Abstract Not Provided / Thesis / Master of Science (MSc)

Environmental gradients can help shed light on the evolution of life history strategies such as parental investment. Parental investment is crucial for the fitness of many species. In this thesis, I examine reproductive investment dynamics in the Eurasian blue tit (Cyanistes caeruleus) in the French Pyrenees and assess potential differences in reproductive measures across an altitudinal gradient that creates variation in environmental “harshness”. Further, I investigate fine-scale aspects of bi-parental care, such as investment tactics in current reproduction, and sex differences in contributions to offspring care. To do so, I used a mixture of observational and experimental data, collected over a total of six breeding seasons from over 500 blue tits nests. I showed that breeding conditions are “harsher” due to colder temperatures with increasing elevation, leading to changes in reproductive timing and output. I found that increasing altitude leads to decreased hatching success. Nevertheless, clutch size and brood mortality is comparable across the gradient. A shift to a lower, but qualitatively comparable reproductive output may be part of a slower “pace of life” strategies pursued at high relative to low altitudes. From experimental data, I also found that parental investment is positively linked across different phases within one reproductive attempt. Finally, in line with theory, a temporary brood manipulation revealed that parents balance the benefits and costs of reproduction by partially compensating for changes in brood size. Parents also responded in similar ways to brood size. Overall, the findings presented in this thesis highlight the importance of mechanisms to fine-tune reproduction to maximise reproductive fitness. I suggest that initial reproductive decisions such as timing and amount of offspring produced heavily shape the success of a reproductive attempt. These results have implications for current versus future reproductive trade-offs in life history theory, in particular for short-lived species.

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