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

Abstract

Background: The gut microbiome forms at an early stage, yet data on the environmental factors infuencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. 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 efects of specifc 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. Results: Gut microbiome diversity showed high variation between individuals, but this variation was not signifcantly explained by brood size or body mass. Additionally, we did not fnd a signifcant efect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no signifcant association between gut microbiome diversity and short-term (survival to fedging) or mid-term (apparent juvenile) survival. Conclusions: We found no clear association between early-life environment, ofspring condition and gut microbiome. This suggests that brood size is not a signifcantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to ofspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental infuences.