The only problem with saying that you don't see any adverse effects is that with heavy metal, it is a "build up" over time in the bloodstream issue. You won't necessarily see any adverse effects right away.
Then after it builds, it manifests much like any other disease so it's hard to pinpoint that's what it is from without taking blood levels.
Signs of zinc poisoning in chickens include:
Loss of appetite / weight loss
Feather picking
Shallow breathing
Depression and lethargy
Weakness and shaking
Loss of balance
Diarrhea (in advanced cases stool may appear black and tarry)
Vomiting
Kidney, liver and pancreatic anomalies
Anemia
Death
Signs of lead poisoning:
Poultry with higher contamination levels may exhibit the following symptoms:
Lethargy
Depression
Failure to grow or to maintain weight
Loose stools – greenish-black in color
Lack of balance
Disorientation
Inability to perch
Vomiting
Brain function disturbance
Loss of coordination
Loss of vision
Seizures
Inability to hold head straight
Death
Article in NY times regarding lead detected in backyard chickens
http://www.nytimes.com/2012/10/10/d...ead-for-new-yorks-garden-fresh-eggs.html?_r=0