Free-living great tits differ considerably in the level of stress hormones in their blood. A research project at the Max Planck Institute for Biological Intelligence studied the birds over several years. The scientists observed large differences in how strongly glucocorticoid hormone levels fluctuated in individual great tits, as the birds experienced different environmental temperatures. Such differences among individuals can make it easier for bird populations to adapt to changing conditions—such as more frequent temperature extremes due to climate change.
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Source: Phys.org