The trade-off between reproduction and innate immunity in the great tit (Parus major): a brood size manipulation experiment

Abstract

According to optimal resource allocation theory, reproductive investment is determined by the trade-off between present and future reproduction and offspring quality and quantity. The immune system, including innate immune function, protects the body against disease-related mortality. As such, it can be representative of the bird´s survival prospects, and has the potential to influence the outcome of both the present and potential future reproductive events. Innate immunity implies some maintenance costs, meaning that it also competes for resource allocation with other life functions, importantly reproductive success in adult birds and growth rate in nestlings. Through brood size manipulation, I experimentally altered resource allocation to the present breeding effort in great tits (Parus major) to test whether it would affect innate immune function (measured as agglutination, lysis and haptoglobin concentration) and mass in the adult birds. In the nestlings, I tested how rearing conditions would affect investment in innate immune function, and the trade-off between developing innate immunity and growth (mass, tarsus, wing). Increasing brood size resulted in an upregulation of the lysis activity in adult great tits, and a significant decrease in adult female body mass, likely associated with the cost of hard work. The development of innate immune function in nestlings was hindered in both experimental treatments, where these nestlings showed lower lysis as compared to the controls. Birds growing in a larger brood, potentially faced a trade-off between resource allocation to growth and immunity. Some evidence for different strategies in the sexes could be found as early as in the nestling stage. Overall, the manipulation affected the immune parameters differently between the treatment groups. This suggests that trade-offs with other activities might be involved. These results might be relevant for a scenario in which climate change, decoupling the synchrony between prey and caterpillar phenologies, results in a higher work rate for the adult birds.

Popular Abstract

Life can be hectic, and it is easy to get caught up in hustle culture. As it turns out, humans are not the only ones to experience this: for birds this translates into striving for reproductive success. During the breeding season a lot of resources are invested into reproduction: from elaborate songs and displays to the foraging effort to feed themselves, and eventually the brood. However, this ambition can come at the expense of the bird’s survival.
In the forest, birds are exposed to several risks, such as that of predation or infection, but they might also need to spend extra energy to warm up during cold nights. The strategies to withstand these challenges are also resource demanding. This means that to thrive while successfully raise a brood, the breeding pair needs to decide how much it is worth investing in reproduction, in other words how big a brood they can afford to raise. This is because the resources that go into reproduction cannot be spent to survive, hence producing a larger brood could be potentially detrimental for the adult birds. Brood size also affects the nestlings: in a large brood each nesting will have access to fewer resources, which will increase the strength of competition and might impact their growth and survival probabilities.
The constitutive immune system provides a good indication of the resources that the bird invests in survival. It is a component of the immune system that the bird maintains while healthy to avoid getting sick. However, not all life stages of a bird are the same, meaning that the need for a stronger constitutive immunity also varies. For example, during the breeding season, when a lot of energy is needed to feed a brood, birds may be forced to reduce the investment in immune function. But how big is this compromise?
I explored this question by carrying out a “brood size manipulation” on a passerine bird, the great tit (Parus major). In this type of experiment the original brood size, set naturally by the breeding pair, is altered by moving some of the nestlings from one nest to another. I also studied some broods that were left unvaried in size to compare the experimental conditions to the way it is “supposed to be “. I quantified the consequence of the brood size manipulation for the survival prospects of both the parents and their offspring by measuring three parameters of constitutive immune function.
The breeding pair was differently affected by the experiment. The body mass loss in the females providing large broods was likely the consequence of higher work rate to feed more nestlings. Males can reproduce with more than one female during the same breeding season. Being uncertain about their relatedness to all nestlings in the broods, they might have been unwilling to go above and beyond to raise them. Interestingly, parents of a large brood showed better immune defences, which could reflect a specific need under those conditions such as increased exposure to disease vectors through contact with more nestlings or more time spent outside the nest foraging.
Despite their hard-working mothers, nestlings from enlarged broods were significantly lighter and showed weaker immunity. This could have been due to limited access to food and higher competition. Male nestlings were larger than females, and likely required more food to sustain their growth. The fact that male nestlings from large broods showed worse immunity suggests that they might have struggled more from resource limitation.
Why do we need to know this? The answer has to do with climate change, but to understand this we need to take a step back: great tits are insectivorous birds, and feed on caterpillars. Due to the prey’s life cycle, following this diet requires some planning. Caterpillars emerge at budburst and stay on the tree for a limited period before they pupate in the soil. To make the most of this time frame, great tits lay eggs at budburst: in this way fully grown caterpillars will be abundant when parents are feeding their brood. This is a very important strategy because it ensures not only that the brood will be well fed, but also that the adult birds will be able to work moderately hard and maintain a good body condition. With spring temperatures on the rise, in some years, budburst and caterpillar emergence are occurring earlier. However, birds struggle to adapt to the new climatic conditions. Preparations for egg laying start around a month before the peak caterpillar abundance. Advancing the onset of these processes might be too challenging because the forest might not be rich enough in resources to sustain egg laying so early in the season. This potential mismatch with the caterpillar life cycle is not good news for the birds, as they would have to work harder to provision the brood and, according to the results of my study, these nestlings might experience delayed growth and weaker constitutive immunity.

• author: Ventura, Sofia
• supervisor: Jan-Åke Nilsson; Arne Hegemann
• organization: Degree Projects in Biology
• course: BION02 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9155747
• date added to LUP: 2024-05-30 14:41:01
• date last changed: 2024-05-30 14:41:01