the hopper. Feathers and eggs shouldn't be (and testedly aren't) able to fall into the hopper.

However, raw chickens don't fall into the hopper, if there are feathers or eggs lying on the hopper.

Edit: Sorry for the duplicate. Searched, but didn't find. 😞