My layer chickens are exhibiting several concerning symptoms, including reduced egg production, lethargy, ruffled feathers, and some birds are showing signs of respiratory distress. I'm worried about the health of my flock and the impact on egg production. What are the potential causes of these symptoms? Could there be underlying health issues, such as infectious diseases, nutritional deficiencies, or parasitic infestations? I need practical advice on disease identification, including how to recognize the symptoms of common poultry diseases. What are the appropriate treatment options for various health issues, and what are the best preventative measures I can take to maintain a healthy flock? I'm also interested in learning about effective biosecurity measures to minimize the risk of disease introduction and spread.