The researchers studied birds given a simulated bacterial infection in order to stimulate their immune system. The birds were then compared with birds whose immune system was not stimulated – and their activity was measured for several weeks using miniature data loggers.
“We found that the birds whose immune system was stimulated had reduced activity for three weeks, which is much longer than we expected. We could also see that the 'sick' blackbirds stopped their activities almost an hour earlier in the evenings compared to the control group”, says Arne Hegemann, biologist at Lund University.
Previously, researchers assumed that effects from a compromised immune system only take a day or two to resolve. The new study shows that it takes much longer to recover; and that it affects the duration of activity per day rather than the level of activity throughout the day.
“First of all, it is important to understand what happens to wild animals when they are affected by disease. Even mild ailments and short disease spans can have far-reaching consequences for animals, not least because it affects their everyday life”, says Arne Hegemann.
Whether the birds were sleeping or just sitting still is unknown, but the study shows that sick birds go to bed earlier, just like sick people do.
“The difference is that when we humans are sick and have symptoms such as fever, reduced appetite or body pain, we may stay at home for a day or two and then return to normal life. Wild animals have the same symptoms but for them the consequences are greater. If small birds get sick and have 45 minutes less time per day to look for food, it can be the difference between life and death for both them and their young ones”, concludes Arne Hegemann.
The study "Immune challenge reduces daily activity period in free-living birds for three weeks" is published in Proceedings of the Royal Society B.