Millions of birds migrate every year to escape winter, but spending time in a warmer climate does not save them energy, according to research by the Max Planck Institute of Animal Behavior (MPI-AB). Using miniaturized loggers implanted in wild blackbirds, scientists recorded detailed measurements of heart rate and body temperature from birds every 30 minutes from fall to the following spring—the first time the physiology of free flying birds has been quantified continuously at this scale over the entire wintering period.