Metabolic strategies of Eurasian blue tits and their influence on survival and reproduction

Abstract

Metabolism plays a crucial role in an animal’s life as it determines the amount of energy that can be allocated to growth, reproduction, and survival. The strategies an individual adopts to manage its energy budget have direct consequences to its fitness. How an animal allocates its energy will determine whether it can ensure its own survival while having a high reproductive success or neglect one for the benefit of the other. Three metabolic measurements, the Basal Metabolic rate (BMR) and the maximum metabolic rate during exercise (MMR) and thermogenesis (Msum), are commonly recorded to study these strategies. These measurements are known to be repeatable and believed to be related to fitness. Theory suggests the existence of a phenotypic link between all metabolic components, and previous empirical evidence provide heterogenous results. A few studies reported that Msum and BMR could influence survival and breeding performance in birds with results varying depending on population, species, or year. In this study, I measured BMR, MMR and Msum in Eurasian blue tits (Cyanistes caeruleus) to determine their metabolic phenotype, focusing on the relationships between these metabolic traits and their influence on overwinter survival and breeding performance. I found positive correlations between whole-animal BMR and maximal metabolic rates. However, when using mass-independent values, I found no significant correlations, suggesting that the relationship was mostly driven by body mass. Therefore, the processes contributing to minimum and maximum metabolic rates may function independently. I found a positive relationship between MMR and Msum which remained significant when controlling for body mass, suggesting that blue tits can devote energy to both the capacity for thermogenesis and exercise without needing to favor one at the expense of the other. This finding also highlights that there are similar mechanisms underlying Msum and MMR. However, none of the metabolic measurements were correlated with survival probability, contrary to other studies. There was no correlation between any of the metabolic rates and the date of first egg-laying or clutch size. Parental Msum was significantly related to nestling wing length but not to tarsus length or body mass. Nestling body mass and tarsus lengths probably had already reached their final size by the time of measurements as they were relatively large and had limited variation between chicks. However, the primaries continue to grow until after nestlings have fledged. This study highlighted the uncoupling of the mechanisms contributing to minimum and maximum metabolic rates which might provide blue tits with great flexibility to thrive in environments with fluctuating requirements. The relationship between parental Msum and nestling wing length is intriguing and would benefit from additional studies to understand the nature of this association.

Popular Abstract

The energy management strategies of blue tits

A bird body can be seen as a car that needs fuel to function. Food is the fuel. It is needed for the function of vital organs (brain, heart or lungs), to keep the body warm and to engage in daily activities such as flying, building nests or feeding offspring. The amount of food a bird can get is limited so it must be efficiently distributed between the different tasks. How do birds distribute their energy? When food is lacking or when they need to work harder, do they neglect certain tasks for the benefit of others? And if they do, does it impact their survival chances and/or their reproductive success?

To answer all these questions, I caught several blue tits in the middle of winter and measured how much energy they use (their metabolic rate) for different tasks. I measured the lowest amount of energy they need to support their organ function when they were resting in a neutral temperature and not digesting (the Basal Metabolic Rate or BMR). I also recorded the maximum amount of energy they can use when they need to maintain their body temperature when exposed to cold temperatures (Summit Metabolic rate or Msum). And, to know the maximum amount of energy they can use for sustained physical activities, I let them exercise in a wheel (Maximum Metabolic rate or MMR).

Now, imagine that you are a blue tit and consider your body works as a car. When the car is on and not moving it uses some fuel (this is BMR). If you turn on the heater or start to drive (Msum and MMR, respectively), the car will need more fuel. How do you handle this increased need for fuel? Do you prioritize the heater (or driving around) by sending most of the fuel to it and reducing the amount of fuel sent to the engine? With this scenario you take the risk of damaging your engine as it will not receive enough fuel to properly function. Or do you get a larger tank allowing to take in more fuel to use the heater (or drive around)? In this case you do not damage your engine, but your expenses will increase as the maintenance costs of your tank will be higher. Lastly, if you use the heater, can you also afford to drive around or do you need to choose between the two? In case you need to choose, which one will be the most important?
Blue tits need to answer these questions and decide how to distribute their fuel (energy/food) to maintain their organs (running the engine), keep their body warm (turn on the heater) and rear offspring (drive around). They must choose carefully as it may affect whether they survive and their reproductive success.

What happens for blue tits is that they have a BMR relying on a different mechanism than MMR and Msum. Some individuals can have a high BMR, MMR and Msum while others have a low BMR but high MMR and Msum and the opposites are also true. On the other hand, MMR and Msum rely on similar mechanisms which means blue tits do not need to choose between keeping a stable body temperature or investing in reproduction. This suggests that when their capacity to produce heat increases, their capacity for intense physical activity also increases, and vice versa.

Now that we have a better understanding of how blue tits manage their energy, it would be interesting to know how it influences their survival chances and their reproduction. In the spring, I tried to find the blue tits who had their metabolic rates measured in the winter and found that about 30% of them survived. There were some variations in their ability to tolerate the cold temperatures, in their BMR and Msum but, none of these parameters seem to have impacted their chances of surviving. However, Msum was linked with nestling wing length. Parents which had a high capacity to produce heat had chicks with long wings. Long-winged offspring leave their nest sooner than those with shorter wings and they are good at flying which increases their chances of surviving.

This study provides a better understanding of the energy management strategies of blue tits but how they directly affect survival and reproduction is still a little mystery. Understanding how birds manage their energy can help understand their behavior, whether their survive or successfully reproduce. By identifying where blue tits spend most of their energy, we can instore conservation policies fitting their needs, whether it involves protecting habitat with abundant food resources or offering shelters protecting from the cold and predators.
Global warming might also affect bird energy management strategies. For example, we saw that Msum is linked with the production of long-wing offspring. If winters become warmer and having a high Msum is no longer necessary, bird might decrease it to save energy. Will this impact the size of their offspring? Or will they be able to redirect the saved energy to maintain the quality of their offspring?
While we still do not have answers to all our questions, we can safely say that blue tits are highly skilled when it comes to managing their energy and thrive in their environment.


Master’s Degree Project in Biology, Animal Ecology, 45 credits, 2024
Department of Biology, Lund University
Advisors: Jan-Åke Nilsson, Elana Rae Engert
Department of Biology, Lund University

• author: Grange, Marion
• supervisor: Jan-Åke Nilsson; Elana Rae Engert
• organization: Degree Projects in Biology
• course: BION02 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9176939
• date added to LUP: 2024-10-22 16:14:01
• date last changed: 2024-10-22 16:14:01