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Magpie Parents Appear To Induce Mortality Of Unwanted Chicks

Posted on 24. June, 2010.

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According to a study published in Journal of Avian Biology, magpies favour some of their nestlings over others but in a strange and unique manner. Dr Sang-im Lee and her colleagues at Seoul National University and Ewha University, South Korea, examined the mortality of magpie nestlings and found 'rarer-sex disadvantage'. This means that sons die out more when more daughters are present in the nest, and daughters die more when more sons are in the nest.

"This pattern is not compatible with previous hypotheses," says Dr Lee. In species such as magpies where one sex is larger than the other (in magpies, sons are larger than daughters), mortality of nestlings was often attributed to two mechanisms: larger sex vulnerability or size dominance. Larger sex vulnerability predicts that the nestlings of the larger sex die when environmental conditions deteriorate and parents cannot deliver sufficient amount of food. Size dominance predicts that the nestlings of the larger sex dominate over the nestlings of the smaller sex.

Dr Lee says that she first expected that magpies, just like many other birds, would fall into one of these previously described mechanisms, but her research unexpectedly revealed 'rarer-sex disadvantage' – a phenomenon not previously observed in birds.

According to Dr Lee's hypothesis, the observed mortality patterns of chicks may be related to how parents behave toward the nestling. Because the nestlings of the rarer sex are more likely to die, the sex composition of the nestlings at the end of the feeding period becomes more biased towards the direction of the original bias observed when the nestlings hatched. This bias is nest-specific. The parents appear to induce mortality of nestlings to reach their ideal brood sex composition during the five-week long feeding period.

It has already been known that several species of birds (e.g. tits, sparrows, etc.) are able to adjust their brood sex ratio as early as egg laying. Magpies are either unable to do this or do not – possibly because of their longer feeding periods making it harder to predict environmental conditions.

According to Dr Lee, evidence suggests that parents in good condition or with better skills rear broods that are biased towards sons, and poorer parents rear broods with more daughters. In many bird species, the quality of parents is reflected in their breeding time. It is usually assumed that the parents who breed earlier are the better ones in terms of health and also at rearing children. When the authors checked the laying dates of the parents, they found that, although the early parents did not significantly produce more sons at the beginning, they induced mortality of daughters to make their broods contain slightly more sons later when the nestlings are ready to leave their nests. It may be that only good parents can afford sons, who are known to require 10% more food than daughters.

The complete article can be found in Journal of Avian Biology, 2010; 41 (2), 139 OI: 10.1111/j.1600-048X.2009.04749.x