Chicken Gallus gallus

The domestic chicken descends from the red junglefowl, whose subspecies Gallus gallus spadiceus — native to northern Thailand, Myanmar and southwestern China — is identified as the main wild ancestor. It is kept for meat and eggs across systems ranging from scavenging village flocks to fully controlled commercial houses. FAO surveys of village birds record 35 to 100 eggs per hen per year and adult weights of roughly 1.14 to 2.3 kg, with hens spending more than 90 percent of the day foraging within 110 to 175 m of the homestead on a diet that was mostly household waste, grass and invertebrates. Eggs hatch in 21 days at 37.2 to 37.8 °C. Clinically the species is defined by avian physiology: a body temperature of 105 to 107 °F, a resting heart rate around 250 to 300 beats per minute, and a respiratory system with nine air sacs, no complete diaphragm and one-way airflow through rigid lungs, so ventilation depends on sternal movement and anaesthetic death risk runs far above that of dogs and cats. Newcastle disease and high-pathogenicity avian influenza are notifiable and dominate flock health policy.