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Great tits’ temperature-related hormone responses did not predict breeding success

Sharp detailed photograph of a wild male great tit in natural habitat
Research: Physiology, Reproduction & development

Great tits (Parus major) use the hormone corticosterone to help regulate their physiological responses to changing demands. Temperature fluctuations can alter hormone levels, potentially influencing how birds cope with their environment while raising young. In a nine-year study of a wild population, researchers asked whether birds whose hormone levels responded differently to temperature also differed in reproductive success.

During nestling rearing, the team repeatedly measured both baseline corticosterone and levels induced by a stress response. The measurements allowed them to distinguish each bird’s average hormone level from its plasticity: how strongly its hormone levels changed as air temperature varied. These individual patterns are known as reaction norms.

Birds consistently differed in their average corticosterone levels and the strength of their temperature-related responses. But neither of those individual differences was associated with how many chicks fledged or with brood mass. In other words, birds with more temperature-responsive hormone levels did not achieve better reproductive results in the measured outcomes.

This is interesting because physiological plasticity could, in principle, provide an advantage in variable conditions and thereby be favoured by natural selection. The study found individual variation on which selection could act, but no evidence that it translated into reproductive advantage under the temperatures experienced. More extreme conditions might produce different relationships; that remains to be tested.

Sources and image credit

  1. Individual variation in glucocorticoid plasticity to temperature is not associated with reproductive success in a wild songbird populationPrimærkilden. Det tilgjengelige sammendraget beskriver niårsstudien, hormon- og reproduksjonsmålene, hovedresultatene og forfatternes tolkning.
  2. Great tits differ in glucocorticoid plasticity in response to spring temperatureTidligere, kortere studie av temperaturrelatert kortikosteronplastisitet i bestanden, omtalt i kandidatens sammendrag som relevant tidligere arbeid.

Image: Male Great Tit at Otmoor RSPB Reserve, England. Photo: Charles J. Sharp / Wikimedia Commons, CC BY-SA 4.0. · CC BY-SA 4.0 · Source

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