A time?lagged association between the gut microbiome, nestling weight and nestling survival in wild great tits

Natal body mass is a key predictor of viability and fitness in many animals. While variation therefore juvenile may be explained by genetic environmental factors, emerging evidence points to the gut microbiota as an important factor influencing host health. The known change during development, but it remains unclear whether microbiome predicts fitness, if does, at which developmental stage affects traits. We collected data on two traits associated with wild nestling great tits Parus major: weight survival fledging. characterised using 16S rRNA sequencing from faeces investigated temporal associations between across development Day-8 (D8) Day-15 (D15) post-hatching. also explored particular microbial taxa were ‘indicator species’ that reflected nestlings survived or not. There was no link diversity D8 D15. However, we detected time-lagged relationship where D15 negatively D8, controlling for reflecting relative gain over intervening period. Indicator species analysis revealed specificity values high fidelity low, suggesting indicator primarily within either not groups, always birds died. Therefore these sufficient, necessary determining mortality, perhaps owing functional overlap microbiota. highlight measuring microbiome-fitness relationships just one time point misleading, especially early life. Instead, microbial-host effects best longitudinally detect critical windows neonatal weight. Our findings should inform future hypothesis testing pinpoint features community impact when this occurs. Such confirmatory research will shed light population level processes could have potential support conservation.