Componentes sociales y ambientales del comportamiento de aprovisionamiento al nido en cornejas negras cooperativas

Resumen

Cooperative breeding in birds arises when individuals, called helpers, forgo or postpone their reproduction to help raising young that are not their own. Hamilton’s inclusive fitness theory provides a firm theoretical basis for understanding the paradox of helping, but does not apply to societies composed of unrelated individuals. Moreover, in some species living in family groups, indirect benefits seem to be insufficient to explain a delay in own reproduction. Therefore, the existence of direct benefits (access to resources, protection from predators and increased social status, among others) has been hypothesized to explain how these societies have evolved and maintained over time. Importantly, some of the direct benefits proposed would imply that helpers signal their contribution to communal tasks to the rest of the social group. Most research on cooperative breeding in bird has focused until now on the amount of care that individuals provide, overlooking, other dimensions of young provisioning behaviour, like the distribution over time of nest visits. This temporal dimension, however, might shed light on social aspects of cooperative behaviour that are related to its ultimate function and mechanism. First, looking at the patterns of overlap of provisioning events by different carers can unveil a signalling component of helping that may serve to enhance individual “social prestige” or to show to the dominants that a” rent” has being paid for access to the resources of the territory. Second, recent theoretical models have postulated that an alternation of nest visits among group members could resolve the conflict over brood care. Testing these two aspects in the cooperative society of the carrion crow Corvus corone are the key objectives of this thesis: addressing the direct benefits and signal function of brood provision will shed light on the ultimate function of helping, and investigating the turn taking among carers will help us understanding the stabilising mechanisms of cooperation in this system. Carrion crows form stable enlarged families in northern Spain, where retained offspring and more distant relatives aid a dominant breeding pair to raise its brood. In this society, male subordinates face strong constraints on independent reproduction and make the best of bad job by investing substantially in helping to increase their indirect fitness. In contrast, the expectation of future reproduction is better for female offspring, because of the higher turnover of breeding females in this population, which explains why on average they are less helpful than males. Under these circumstances, dominants are expected to be more vigilant over the contribution to brood provisioning by female subordinates, which, in turn, might need to show their helpfulness to retain group membership. In accordance with this idea, we found that female retained offspring increased the ‘visibility’ of their contribution by provisioning in front of the dominant breeders significantly more than any other category of group members. In addition, retention of group membership for female offspring was associated with their perceived effort in chick feeding. Our results are consistent with the hypothesis of ‘pay to stay’ and suggest that a signalling component of helping can arise in a ‘typical’ kin-based cooperative society. Cooperative food provisioning might arise incidentally as a result of the individual efforts of multiple carers that ‘act apart together’, but recent studies on birds suggest that individuals may also engage in active turn taking. However, evidence for individuals responding to each other’s behaviour is controversial, and the potential benefits of such coordination remain unknown. Our results suggested that crow carers alternate in provisioning the nestlings. Importantly, we found that the body mass of nestlings increased significantly with the degree of carers’ alternation, possibly because coordinated groups provided food at more regular intervals. The observed increase in body mass in crows substantially boosted estimated post-fledgling survival rates. Our study demonstrates that alternation in nestling provisioning has measurable fitness benefits in this system. This implies that any mechanism that causes and/or enhances nest alternation would be favoured by natural selection. However, not only intrinsic and social factors determine parental care in cooperatively breeding birds, but the effects of environmental factors (such as the weather) may also play a relevant role. Meteorological conditions, like temperature and drought, can constrain brood provisioning in some bird species, but the consequences on reproductive success have been rarely quantified. The third objective of this thesis addresses this question in the cooperatively breeding carrion crow by analysing a long-term data set, which covers the last 26 years. Our results showed that individual feeding rates decreased significatively with rising temperatures both in breeders and helpers, while drought was associated with a significant reduction in the effort of the male helpers as compared to the other social categories. The degree of alternation of nest visits by carers was also negatively affected by rising temperature. Consequences on crow reproductive success were detected in terms of body condition of the nestlings, which worsened when temperatures were high during the rearing period. Interestingly, a significant temporal trend was found, with nestling body condition steadily deteriorating over the 26-years study period. Although many factors may concur in causing population changes, our data suggest a possible causal link between global warming, brood caring behaviour and the decline of carrion crow population in the Mediterranean climatic areas of Spain.