Sibling conflict and dishonest signaling in birds

Abstract

Offspring survival can often depend on successful communication with parents about their state of need. Theory suggests that offspring will be less likely to honestly signal their need when they experience greater competition from either a greater number of nestmates or less-related nestmates. We found support for this hypothesis with a comparative analysis, examining data from across 60 species of birds. We found that offspring are less honest about their level of need when (i) they face competition from current siblings; (ii) their parents are likely to breed again, and so they are in competition with future siblings; and (iii) parental divorce or death means that they are likely to be less related to future siblings. More generally, these patterns highlight the sensitivity of communication systems to conflict between signaler and receiver while also suggesting that when there is little conflict, natural selection favors the honest.

Baby birds do not always tell the truth

“…Competition with future siblings drives a similar pattern: when parents have the capacity to breed again, begging intensity less honestly reveals need. Supporting the prediction that high relatedness with competitors favors some restraint, Caro et al (5) find that low adult mortality and low parental divorce (both of which increase a focal chick's relatedness to future siblings) are associated with more honest signals of need. The same, however, was not true with respect to concurrent siblings: low within-brood relatedness, caused by frequent extrapair mating by parents, was not associated with a decline in honesty.…”

“…This incomplete overlap in relatedness results in an incomplete overlap in desired food allocation, setting the stage for a selfish nestling to benefit from deception (4). In PNAS, Caro et al (5) shed light on several factors that shape how honest offspring signals are.…”

“…How could such deference evolve? By allowing nutrients to be directed to needier siblings (current or future), a modest signaler can improve its own inclusive fitness via the survival of these brothers and sisters, and, eventually, the existence of nieces and nephews (3)(4)(5)(6)(7). Because the benefits of this modesty are paid in the currency of genetic relatives, it stands to reason that honest signals of need should be more common when the relatives in question are more closely related.…”

“…Because the benefits of this modesty are paid in the currency of genetic relatives, it stands to reason that honest signals of need should be more common when the relatives in question are more closely related. It is this prediction that Caro et al (5) test, alongside the related prediction that competition with siblings always reduces the incentives for honesty because a chick cannot control what happens to food it passes up (2,7,10).…”

“…Nestling birds are not the only animals that solicit feedings from parents using complex displays (1, 3), but they are by far the most studied, allowing Caro et al (5) to perform a metaanalysis in this group. The authors operationally define an honest signal of need as one for which signal intensity is positively and highly correlated with need, itself defined by a suite of proxies including low body condition or within-brood rank.…”