1) It supports industrial animal agricultural operations. Most backyard chicken farmers purchase baby chicks from commercial hatcheries. There is much cruelty in commercial hatcheries. For example chicks who get under the wheels of carts are left half-smashed and still alive for hours on the floor. Others get their tender innervated beaks singed off with a hot iron. They are slammed into stationary vaccination needles so rapidly that some get the shots in the eye or die from the roughness.
2) Most backyard farmer's chicks have been sent through the US Postal service. This may take up to 72 hours during which time they have no food, no water, no warmth. Not all of them survive this abuse -- and the hatcheries know this -- so they usually include a few extras to replace those killed in the transport process.
3) Most hatcheries suffocate or grind up alive the male chicks. There is no market for them as they are the wrong breed to use for meat production. But some take a more humane approach and use the males as live, "packing material" in their shipments of female chicks to farmers -- letting their customers figure out what to do with these unwanted individuals.
4) Even if you can find backyard farmers who do not get their chicks from commercial hatcheries you are still perpetuating violence and exploitation of other beings. Most cities only allow the keeping of hens, not roosters. Since about half of eggs hatch into males, these birds are not wanted. Even in places where keeping them is allowed, most urban egg farmers don't want them, as they are not useful for producing eggs, take up space and cost money to feed and house. And they crow loudly. So most males are either killed at birth, allowed to live for awhile and then killed and eaten, or set loose in areas that they are not adapted to survive in, or they end up at animal shelters.
5) Backyard poultry increase rodent populations in urban areas. Even under the most well-maintained conditions, the straw and feed that is put out for the birds, is a huge draw for mice and rats.
6) The natural reservoir for influenza is birds. All strains of human flu started out as bird flu. Bringing human populations into close repeated contact with birds (chickens and ducks) exposed to wild bird populations increases the possibility of a new strain of influenza making the cross species jump.
7) Chickens and their feces often contain pathogenic organisms like Salmonella, and Campylobacter. When it rains, runoff from a yard with chickens can find its way into neighboring yards spreading these bacteria. Privacy fences do not prevent this, and children playing in other yards are more likely to be exposed via playing in the dirt or cuddling with cats and dogs who have also been exposed.
We not only don't need to raise and kill animals for our survival, doing so contributes to the major environmental problems plaguing humanity, is an inefficient use of limited resources, is exploitative of other beings and harms human health. If you are reading this, please go vegan. For more on this topic visit:
What's Wrong With Backyard Eggs?