Prenatal maternal effects appear to be insensitive to experimental or natural environmental variation

Maternal allocation in relation to weather, predation and social factors in a colonial cooperative bird

Females may adjust prenatal allocation in relation to ecological conditions that affect reproductive success, such as weather conditions or predation risk. In cooperative breeders, helpers might also influence reproductive success, and previous studies suggest that females can lay smaller eggs or larger clutches when breeding with more helpers. Although recent work suggests that helper effects can vary according to climatic variables, how social and ecological factors interact to shape prenatal allocation is poorly understood. Here, we examine how ecological and social components of the breeding environment covary with egg mass and clutch size, using as a model the sociable weaver Philetairus socius, a colonial, cooperatively breeding passerine. The study spanned 9 years and included over 1,900 eggs from over 550 clutches. Our analyses combined natural variation in weather conditions (rainfall before each reproductive event) with a nest predator-exclusion experiment and continuous monitoring of the mother's social environment, allowing us to estimate how individual females adjust allocation to reproduction as their number of helpers varies. We found that egg mass varied consistently within females and did not clearly differ in relation to rainfall or predation risk. Contrary to previous studies, there was no evidence for plastic adjustments as females gained and lost helpers, and egg mass was instead better predicted by mother size and identity. Females laid larger clutches when breeding in environments where predation risk was experimentally reduced and after higher rainfall levels. Yet, there was no evidence for increasing clutch size as the number of helpers increased, nor for an interaction between helper effects and ecological factors. We conclude that while sociable weaver females can vary their clutch size, they show high individual consistency in egg mass. In addition, we found no evidence that females may maximize fitness through plastic prenatal allocation in relation to the number of helpers, or that the presence/absence of helper effects is modulated by rainfall levels or predation risk. These results challenge our current knowledge on some of the possible benefits of breeding with helpers and call for more long-term analyses on reproductive allocation adjustments in other cooperative systems.

The Effects of Weather on Avian Growth and Implications for Adaptation to Climate Change

Climate change is forecasted to generate a range of evolutionary changes and plastic responses. One important aspect of avian responses to climate change is how weather conditions may change nestling growth and development. Early life growth is sensitive to environmental effects and can potentially have long-lasting effects on adult phenotypes and fitness. A detailed understanding of both how and when weather conditions affect the entire growth trajectory of a nestling may help predict population changes in phenotypes and demography under climate change. This review covers three main topics on the impacts of weather variation (air temperature, rainfall, wind speed, solar radiation) on nestling growth. Firstly, we highlight why understanding the effects of weather on nestling growth might be important in understanding adaptation to, and population persistence in, environments altered by climate change. Secondly, we review the documented effects of weather variation on nestling growth curves. We investigate both altricial and precocial species, but we find a limited number of studies on precocial species in the wild. Increasing temperatures and rainfall have mixed effects on nestling growth, while increasing windspeeds tend to have negative impacts on the growth rate of open cup nesting species. Thirdly, we discuss how weather variation might affect the evolution of nestling growth traits and suggest that more estimates of the inheritance of and selection acting on growth traits in natural settings are needed to make evolutionary predictions. We suggest that predictions will be improved by considering concurrently changing selection pressures like urbanization. The importance of adaptive plastic or evolutionary changes in growth may depend on where a species or population is located geographically and the species’ life-history. Detailed characterization of the effects of weather on growth patterns will help answer whether variation in avian growth frequently plays a role in adaption to climate change.

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.