Thermoregulatory Mechanism in Birds
In the current trend of changing climate, birds are posed with challenges to maintain thermoregulation such as increased occurrence of extreme weather events, lower ability to predict climate and much increased mean temperature.
Across wide ranges of changes in environment, birds usually maintain a stable body temperature of about 41.02 ± 1.29 °C during active phase at rest, through various physiological, behavioural and morphological adaptations. Thermoregulatory system of birds is comprised of three parts, namely a sensory part comprising of thermoreceptors, osmoreceptors and baroreceptors, an integrating part consisting of thermoregulatory centre occupying preoptic anterior hypothalamus and the command part involving temperature control mechanisms such as non-shivering thermogenesis, shivering thermogenesis, evaporative heat loss mechanism (cutaneous and respiratory heat loss), peripheral vasodilatation or constriction and behavioural mechanism of adaptation. By evolution, birds are benefitted with its morphological features such as feathers and plumage, changes in peripheral blood circulation and bodily fat distribution for thermoregulation.Majorly, behavioural method of thermoregulation is expressed by the unfeathered portions of foot, leg, head comb, wattle and distal segments of wings. For heat loss, the behavioural modifications exhibited by the birds include separating and holding the wings from the body for enhanced air circulation, lying down with forward stretched head and neck, increased preference for sitting on the soil and increased frequency of squashing of breast feathers. Thermal panting behaviour is commonly observed in the birds experiencing heat stress. Ruffling of head and back feathers, avoiding sun’s direction and facing away from the sun and shade-seeking are the kind of behaviours observed in many birds.
Excessive drinking of water, tucking of head under the wings and splashing excess quantities of water over and above their feathers are also expressed by birds as a method of behavioural adaptation to heat stress. In contrary, there are also means of heat conservation behaviour elicited by birds such as hunching up with feathers stretched out, completely fluffing out and sleeping most of the time with tucked head under the wing and sunbathing. A unique phenomenon of torpidity or hibernation is shown by few bird species. Huddling and seeking warm environment nature of behaviour are observed in case of small birds. At extreme conditions, birds also utilize migration as a method of adaptation to tackle changing weather.Basically, below the low critical temperature, heat generation is attained by non-shivering and shivering thermogenesis and in conditions above critical temperature, heat loss happens by evaporative cooling mechanism majorly through panting, cutaneous evaporation, gular flutter and lingual flutter. A novel thermoregulatory mechanism that enhances thermal tolerance was identified in desert inhabited passerines, called as vocal panting. It is a very high pitched, quick rhythm vocals generated during panting. These heat-calls are observed to be produced as a side effect of vibration of a segment of respiratory tract. This behaviour increases the area available for evaporative water loss and believed to be associated with thermoregulatory mechanism for improving heat tolerance.
Thermogenesis is governed by thyroid hormones. Particularly, thyroid hormones regulate thermoregulation below thermoneutral zone (facultative thermogenesis) and are involved mainly in non-shivering thermogenesis. Association of corticosterone with body temperature and environmental temperature extended over summer and winter. Other hormones such as neuropeptide Y, melatonin, leptin, ghrelin and arginine vasotocin are also reported to have a role in avian thermoregulation.
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