With a little help from my kin: barn swallow nestlings modulate solicitation of parental care according to nestmates' need

Elder Barn Owl Nestlings Flexibly Redistribute Parental Food according to Siblings' Need or in Return for Allopreening

Kin selection and reciprocation of biological services are distinct theories invoked to explain the origin and evolutionary maintenance of altruistic and cooperative behaviors. Although these behaviors are not considered to be mutually exclusive, the cost-benefit balance of behaving altruistically or cooperating reciprocally and the conditions promoting a switch between such different strategies have rarely been tested. Here, we examine the association between allofeeding, allopreening, and vocal solicitations in wild barn owl (Tyto alba) broods under different food abundance conditions: natural food provisioning and after an experimental food supplementation. Allofeeding was performed mainly by elder nestlings (hatching is asynchronous) in prime condition, especially when the cost of forgoing a prey was small (when parents allocated more prey to the food donor and after food supplementation). Nestlings preferentially shared food with the siblings that emitted very intense calls, thus potentially increasing indirect fitness benefits, or with the siblings that provided extensive allopreening to the donor, thus possibly promoting direct benefits from reciprocation. Finally, allopreening was mainly directed toward older siblings, perhaps to maximize the probability of being fed in return. Helping behavior among relatives can therefore be driven by both kin selection and direct cooperation, although it is dependent on the contingent environmental conditions.

An evolutionary switch from sibling rivalry to sibling cooperation, caused by a sustained loss of parental care

Sibling rivalry is commonplace within animal families, yet offspring can also work together to promote each other's fitness. Here we show that the extent of parental care can determine whether siblings evolve to compete or to cooperate. Our experiments focus on the burying beetle Nicrophorus vespilloides, which naturally provides variable levels of care to its larvae. We evolved replicate populations of burying beetles under two different regimes of parental care: Some populations were allowed to supply posthatching care to their young (Full Care), while others were not (No Care). After 22 generations of experimental evolution, we found that No Care larvae had evolved to be more cooperative, whereas Full Care larvae were more competitive. Greater levels of cooperation among larvae compensated for the fitness costs caused by parental absence, whereas parental care fully compensated for the fitness costs of sibling rivalry. We dissected the evolutionary mechanisms underlying these responses by measuring indirect genetic effects (IGEs) that occur when different sibling social environments induce the expression of more cooperative (or more competitive) behavior in focal larvae. We found that indirect genetic effects create a tipping point in the evolution of larval social behavior. Once the majority of offspring in a brood start to express cooperative (or competitive) behavior, they induce greater levels of cooperation (or competition) in their siblings. The resulting positive feedback loops rapidly lock larvae into evolving greater levels of cooperation in the absence of parental care and greater levels of rivalry when parents provide care.

No evidence for sibling or parent-offspring coadaptation in a wild population of blue tits, despite high power

Parent and offspring behaviors are expected to act as both the agents and targets of selection. This may generate parent-offspring coadaptation in which parent and offspring behaviors become genetically correlated in a way that increases inclusive fitness. Cross-fostering has been used to study parent-offspring coadaptation, with the prediction that offspring raised by non-relatives, or parents raising non-relatives, should suffer fitness costs. Using long-term data from more than 400 partially crossed broods of blue tits (Cyanistes caeruleus), we show that there is no difference in mass or survival between crossed and non-crossed chicks. However, previous studies for which the evidence for parent-offspring coadaptation is strongest compare chicks from fully crossed broods with those from non-crossed broods. When parent-offspring coadaptation acts at the level of the brood then partial cross-fostering experiments are not expected to show evidence of coadaptation. To test this, we performed an additional experiment (163 broods) in which clutches were either fully crossed, non-crossed, or partially crossed. In agreement with the long-term data, there was no evidence for parent-offspring coadaptation on offspring fitness despite high power. In addition there was no evidence of effects on parental fitness, nor evidence of sibling coadaptation, although the power of these tests was more modest.

Size matters but hunger prevails-begging and provisioning rules in blue tit families

It is commonly observed in many bird species that dependent offspring vigorously solicit for food transfers provided by their parents. However, the likelihood of receiving food does not only depend on the parental response, but also on the degree of sibling competition, at least in species where parents raise several offspring simultaneously. To date, little is known about whether and how individual offspring adjusts its begging strategy according to the entwined effects of need, state and competitive ability of itself and its siblings. We here manipulated the hunger levels of either the two heaviest or the two lightest blue tit (Cyanistes caeruleus) nestlings in a short-term food deprivation experiment. Our results showed that the lightest nestlings consistently begged more than the heaviest nestlings, an effect that was overruled by the tremendous increase in begging behaviour after food deprivation. Meanwhile, the amplified begging signals after food deprivation were the only cue for providing parents in their decision process. Furthermore, we observed flexible but state-independent begging behaviour in response to changes in sibling need. As opposed to our expectations, nestlings consistently increased their begging behaviour when confronted with food deprived siblings. Overall, our study highlights that individual begging primarily aims at increasing direct benefits, but nevertheless reflects the complexity of a young birds’ family life, in addition to aspects of intrinsic need and state.

Sibling conflict and dishonest signaling in birds

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.

Parental care masks a density-dependent shift from cooperation to competition among burying beetle larvae

Studies of siblings have focused mainly on their competitive interactions and to a lesser extent on their cooperation. However, competition and cooperation are at opposite ends on a continuum of possible interactions and the nature of these interactions may be flexible with ecological factors tipping the balance toward competition in some environments and cooperation in others. Here we show that the presence of parental care and the density of larvae on the breeding carcass change the outcome of sibling interactions in burying beetle broods. With full parental care there was a strong negative relationship between larval density and larval mass, consistent with sibling competition for resources. In the absence of care, initial increases in larval density had beneficial effects on larval mass but further increases in larval density reduced larval mass. This likely reflects a density-dependent shift between cooperation and competition. In a second experiment, we manipulated larval density and removed parental care. We found that the ability of larvae to penetrate the breeding carcass increased with larval density and that feeding within the carcass resulted in heavier larvae than feeding outside the carcass. However, larval density did not influence carcass decay.

Parent-absent calls are related to nestling reaction time and parental food allocation in the spotless starling

Absent repeat calls (ARC) are produced by nestlings of some bird species when parents are not at the nest, and play a role in sibling interactions and parental investment. We explored if individual traits influencing begging also determine ARC in the spotless starling (Sturnus unicolor), and whether this behaviour explains nestling feeding success. We video-taped natural broods and examined the effects of experimental feeding in this behaviour. Experimentally fed chicks stopped calling and received fewer feedings. Among un-fed chicks, absence calls were more frequent in smaller nestlings. We found a positive relationship between nestling reaction time to parental arrival and food acquisition: chicks that reacted first received more feedings that slower chicks. ARC performance was also positively related to reaction time: chicks that produced more calls also reacted first to parents. These results suggest that ARC may have important effects on resource allocation and family interaction networks.

Sibling cooperation influences the age of nest leaving in an altricial bird

In altricial birds, siblings raised within a nest usually leave the nest within hours of each other, despite often differing considerably in age. The youngest members of the brood are typically underdeveloped at this time and less likely than their older siblings to survive outside the nest, yet they risk abandonment if they do not fledge with their older siblings. Nest leaving is usually initiated by the older offspring, which may delay this process to provide more time for their younger siblings to mature, increasing the younger siblings' postfledging survival and their own inclusive fitness. We tested this hypothesis in a population of house wrens Troglodytes aedon and found that broods with broad age spans among siblings had longer nestling periods than broods with narrow age spans and that delayed fledging improves the survival and reproductive prospects of younger siblings, although at a potential cost to future siblings. We also manipulated age spans through cross-fostering and found that older foster nestlings postponed fledging when raised with younger broodmates, as predicted if the age of younger nestlings determines the time of fledging. Our results support kin-selection theory and demonstrate that the exact time of fledging is attributable, in part, to sib-sib interactions.