A team of researchers has found that sibling chicks are more likely to exaggerate their need for food when there’s conflict in the family. The team was studying sibling rivalry in birds and noticed that dishonesty increased during events such as the parents divorcing, breeding again, or dying. The findings are in a paper that was just published in the journal Proceedings of the National Academy of Sciences.
Researchers from Oxford University studied sibling bird behavior across 60 bird species. Chicks sometimes ask for extra food that they don’t need, potentially harming siblings. This form of dishonesty only makes evolutionary sense if they’re competing with half-siblings or if the entire nest is at risk. The research team analyzed data from over 100 individual studies, comparing sibling begging behaviors with factors such as nest size, the possibility of new siblings in the future, and parental conflict.
The team found that the chicks were more dishonest when there was family conflict. If the parents were likely to divorce or die, the chicks engaged in more exaggerated begging behaviors. This makes sense because the loss of one parent means that future babies won’t be full siblings to the existing chicks. Dishonesty was rarest in long-lived, hardy species that mate for life, including the albatross. Exaggerated begging behaviors were common in species that lay a lot of eggs and mate with multiple individuals, such as blackbirds. This shows that evolution has resulted in sibling dishonesty when it would help chicks compete with potentially unrelated siblings.
The findings explain the variance in dishonest begging behaviors between different bird species. Exaggerated begging increases in cases where the siblings may face competition in the future from unrelated chicks or half-siblings. Times of conflict, such as divorce or the death of a parent, lead to increased sibling competition. If the home is stable and the chicks are full siblings, dishonest begging becomes rare.
Caro SM, West SA, Griffin AS. Sibling conflict and dishonest signaling in birds. Proceedings of the National Academy of Sciences (2016).