Hmmm. So bread has no nutritional value? I guess I don't see how it could be good for people, but has no value for chickens. They sure like it. ???
Rather than calling out the logical fallacy in the above, I'll illustrate.
Bread has limited benefit, and to the extent it displaces other things in the diet, it is a net negative.
Imagine if, you will, that you have a pocket full of quarters. That pocket full of quarters represents a full chicken, its nutritional needs met for the day. There's good value in it.
Now imagine that you start replacing quarters with pennies, your bread. Suddenly that pocket full of money isn't worth as much. Their nutritional needs no longer met.
Worse, just as you can't put pennies in the local vending machine (unlike quarters, in at least a few places still), the few beneficial things in bread (mostly carbs - energy, and a limited number of vitamins they easily obtain from other sources) can't be used to make better or other things critical to their development.
To the extent that the bread adds to the existing diet, their energy intake exceeds their energy spend - resulting in deposited fat. Unlike humans, chickens don't deposit intramuscular fat well. Instead it collects primarily in the organ cavity where it can cause pressure on the heart and lungs where it restricts blood flow and oxygenation as well as causing the liver to become increasingly friable until it ruptures in a condition called fatty liver hemorrhagic syndrome. Which is generally first noted by sudden chicken death.
When you look at a bag of chicken feed, they generally don't tell you the carb count. CHickens will try and eat till their energy needs are met. What they do tell you is Crude Protein, and a tiny bit about the amino acid profile of that protein. Also, Fat, Fiber, and a few key vitamins & minerals.
For adult laying hens, whose nutritional needs are the lowest of any typical chicken "category" (i.e. pullets, Meat birds, dual purpose adolescents, etc) the general minimal dietary recommends are 16% crude protein*, 3.5% fat +/- 3,5% fiber +/- , 1% calcium +/-, .5% phosphorus (to slightly more), 0.25% sodium with key amninos Methionine 0.3%+ and Lysine (about) 0.7%.
How does bread compare?
9% crude protein
2.5% fat
5% fiber (fiber is NOT beneficial to chickens in the way its beneficial to humans - too much of a good thing is not a good thing)
0.8% calcium
0.1% phos
too much sodium
negligible Met, Lys
In short, its a net negative on every single metric.
and at least in waterfowl, high bread intake is associated with developmental abnormalities in ducks, geese, and the like, primarily in the form of "angel wing".
I hope that helps your understanding. Sources available at request.
* you can get by with lower total crude protein if you adjust the amino acid profile, primarily by addition of DL-Methionine and L-Lysine. The EU is on the cutting edge of such research and does reasonably well around 14% CP for adult layers with such adjustments. The US, largely, still gets by with an excess of low quality protein to meet minimal Met and Lys targets.