Longitudinal Associations Between Emotional Abuse, Family Functioning, and Depressive Symptoms in Adolescence.

Bacterial symbionts are ubiquitous across the animal tree of life (McFall‐Ngai et al., ), and both beneficial (mutualistic) and harmful (pathogenic and parasitic) symbionts significantly impact the ecology and evolution of hosts (Flint et al., ; Fraune & Bosch, ; McFall‐Ngai et al., ). Beneficial bacterial symbionts can provide protective functions against pathogens (Vorburger & Perlman, ), as bacterial communities on the skin, hair and feathers can act as a critical first line of defence against infections (Flechas et al., ; Javůrková et al., ; Longo & Zamudio, ). Such protection may involve beneficial bacteria residing within exocrine glands or growing in association with gland secretions that the hosts can then smear onto their bodies (Currie et al., ; Kaltenpoth et al., ; Martín‐Vivaldi et al., ; Soler et al., ). Therefore, these bacteria, or their antimicrobial compounds, must reach the integument (see examples in Flórez et al., ). This may be facilitated by host grooming (Martínez‐García et al., ), which may increase with increasing pathogen pressure (Fernández‐Marín et al., ). In birds, the uropygial gland (a unique exocrine gland located on the upper rump) produces secretions with species‐specific chemical compositions (Jacob & Ziswiler, ; Mazorra‐Alonso, Peralta‐Sánchez, et al., ). The defensive properties of these secretions are likely partially mediated by gland‐associated bacterial symbionts (Bodawatta et al., ; Mazorra‐Alonso et al., ) that produce ectoparasite‐repellent volatiles (Mazorra‐Alonso, Peralta‐Sánchez, et al., ; Tomás et al., ) or antimicrobial substances (Seibel et al., ; Soler et al., ). During preening, birds spread these secretions along with beneficial bacteria to the skin, feathers and other body parts (Martín‐Vivaldi et al., ; Ruiz‐Rodríguez et al., ; Soler et al., ). As a result, the bacterial community of the uropygial gland is connected with bacterial communities in preened body locations (Martínez‐García et al., ; Soler et al., ). Preening activities should be optimized according to pathogen exposures, which in turn may be driven by variation in nest types that affect the microclimate and microbial exposure of adults and their developing chicks. Cavity nesting, for instance, provides a partially isolated environment that is less exposed to environmental microbes than open‐cup nesting (Godard et al., ). Cavity nests also exhibit lower thermal variation, slightly reduced oxygen levels and higher carbon dioxide concentrations (Deeming & Mainwaring, ). Since these factors influence the composition of bacterial communities in avian nests, eggs and nestlings (West et al., ), it could be expected that cavity nesters show a less variable and compositionally distinct gland microbiota compared to open‐cup nesters.