Birds like female blue tits control the sex of the eggs they lay, but this crucial decision hinges on the health of their mates. If the males are more active and healthy, the females produce more sons than daughters. This was revealed in a study conducted by researchers from Lund University in Sweden. While studying blue tit-nestlings, they recorded the sex of the nestlings in each brood, and directly related it to the state of the father's health and on the chances of their survival till the next winter.
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