This blog post is provided by Eva Trapote and tells the #StoryBehindthePaper for the paper ‘Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows’, which was recently published in Journal of Animal Ecology. Trapote and colleagues investigated brood provisioning in breeding carrion crows to understand whether the degree of alternation at the nest by caregivers increases their reproductive success and is thereby favoured by natural selection.

Cooperation, typically defined as a behaviour that provides a benefit to another individual and has evolved at least partly because of this benefit, is a central theme in evolutionary biology.

Cooperation can arise incidentally, when individuals acting on their own “involuntarily” end up helping each other, or through active coordination, in which individuals adjust their behaviour in response to that of their group mates. Distinguishing these two processes is a critical first step for studying the evolution of specialized behavioural or cognitive capacities that may be required to achieve the most complex forms of cooperation. Cooperatively breeding bird societies provide excellent models for addressing this dichotomy.  

Cooperative breeding in birds generally occurs when, in a social group, subordinate individuals, called helpers, forgo or postpone their own reproduction to help raise the offspring of the dominant breeding pair.

Traditionally, most studies have focused on the amount of care that carers provide to the young, overlooking the distribution over time of those contributions. This temporal dimension of care, however, may be crucial to understanding how cooperation works.

Pioneering studies on bird species with biparental care (i.e., breeders without helpers) have recently revealed that, in some species, parents take turns in feeding the young, meaning they tend to alternate their visits with other carers rather than making repeat visits. This alternation has been suggested to help resolve the conflict over biparental care. This conflict arises because each parent stands to gain in terms of survival or future fecundity if the other does more of the work. By strictly taking turns, the male and female would ensure that any increase of their own feeding rate is followed by a matched increase of their partner’s efforts leading to an egalitarian share of duties. Conceivably, such active turn taking could also be an effective mechanism for stabilizing cooperation among multiple carers (breeders and helpers), where the conflict is likely to be higher than in species with only two caregivers.

Despite these notable progresses, questions were nevertheless recently raised about whether turn-taking really implies that group mates monitor each other’s behaviour and adjust their own behaviour accordingly. The alternative explanation is that coordination arises “passively”, for example because of the “refractory time” that each bird must invest in foraging before returning to feed the brood. This hypothesis has proven hard to dismiss, casting doubts on the actual adaptive values of turn-taking. In our work, we addressed this problem from a different perspective, namely by investigating if the degree of alternation at the nest increases the reproductive success of the caregivers and can therefore be favoured by natural selection.

To do so, we investigated brood provisioning in a population of cooperatively breeding carrion crows Corvus corone, in northern Spain. Weanalysed first if carrion crows took turns in visiting the nest, and second, if the degree of alternation improved the number of fledglings produced and/or their quality.

Our study system: the carrion crow population

Carrion crow groups are enlarged families, up to 9 individuals, comprising a dominant breeding pair, some non-dispersing offspring of both sexes, and/or individuals (mostly males) fledged in other territories (called ‘immigrants’), that are related to the same-sex breeder. It has been shown that this kin structure arises because crows actively choose relatives to cooperative with, proving the role of kin selection in shaping this complex society.

Data collection and video-analyses

We collected the data as part of a long-term population study. We measured and banded all nestlings just before fledging, and we also captured adult group members and marked them. We collected blood from all individuals (adults and nestlings) for parentage analysis and sex determination. During the breeding seasons of 1999–2000, 2003–2007, 2015, 2018 and 2019, we monitored a total of 50 nests from 37 different territories with camouflaged small video-cameras. We collected a total of 1,622 hours of footage documenting 8,693 nest visits. For each visit, we noted the identity of the carer, the exact time of entrance and departure from the nest, the nestlings that were fed and the time allocated to any other duty.

Results and implications

Our results showed that cooperatively breeding carrion crows alternated in provisioning the brood, regardless of group size and social category. Importantly, we found a strong positive effect of the degree of alternation on chick body mass that substantially increased their post-fledging survival rate. At this point, we also asked what mechanism could underlie the correlation between the degree of alternation and the body mass of the chicks. In subsequent analysis, we observed that chicks eat more regularly in well-coordinated groups, hence developing better than chicks raised in poorly coordinated groups.

In conclusion, our results show that the way cooperation is achieved (coordinated vs. uncoordinated brood provisioning) affects reproductive success in carrion crows, which implies that any mechanism that causes and/or improves alternation at the nest would be favoured by natural selection. Among these mechanisms, active communication between group members is a strong candidate. Interestingly, cooperative breeding is associated with richer vocal repertoires in birds and, in particular, with a larger variability of contact calls. In light of the findings presented here, cooperatively breeding carrion crows are a particularly valuable model system for investigating communicative cooperation in non-human animals.

We are currently working on this, using advanced machine-learning methods for charting the vocal repertoires of pairs with and without helpers, detecting context dependence in crows’ communicative behaviour, and ultimately, decoding call types that may be involved in coordination.

Read the full paper

Trapote, E., Moreno-González, V., Canestrari, D., Rutz, C., & Baglione, V. (2023). Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows. Journal of Animal Ecology, 00, 1–14 . https://doi.org/10.1111/1365-2656.14033