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

Abstract

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.

Disentangling climatic and nest predator impact on reproductive output reveals adverse high‐temperature effects regardless of helper number in an arid‐region cooperative bird

“…Most colonies in the study area have been captured since 1993 and birds were marked with numbered metal rings and unique colour combinations (Fortuna et al 2021 ). The data set used here includes the breeding seasons of 2011/2012 and from 2013/2014 to 2018/2019.…”

Contrasting associations between nestling telomere length and pre and postnatal helpers’ presence in a cooperatively breeding bird

“…The direction and magnitude of the response are expected to be inluenced by the species' life-history strategy and especially by the efects of maternal investment on mother's survival prospects and chances of breeding successfully in the future (Russell and Lummaa 2009). In birds, cooperatively breeding species tend to be long-lived (Arnold and Owens 1998;Downing et al 2015), and for long-lived species, maximization of lifetime reproductive success was suggested to be more eiciently achieved through increasing the number of breeding events in life than through increased investment in each breeding event (Ghalambor and Martin 2001;Drent and Daan 2002;Reid et al 2003), In agreement, a recent meta-analysis on 10 cooperatively breeding birds highlighted a tendency for mothers to reduce egg size in larger groups in most species studied (Dixit et al 2017; but see Fortuna et al 2021).…”

“…These weavers are facultative cooperative breeders, with up to eight helpers assisting a breeding pair in raising the chicks, notably by feeding them (Covas et al 2008). This lighter parental workload may increase the survival of young female breeders (Paquet et al 2015b) but has no detectable association with clutch size (Covas et al 2008;Fortuna et al 2021). Helpers are most often the ofspring of one or both breeders (Covas et al 2006).…”

Mothers front-load their investment to the egg stage when helped in a wild cooperative bird

“…1101/2021.11.11.468195 doi: bioRxiv preprint fish). Three studies of cooperative birds have reported no evident relationship between egg size and the availability of help (Koenig et al 2009;Lejeune et al 2016;Fortuna et al 2021), and just one study has reported the reverse relationship. Iberian magpie (Cyanopica cooki) mothers with more helpers lay larger eggs and feed their nestlings at higher rates, consistent with the predictions of the differential allocation hypothesis (Valencia et al 2006(Valencia et al , 2017Savage et al 2015).…”

“…Several studies of cooperative birds have now reported that, after controlling for variation in clutch size, mothers with (more) helpers lay smaller eggs; the pattern predicted by the load-lightening hypothesis (e.g., Malurus cyaneus (Russell et al 2007); Corvus corone corone (Canestrari et al 2011); Vanellus chilensis (Santos & Macedo 2011); Philetairus socius (Paquet et al 2013); see also Taborsky et al (2007) for an experimental demonstration in fish). Three studies of cooperative birds have reported no evident relationship between egg size and the availability of help (Koenig et al 2009;Lejeune et al 2016;Fortuna et al 2021), and just one study has reported the reverse relationship. Iberian magpie (Cyanopica cooki) mothers with more helpers lay larger eggs and feed their nestlings at higher rates, consistent with the predictions of the differential allocation hypothesis (Valencia et al 2006(Valencia et al , 2017Savage et al 2015).…”