Heat stress in poultry — signs and how to prevent it
Birds have no sweat glands and don’t sweat — they shed excess heat mainly by panting. When temperature and humidity rise together, this cooling stops being enough and the flock begins to suffer. We explain how to spot heat stress, what makes it worse, and what to do so a heatwave doesn’t end in losses.
verifiedFrom the team that has organised work on poultry farms for years.
Heat stress is a state in which a bird produces more heat than it can give back to its surroundings. Poultry don’t sweat — their skin is covered in feathers and they have no sweat glands at all. They shed excess heat mainly through the respiratory tract: they speed up their breathing and pant with an open beak, evaporating water from the airways. As long as the air is cooler and dry, this works. In the heat, especially at high humidity, it can no longer keep up — and that is when the trouble starts.
Why is heat so dangerous for poultry?
A heavy, fast-growing bird produces the most heat and finds it hardest to shed — which is why broilers in the final phase of the cycle and hens at peak production are the most exposed. The risk is not set by temperature alone, but by its combination with humidity. The more humid the air, the harder it is to evaporate water by panting, so the same thermometer reading means a completely different load on a dry day and on a muggy one. This is captured by the THI (temperature plus humidity): on a humid day heat stress appears at a lower temperature than on a dry one. That is why a poultry house is judged by temperature and humidity together, not separately.
How to spot heat stress and what makes it worse
Behaviour changes first, production follows. The sooner you read the signals and the risk factors, the smaller the losses.
Panting and spread wings
The first and clearest sign. Birds breathe fast with an open beak (panting), move apart and hold their wings away from the body to bare less feathered skin and shed more heat. A flock that is normally active goes quiet and sluggish.
Less feed, more water
In the heat birds cut their feed intake — digestion itself produces heat, so the body avoids it. At the same time water intake rises sharply. A sudden jump in water use together with a drop in feed eaten is one of the surest signs that a flock is overheating.
Apathy, crowding along the walls
Overheated birds stop walking, lie down and seek cooler spots — along the walls, under the drinking lines, in a draught. Activity falls, and the flock may crowd wherever it feels air moving. That is a sign the cooling is spread unevenly across the house.
Lower lay and growth
The production effects come quickly: hens lay fewer eggs with thinner shells, and broilers grow slower and convert feed worse. Some of the loss only shows up afterwards, in weaker cycle results — which is why heat costs money even when there are no deaths.
Severe overheating and deaths
If the bird’s body temperature keeps rising, heat stroke sets in: tremors, loss of balance and finally deaths — most often in the heaviest birds and in the hottest afternoon hours. This is the ultimate and most expensive outcome of uncontrolled heat stress.
Risk factors: THI, age, stocking
The severity depends on several things at once: on the THI (temperature plus humidity), on the age and weight of the birds (heavier ones heat up more), on stocking density (more birds means more heat and moisture in the same air) and on the building’s insulation and ventilation. The more of these act together, the faster and harder the heat hits.
Protecting the flock from heat step by step
- 1
Lead with ventilation and air movement
The foundation is air exchange and its speed over the birds. Moving air acts like wind on the skin — it carries heat away and makes the bird feel a lower temperature than the thermometer shows (the wind-chill effect). In the heat you raise the capacity of lengthwise (tunnel) ventilation, so the air really flows over the flock and doesn’t just turn over near the ceiling.
- 2
Switch on evaporative cooling
When ventilation alone is not enough, evaporative cooling helps: cooling pads at the air inlet or misting. Evaporating water lowers the temperature of the incoming air. It works best in dry air — on muggy days take care not to raise the humidity too far, because that worsens rather than improves the birds’ cooling.
- 3
Mind the insulation, the roof above all
Most heat enters a poultry house through a sun-baked roof. Good insulation of the shell, a light-coloured covering and effective shading of the inlets limit how much the building heats up, before the fight with ventilation even begins. This is the cheapest line of defence — it works every day, with no electricity.
- 4
Provide cool water and clean lines
In the heat water matters more than feed. Make sure it is cool and freely available along the whole line. Flush water that has warmed up in the pipes (run the lines through), because birds drink less when it is warm. Check that the pressure and the number of drinkers keep up with the higher intake — a thirsty flock can’t queue at the drinker.
- 5
Support watering and feeding
Panting makes birds lose salts and upsets the body’s balance, so in the heat electrolytes are used, and often betaine in the water too, to help birds hold their water balance. On the feed side: move feeding to the cooler hours (early morning and evening), because digestion generates heat, and avoid large portions during the peak of the heat.
- 6
Have a heatwave plan ready
Don’t improvise at 35 degrees. Decide in advance at what temperature and THI you raise ventilation, when you switch on cooling, how you shift feeding hours and who checks the flock in the hottest part of the day. Test the fans, alarms and backup power ahead of time — in the heat a ventilation failure is a matter of minutes, not hours.
Frequently asked questions about heat stress in poultry
At what temperature does heat stress begin?add
There is no single threshold, because it is set by the combination of temperature and humidity, as well as the age and weight of the birds. Adult, heavy poultry are comfortable at lower temperatures than chicks and start to overheat just above the comfort zone, especially when the air is humid. So instead of looking for one number, watch the birds’ behaviour and read temperature together with humidity (THI).
What is the quickest way to tell birds are overheating?add
Panting with an open beak and wings held away from the body, a quiet and motionless flock, and a sudden jump in water intake while feed eaten falls. These signals come before any drop in lay or growth, so if you see them, react at once — raise ventilation and check access to cool water, don’t wait for worse symptoms.
Why does humidity matter as much as temperature?add
Because a bird cools itself by evaporating water as it pants, and in humid air that evaporation goes slower. So the same thermometer reading means a different heat load on a dry day and a muggy one. The THI ties this together: at high humidity heat stress appears at a lower temperature. It is also why misting on a humid day must be dosed carefully — by adding moisture you can make things worse.
What should I do during a sudden heatwave?add
Act on a plan set in advance: raise ventilation and air speed over the birds, switch on evaporative cooling if needed, secure cool water and flush the warmed-up drinking lines, add electrolytes, and shift feeding to the cooler hours. Check the flock more often in the afternoon peak and make sure the ventilation and backup power are working — they are what save the flock when the temperature won’t drop.
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