Heat stress remains a challenge for all living organisms on the globe, especially in the summer months. Heat stress is one of the most critical environmental stressors impairing the welfare and productivity of avian species, especially poultry. Heat stress evokes adverse effects, which include but are certainly not limited to immunosuppression, compromised gut health, oxidative damage, and reduced growth performance in birds. It is now increasingly evident that accurate measurements of heat stress before adverse effects are manifested signify a critical step towards developing effective heat stress mitigative strategies. Despite the growing body of knowledge on the impacts of heat stress on avian species and possible mitigative strategies, a comprehensive overview of the methods and biomarkers available to evaluate the occurrence of heat stress is lacking in the literature. Accordingly, this review provides a detailed overview of the physiological mechanisms underlying heat stress in birds. Additionally, the merits and limitations of current invasive and non-invasive measures of heat stress are highlighted. Furthermore, justifications for the adoption of novel non-invasive biomarkers of heat stress are also presented. Practical recommendations on the use of these biomarkers are equally offered. There is no doubt that the information provided herein would benefit both avian physiologists and ecologists by sparking the much-needed conversation in the quest for reliable heat stress biomarkers.