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.