Vertebrate gut microbiota (GM) is comprised of a taxonomically diverse consortium of symbiotic and commensal microorganisms that have a pronounced effect on host physiology, immune system function and health status. Despite much research on interactions between hosts and their GM, the factors affecting inter- and intraspecific GM variation in wild populations are still poorly known. We analysed data on faecal microbiota composition in 51 passerine species (319 individuals) using Illumina MiSeq sequencing of bacterial 16S rRNA (V3-V4 variable region). Despite pronounced interindividual variation, GM composition exhibited significant differences at the interspecific level, accounting for approximately 20%-30% of total GM variation. We also observed a significant correlation between GM composition divergence and host's phylogenetic divergence, with strength of correlation higher than that of GM vs. ecological or life history traits and geographic variation. The effect of host's phylogeny on GM composition was significant, even after statistical control for these confounding factors. Hence, our data do not support codiversification of GM and passerine phylogeny solely as a by-product of their ecological divergence. Furthermore, our findings do not support that GM vs. host's phylogeny codiversification is driven primarily through trans-generational GM transfer as the GM vs. phylogeny correlation does not increase with higher sequence similarity used when delimiting operational taxonomic units. Instead, we hypothesize that the GM vs. phylogeny correlation may arise as a consequence of interspecific divergence of genes that directly or indirectly modulate composition of GM.

It has been proposed that life histories have co‐evolved with a suite of physiological and behavioural adaptations, termed pace‐of‐life syndromes. Here, we hypothesise that basal concentration of blood glucose (G0), a major source of energy circulating in blood, may constitute a key component of pace‐of‐life syndromes. To test this hypothesis, we measured G0 in 30 passerine species and tested its covariation with body mass and other life‐history traits. Importantly, body mass is a major life‐history determinant and, when its effect is controlled for, there may be no single fast–slow life‐history continuum in birds comprising both fecundity and life span. Hence, we used individual life‐history traits, rather than principal component analysis, to characterise life‐history variation in our analysis. In support of G0‐life‐history co‐evolution, we found G0 to be negatively correlated with body mass and positively with reproductive investment in a single clutch across 30 passerine species. Higher G0 in females suggests that the energy demands of clutch production and incubation may be an important selection force driving co‐evolution of G0 with reproductive output. In contrast, G0 was not associated with maximum life span, suggesting that high G0 may not constrain evolution of longevity. This implies that long‐lived species can evolve physiological adaptations preventing harmful effects of high glucose concentrations, known to cause pathologies and accelerate ageing. In addition, G0, but not basal metabolic rate (BMR), was negatively correlated with migration distance, attesting to evolutionary changes in energy metabolism in long‐distance migrants. Our results further suggest that the links between body mass, reproduction and G0 are not mediated by BMR and that G0 is associated with fast–slow life‐history variation more closely than available BMR data. A species life history is determined to a great extent by body mass. When this effect is controlled for, only those traits related to reproduction (but not life span) constitute the principal axis of life‐history variation in birds. Hence, the co‐evolution of G0 with body mass and reproductive output evidenced in our study indicates that G0 constitutes an important physiological component of pace‐of‐life syndromes. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13238/suppinfo is available for this article.

Species evolve life histories and phenotypes that are adapted to the environments they inhabit. Consequently, latitudinal and elevational gradients in life-history strategies emerged through evolution, shaped by geographic variation in environmental conditions. Since similar life histories co-evolve with suites of convergent physiological adaptations termed pace-of-life syndromes (POLS), corresponding latitudinal and elevational gradients in POLS should also emerge (Ricklefs & Wikelski, 2002).Physiological adaptations are further shaped by other environmental demands, such as thermogenic needs. Research into patterns of physiological variation over large geographical and phylogenetic scales, known as macrophysiology, is therefore essential to our understanding of the mechanisms underpinning global variation in demographic rates and life histories, species distribution and abundance or physiological adaptation under diverse environmental conditions (

Tropical bird species are characterized by a comparatively slow pace of life, being predictably different from their temperate zone counterparts in their investments in growth, survival and reproduction. In birds, the development of functional plumage is often considered energetically demanding investment, with consequences on individual fitness and survival. However, current knowledge of interspecific variation in feather growth patterns is mostly based on species of the northern temperate zone. We evaluated patterns in tail feather growth rates (FGR) and feather quality (stress-induced fault bar occurrence; FBO), using 1518 individuals of 167 species and 39 passerine families inhabiting Afrotropical and northern temperate zones. We detected a clear difference in feather traits between species breeding in the temperate and tropical zones, with the latter having significantly slower FGR and three times higher FBO. Moreover, trans-Saharan latitudinal migrants resembled temperate zone residents in that they exhibited a comparatively fast FGR and low FBO, despite sharing moulting environments with tropical species. Our results reveal convergent latitudinal shifts in feather growth investments (latitudinal syndrome) across unrelated passerine families and underscore the importance of breeding latitude in determining cross-species variation in key avian life-history traits.