Ecological harshness mediates reproductive trade-offs in a great tit population

2025 · doi:10.32942/x26629
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The paper analyzes 58 years of detailed demographic and reproductive data from a great tit population to study how continuous variation in environmental harshness affects reproductive trade-offs, using the covariance reaction norm model to characterize changes in the offspring quantity–quality correlation across years. The authors find that the correlation indicative of an offspring quantity–quality trade-off is largely stable over time, with only minimal association to ecological harshness during the breeding season. However, the model indicates that correlations between offspring mass and future offspring recruitment are positive only under harsh environmental conditions, suggesting context-dependent fitness benefits of producing larger offspring. This paper does not explicitly discuss endometriosis or adenomyosis; it was included in the corpus via a keyword match in the upstream search index.

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Abstract

Lack’s seminal work on bird clutch sizes has spurred expansive research on reproductive trade-offs, especially focusing on offspring quantity–quality trade-offs and the potential fitness consequences for the parents. The environment is a critical driver of the expression of individual reproductive traits, influencing them through plastic responses. However, the plasticity of reproductive trade-offs themselves across environments has seldom been studied, and these studies were often limited to experimental approaches and dichotomous environments. Using 58 years of detailed data from a great tit population, we employ the recently developed ‘covariance reaction norm’ (CRN) model to explore how continuous environmental variation influences the shape of reproductive trade-offs among individuals. Our analysis reveals that the offspring quantity–quality trade-off is predominantly stable across years, with minimal variation linked to ecological harshness during the breeding season. However, the CRN also demonstrated that the among-mother correlation between offspring mass and future offspring recruitment was positive, but only under harsh environmental conditions, suggesting that producing larger offspring provides fitness benefits when breeding conditions are suboptimal, which may reflect the importance of size for early-life competition. Altogether, this work highlights that there is temporal variation in some of the phenotypic correlations, mostly driven by environmental conditions, which shape the expression of offspring investment across breeding seasons. Our study shows the benefits of exploring old ecological questions in the light of new statistical methods, highlighting the importance of understanding how environmental variation shapes the expression of life history trade-offs and the evolution of plasticity in reproductive strategies.
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Abstract

Lack’s seminal work on bird clutch sizes has spurred expansive research on reproductive trade-offs, especially focusing on offspring quantity–quality trade-offs and the potential fitness consequences for the parents. The environment is a critical driver of the expression of individual reproductive traits, influencing them through plastic responses. However, the plasticity of reproductive trade-offs themselves across environments has seldom been studied, and these studies were often limited to experimental approaches and dichotomous environments. Using 58 years of detailed data from a great tit population, we employ the recently developed ‘covariance reaction norm’ (CRN) model to explore how continuous environmental variation influences the shape of reproductive trade-offs among individuals. Our analysis reveals that the correlation potentially indicative of the offspring quantity–quality trade-off is predominantly stable across years, with minimal variation linked to ecological harshness during the breeding season. However, the CRN also demonstrated that, despite some uncertainty associated with the results, the correlation between offspring mass and future offspring recruitment was positive, but only under harsh environmental conditions, suggesting that producing larger offspring provides fitness benefits when breeding conditions are suboptimal, which may reflect the importance of size for early-life competition. Altogether, this work highlights that there is temporal variation in some of the phenotypic correlations. This is a consequence of variation in offspring investment across breeding seasons, which is mostly driven by environmental conditions. Our study shows the benefits of exploring old ecological questions in the light of new statistical methods, highlighting the importance of understanding how environmental variation shapes the expression of life history trade-offs and the evolution of plasticity in reproductive strategies. DOI https://doi.org/10.32942/X26629 Subjects Ecology and Evolutionary Biology, Evolution, Population Biology

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Dates Published: 2025-04-06 00:44 Last Updated: 2026-02-20 18:20 Older Versions License CC BY Attribution 4.0 International Additional Metadata Data and Code Availability Statement: The formatted data, as well as the R and Stan code necessary to reproduce the results are available on GitHub https://github.com/lbiard/tradeoffs_parus_major and will be archived on Zenodo. The raw datasets analyzed in the current study are available in the SPI-Birds Database (study name: Wytham Woods, study ID: WYT, version: 1.0.0, data custodian: Ben Sheldon). Language: English

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