No evidence for associations between brood size, gut microbiome diversity and survival in great tit (Parus major) nestlings

Abstract

The gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured. Gut microbiome diversity showed high variation between individuals, but this variation was not explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival. Early-life environment can lead to variation in offspring condition and gut microbiome and therefore, understanding how and which changes in the rearing environment are associated with offspring development is of importance. However, we did not find an association between brood size, gut microbiome diversity and survival, indicating that future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.

Individual variation in the avian gut microbiota: The influence of host state and environmental heterogeneity

“…Cross fostering experiments that manipulate the size of broods and compares broods with nestlings from multiple source nests against broods with nestlings from a single source, would test the relative importance of these reservoir and competition hypotheses. Just one other study, to our knowledge, has tested the link between brood size and the gut microbiota in great tits but found no evidence for an effect (Liukkonen et al, 2022). The disparity between these results probably shows how relationships between host and environmental variables and the microbiota may change in different settings or contexts, which will not be apparent in studies taking place during a single season over a relatively small geographic area.…”

“…While in nestlings, larger brood sizes lead to increased competition for food among siblings (Smith et al, 1989) and higher glucocorticoid hormone concentrations (Greggor et al, 2017; Smith et al, 1988). Thus brood size could influence the microbiota of the parents and their offspring through energetics or stress, though no link has been found to date (Liukkonen et al, 2022). Additionally, the gut microbiota in adults is mediated by a variety of sex hormones (Mallott et al, 2020), and can lead to sex‐specific associations between the gut microbiota and environmental sources of variation (Org et al, 2016).…”