The incubation of eggs is a critical stage in the life of a bird. Although the eggs need the warmth of their parent's body to develop, most birds need to leave their eggs for brief periods to forage for food. When the adults are gone, the egg's temperature will fall and might even reach the temperature of the surroundings if the parents don't return soon. And being exposed to the cold is no fun for the embryos; studies have found that suboptimal incubation temperatures can lead to fewer chicks hatching and lower survival rates. However, most studies on this topic have investigated the effects of a stable incubation temperature, which is seldom experienced by the embryos under natural conditions; little is known about the impact of the parent's periodic absences on their embryos. So, Benjamin Burrows and colleagues from Trent University in Canada set out to test the influence of a variable incubation temperature on the life performance and immune function of Japanese quails.
The team incubated quail eggs at temperatures that fluctuated around an average temperature of 36°C over a continuously repeated 3 h cycle. This was achieved by switching off the incubator – which was set to 37.5°C – for less than an hour, allowing the incubation temperature to drop to 28°C, before the incubator was switched back on again to warm up. They then sampled blood from the chicks at three time points – 5 days after hatching, when the birds reached 20 days of age and after reaching adulthood – and tested the ability of the chicks’ blood to kill Escherichia coli, a common bacterium that can cause diarrhea. The researchers then compared the immune function of the chicks that had been warmed and chilled alternately with that of chicks that were raised at a constant low incubation temperature of 36°C or at a constant 37.5°C, which is commonly used as incubation temperature in quail farms.
When the researchers compared how the immune system of the three groups of chicks developed, they found that the birds’ ability to fight the E.coli infection increased as the chicks grew and initially they could see no clear difference across the groups; all of the chicks had similarly strong immune systems at the age of 20 days. However, when they focused on the long-term effects that the incubation temperature had on adult birds, they saw that the quails that had been exposed to the cycling temperature as embryos had the least ability to fight infections. Their immune function was markedly lower than that of the quails that were raised at constant 37.5°C, while the immune function of embryos incubated at a constant 36°C was only slightly worse than that of birds raised at the optimal temperature.
Although it is not yet understood how variation of the incubation temperature causes a decline in immune function, it is clear that incubation conditions can have a large effect on the chick’s immune system and will probably affect the survival of the adult birds. The study emphasizes the importance of environmental conditions, such as food availability or predator presence, which influence the amount of time that parents spend on the nest, with implications for the life performance of their offspring as adults.