The role of immune function in migrating birds

Abstract

The immune system is the main defense barrier against infections but exerts selection pressures on other life history traits. Recent studies have given importance to understanding the role of the immune system in birds, and it has been hypothesized that during migration, birds either increase their immune function, as they might encounter novel pathogens, or decrease it to account for the cost of migration. Regulating immune function along with long distance flight is costly and requires birds to stop periodically to refuel and recover. Evidence suggests that as they refuel, their immune parameters improve over time. However, relatively few studies investigated if there is a relationship between fuel load and baseline immune function. The first part of my thesis investigates if there is a relationship between fuel load and baseline immune function (innate and acquired) in four bird species during spring migration. There was no correlation between fuel load and innate as well as acquired immune parameters. Similarly, no sex differences were found in immune function in any of the species. However, there was a significant negative effect of Julian day on immune function in some species. Besides the cost maintaining the baseline immune function, there is an energetic cost associated with activating the immune system in case a bird encounters a pathogen. Birds undergo physiological and behavioural changes while mounting an immune response and these effects can influence other fitness-related traits. The second part of my thesis investigates if there are short-term and long-term effects of mounting an immune response on the activity of free-living blackbirds (Turdus merula) in Lund using accelerometer data. Immune challenged birds exhibited an overall lower activity compared to control birds for up to 20 days post immune challenge. However, this effect was only significant in birds that were immune challenged in the morning rather than evening, and in birds that were immune challenged during early autumn rather than mid-summer. The combination of these two case studies shows that, to understand the life history of birds, it is important to consider how much they invest into various aspects of the immune function in the presence or absence of an infection

Popular Abstract

How birds migrate without falling sick

Every year billions of birds migrate between their wintering and breeding grounds, and they face many challenges including harsh weather conditions, scarcity of food and increased risk of predators. Furthermore, they may also encounter novel pathogens. Like all other animals including us humans, birds need to have a well functioning immune system to protect themselves against pathogens. The immune system has two states, it can either be a baseline state, which indicates the capacity to fight an infection or it can be in an activated state, which is when an animal is actively fighting an infection.

Migrating takes energy and birds need to stop periodically to eat (refuel) and recover. It is known that during autumn migration, birds improve their immune system as they refuel and birds with bigger fat stores (the main energy source in birds), have better immune function compared to birds with smaller fat stores. However, it is still unclear if this is true for spring migration as well. Autumn and spring migration pose different challenges to birds. In autumn, birds migrate to better environmental conditions with enough food availability. In spring, birds migrate back to their breeding grounds, and birds arriving earlier have an advantage of finding a better mate and territory than birds arriving later. Thus, birds stop over less often to migrate faster. I studied if fat stores and immune function are linked in spring migration. Four bird species, Chaffinches, Dunnocks, Wheatears and Song thrushes, were captured on Helgoland, which is a small island where birds stop over on their spring migration. I found no correlation between fuel load and immune parameters in any of the species. This could mean that birds migrating in spring invest more energy into other traits, e.g. that aid in reproduction, rather than regulation of immune function. However, I also found that birds arriving at the stop over earlier had better immune function than birds arriving later, which suggests that immune function is also linked to fitness and overall genetic quality.

Besides maintaining baseline immune function, birds also need to activate their immune system when being infected with a pathogen. During such an immune response, birds undergo physical, physiological and behavioural changes. I studied the activity of birds during a mimicked infection using a small data logger that recorded the activity of birds. I found reduced activity in immune challenged birds for up to 48 hours, which is in line with previous studies. Interestingly, I also found this effect lasting for up to 20 days. No study so far has found such a long lasting effect, and it reinforces the severity of the costs of an immune response and how birds trade off their energy from other activities when they are infected.

In conclusion, my studies show that in spring migration immune function is not linked to energy levels in birds. However when birds get sick, they reduce their activity levels for up to 20 days, which then may influence their other activities. Understanding those costs and benefits of immune function may help to understand the behaviour and ecology of birds, especially during migration. .


Master’s degree project in biology, credits: 60
Department of Biology, Lund University
Advisor: Arne Hegemann (Division of aquatic ecology)

• author: Ronanki, Shivani
• supervisor: Arne Hegemann
• organization: Degree Projects in Biology
• course: BION03 20202
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9066765
• date added to LUP: 2021-10-12 15:36:57
• date last changed: 2021-10-12 15:36:57