Diet
Dietary shifts throughout the annual cycle correspond to changes in gut microbiota (Ren et al. 2017; Smits et al. 2017; Drovetskiet al. 2019). Kirtland’s Warblers shift from a fruit-rich diet in The Bahamas to a diet composed primarily of insects in Michigan (Deloria-Sheffield et al. 2001; Wunderle et al. 2010; Wunderle et al. 2014). Firmicutes and Actinobacteria, which are often associated with frugivorous diets and known to aid in digestion through cellulose and carbohydrate degradation, were more abundant in The Bahamas where the Kirtland’s diet is rich in fruit (Anand and Kandula 2012; Segawa et al. 2019). The abundance of Cyanobacteria throughout the sampling periods similarly shifts with decreased frugivory exhibited by the Kirtland’s from The Bahamas to Michigan. Cyanobacteria may be acquired as environmental byproduct; it may also be represented by ingested chloroplasts (Brice et al. 2019). Though Cyanobacteria is often removed from gut microbial studies (Knightet al. 2018), the proportional variation between sampling periods further illustrates environmental and diet related differences throughout the annual cycle.
Proteobacteria, often abundant in insectivorous species (Edenboroughet al. 2020) more than doubled in relative abundance from The Bahamas to the second Michigan recapture period. This may be in response to the shift in diet between locations. Additionally, specific lineages of this phylum, such as genus Serratia , are known to produce chitinase which facilitates the degradation of insects’ exoskeletons (Rathore and Gupta 2015). We identified Serratia as significantly abundant in the second Michigan sampling period, corresponding to the insect-rich diet of that time.