Host
Bacterial taxa presence and abundance may fluctuate in response to host requirements. Phylum Firmicutes has been linked to weight gain, increased nutrient uptake, and metabolic efficiency in birds (Angelakis and Raoult 2010; Teyssier et al. 2018). The abundance of this phylum was lower in the second recapture period in Michigan than in the first recapture period or The Bahamas. Initial capture in The Bahamas occurred within the two months prior to the start of spring migration. During this time birds accumulate fat deposits to sustain them throughout long-distance migration (Fox and Walsh 2012). At the first recapture in Michigan, individuals are actively seeking and defending breeding territories. Both activities are energetically expensive and associated with increased metabolism, potentially associated with higher abundance of Firmicutes in gut microbiota. It is also possible that the bacteria in early Michigan are residual from The Bahamas and stopover sites (Lewis et al. 2017). Further research is needed to better identify bacterial lineages associated with specific metabolic demands of birds throughout the annual cycle.
Sex specific conditions, such as hormones, behaviors, and reproductive physiology may influence or be influenced by the microbiome (Pearceet al. 2017; Escallón et al. 2019). In the breeding season, close proximity of male and female birds can lead to convergence of microbial composition resulting in reduced variation between males and females (White et al. 2010). We found no significant variation in overall beta diversity between sexes, although female showed slightly higher alpha diversity than males. In Rufous-collared Sparrows (Zonotrichia capensis ), cloacal microbiome diversity increased as males transitioned from non-breeding to breeding condition (Escallón et al. 2019), which is the opposite of what we observed in the fecal microbiome of Kirtland’s Warblers, which showed a decrease in diversity. These sparrows are non-migratory and do not experience the same extreme habitat change that the Kirtland’s do, which could potentially explain the alpha diversity differences between species.
We generally found no significant compositional differences between SY and ASY age groups in the full dataset implying that adult age does not influence the microbiome of these birds. However, we did see a difference in beta diversity between SY and ASY in the first recapture period in Michigan. Second year males often do not successfully establish and defend breeding territories against older males which in turn results in these individuals moving at larger spatial scales than territorial adults (Cooper and Marra 2020). The lack of an established breeding territory and subsequent floating behavior could result in those individuals being exposed to a different suite of environmental bacteria.