Associations between brood size, gut microbiome diversity and survival in great tit (Parus major) nestlings

Background 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. Results 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. Conclusions 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.