Impact of temperature-humidity index on heat stress behavior and welfare of battery-caged yellow-feathered native broilers during summertime in Hong Kong: An observational study

This observational study investigated the effects of the temperature-humidity index (THI) on heat stress behavior, hormonal response, and body temperature of battery-caged yellow-feathered broilers on a Hong Kong poultry farm equipped with ventilation fans and foggers from August to September 2023. Twenty-six units distributed in the sidewall and inner zones of the chicken house were randomly selected for real-time THI measurement using data loggers. Each unit comprised six cages with three birds each. All chickens in the selected cages were scored for heat stress behaviors (n = 468). The body temperature and live weight of one randomly selected chicken per cage were measured on days 72 and 90 of age (n = 156). Two broilers per unit were randomly selected for blood corticosterone (n = 52). Although the sidewall zones were more exposed to sunlight than the inner zones, the data loggers showed no difference in the THI between these zones. Forty-five out of 468 chickens (9.6 %, 95 % CI 44.97–45.03) exhibited high heat stress behavior under high THI conditions. The THI and the zones showed no statistically significant associations with our observations of heat stress behavior and body weight change. The serum corticosterone levels of individual broilers measured in the afternoon were consistently lower than those measured in the morning; however, no significant difference was detected between the different zones of the house. In conclusion, the relatively small proportion of birds exhibiting heat stress behavior suggests that the native yellow-feathered broilers had considerable adaptive capacity to tolerate the hot and humid climate during summer in Hong Kong.