Eurasian blackbirds rest earlier when they are sick.

When animals are invaded by pathogens, the immune system initiates a series of coordinated responses involving behavioral, physiological, immunological, and metabolic adjustments, collectively known as the acute phase response (APR). While this response helps combat infection, it also incurs substantial energetic costs, increasing metabolic demands by approximately 5–15%. For free-living animals with unpredictable food resources, such energy expenditure can represent a significant burden.

As a result, animals facing infection must often balance the trade-off between fighting disease and maintaining daily activities. Individuals undergoing the APR may reduce behaviors such as foraging, courtship, or migration in order to conserve energy. Although these adjustments may enhance short-term survival, they can negatively affect long-term fitness. Therefore, understanding the behavioral trade-offs induced by immune responses is crucial for interpreting how host–pathogen interactions influence the ecology and evolution of different species.

House finch（image source：Quist，CC0 1.0 公共領域）

A study on captive House finch（Haemorhous mexicanus）showed that infected individuals exhibited a marked reduction in perching, feeding, walking, and flying activity six days after bacterial infection, but these effects disappeared by day 22. In contrast, studies on free-living animals remain limited. So far, observations suggest that some passerine birds show reduced activity, decreased territorial aggression, and less singing during the early stages of infection. Other studies have also indicated that natural infections in wild birds and mammals may lead to reduced movement, foraging activity, and migration.

However, most of these studies lack continuous data spanning from the onset of infection to recovery. As a result, the duration, magnitude, and regulation of sickness behaviors in wild animals remain poorly understood.

To address this gap, a field-based study employed accelerometers to continuously monitor behavioral changes in birds experiencing a simulated bacterial infection for up to 48 days, aiming to assess the effects of the APR on behavior.

Eurasian blackbirds（image source：Dion Art，採用 CC BY-SA 4.0 授權）

The study focused on populations of Eurasian blackbirds（Turdus merula） in Lund, Sweden. Birds were captured during late summer to autumn in 2019 and 2020. At the time of capture, most individuals had already begun molting, while some that had not may still have been caring for nestlings. Upon capture, blood samples were collected, body measurements were taken, and individuals were aged and sexed based on plumage characteristics.

Each bird was then equipped with an accelerometer and a radio transmitter to record long-term activity patterns. The accelerometers recorded movement five times per hour and converted this into an activity index ranging from 0 to 60, with higher values indicating greater activity. This allowed researchers to precisely track daily behavioral changes.

To simulate infection, the 45 captured birds were divided into control and experimental groups with balanced sex ratios. Birds in the experimental group received an injection of lipopolysaccharide (LPS) derived from Escherichia coli to induce an immune response, whereas control birds received no injection to avoid confounding effects caused by the injection procedure itself. The focus of the experiment was not on a specific pathogen, but rather on comparing behavioral differences between individuals with and without an immune response.

Of the 45 deployed accelerometers, 23 were successfully retrieved, though one failed to record data, leaving usable data from 22 individuals. Among these, 12 belonged to the control group (3 females and 9 males), and 10 to the experimental group (5 females and 5 males).

Notably, one individual in the experimental group exhibited unusually high activity—approximately three times higher than others—during the first 20 days, particularly within the first 24 hours. As no methodological issues were identified, this individual was retained in the analysis, making the results more conservative.

The results showed that sickness behaviors in wild Eurasian blackbirds lasted much longer than previously assumed. On average, activity levels in the experimental group decreased by 19% and this reduction persisted for more than 20 days. The most pronounced decline occurred within the first 24 hours, reaching up to 59%. Importantly, these results reflect changes in activity patterns rather than activity intensity during active periods. In other words, when active, experimental and control birds exhibited similar levels of activity, but immune-challenged birds tended to cease activity earlier in the evening. This suggests that changes in activity patterns may be a more sensitive indicator of health than activity intensity alone.

Previous studies have shown that sickness-induced behavioral changes in birds typically last between 3 and 9 days, regardless of body size. Several explanations have been proposed for this discrepancy. First, many earlier studies were conducted on captive individuals, where spatial constraints may limit the expression of natural activity, thereby reducing observable differences between control and experimental groups. Second, day length may influence sickness responses; longer daylight hours, such as those present during late summer when this study was conducted, may amplify behavioral changes. Finally, no previous studies have monitored free-living individuals continuously over several weeks, resulting in a lack of comparable long-term data.

For wild animals, repeated occurrences of infection-induced behavioral changes may have significant ecological consequences. These include reduced parental care, insufficient energy for migration, inadequate energy storage for night-time or winter survival, and increased vulnerability to predators during dawn and dusk. The observation that Eurasian blackbirds maintained normal activity levels during peak activity periods but reduced their overall active time may represent an adaptive strategy. This pattern allows infected individuals to maintain essential behaviors such as foraging and social interactions during critical periods, while conserving energy at other times. Compared to remaining lethargic during active periods, this strategy may also reduce predation risk.

Overall, this study demonstrates that behavioral changes following infection in wild animals may persist much longer than previously recognized. However, it is important to note that this study simulated infection rather than involving actual pathogenic infection; therefore, the effects of real infections may be even more pronounced.

（Author: Bai Leng）

Reference：

Lennon, R. J., Ronanki, S., Hegemann, A . (2023). Immune challenge reduces daily activity period in free-living birds for three weeks. Proceedings of the Royal Society B Biological sciences.