Songbirds avoid the oxidative stress costs of high blood glucose levels: a comparative study.

Chronically high blood glucose levels (hyperglycaemia) can compromise healthy ageing and lifespan at the individual level. Elevated oxidative stress can play a central role in hyperglycaemia-induced pathologies. Nevertheless, the lifespan of birds shows no species-level association with blood glucose. This suggests that the potential pathologies of high blood glucose levels can be avoided by adaptations in oxidative physiology at the macroevolutionary scale. However, this hypothesis remains unexplored. Here, we examined this hypothesis using comparative analyses controlled for phylogeny, allometry and fecundity based on data from 51 songbird species (681 individuals with blood glucose data and 1021 individuals with oxidative state data). We measured blood glucose at baseline and after stress stimulus and computed glucose stress reactivity as the magnitude of change between the two time points. We also measured three parameters of non-enzymatic antioxidants (uric acid, total antioxidants and glutathione) and a marker of oxidative lipid damage (malondialdehyde). We found no clear evidence for blood glucose concentration being correlated with either antioxidant or lipid damage levels at the macroevolutionary scale, as opposed to the hypothesis postulating that high blood glucose levels entail oxidative costs. The only exception was the moderate evidence for species with a stronger stress-induced increase in blood glucose concentration evolving moderately lower investment into antioxidant defence (uric acid and glutathione). Neither baseline nor stress-induced glucose levels were associated with oxidative physiology. Our findings support the hypothesis that birds evolved adaptations preventing the (glyc)oxidative costs of high blood glucose observed at the within-species level. Such adaptations may explain the decoupled evolution of glycaemia and lifespan in birds and possibly the paradoxical combination of long lifespan and high blood glucose levels relative to mammals.

Gut microbiota variation between climatic zones and due to migration strategy in passerine birds.

Introduction: Decreasing biotic diversity with increasing latitude is an almost universal macroecological pattern documented for a broad range of taxa, however, there have been few studies focused on changes in gut microbiota (GM) across climatic zones. Methods: Using 16S rRNA amplicon profiling, we analyzed GM variation between temperate (Czechia) and tropical (Cameroon) populations of 99 passerine bird species and assessed GM similarity of temperate species migrating to tropical regions with that of residents/short-distance migrants and tropical residents. Our study also considered the possible influence of diet on GM. Results: We observed no consistent GM diversity differences between tropical and temperate species. In the tropics, GM composition varied substantially between dry and rainy seasons and only a few taxa exhibited consistent differential abundance between tropical and temperate zones, irrespective of migration behavior and seasonal GM changes. During the breeding season, trans-Saharan migrant GM diverged little from species not overwintering in the tropics and did not show higher similarity to tropical passerines than temperate residents/short-distance migrants. Interestingly, GM of two temperate-breeding trans-Saharan migrants sampled in the tropical zone matched that of tropical residents and converged with other temperate species during the breeding season. Diet had a slight effect on GM composition of tropical species, but no effect on GM of temperate hosts. Discussion: Consequently, our results demonstrate extensive passerine GM plasticity, the dominant role of environmental factors in its composition and limited effect of diet.

Latitudinal but not elevational variation in blood glucose level is linked to life history across passerine birds.

Macrophysiological research is vital to our understanding of mechanisms underpinning global life history variation and adaptation to diverse environments. Here, we examined latitudinal and elevational variation in a key substrate of energy metabolism and an emerging physiological component of pace-of-life syndromes, blood glucose concentration. Our data, collected from 61 European temperate and 99 Afrotropical passerine species, revealed that baseline blood glucose increases with both latitude and elevation, whereas blood glucose stress response shows divergent directions, being stronger at low latitudes and high elevations. Low baseline glucose in tropical birds, compared to their temperate counterparts, was mainly explained by their low fecundity, consistent with the slow pace-of-life syndrome in the tropics. In contrast, elevational variation in this trait was decoupled from fecundity, implying a unique montane pace-of-life syndrome combining slow-paced life histories with fast-paced physiology. The observed patterns suggest that pace-of-life syndromes do not evolve along the single fast-slow axis.

Within-community variation of interspecific divergence patterns in passerine gut microbiota.

Gut microbiota (GM) often exhibit variation between different host species and co-divergence with hosts' phylogeny. Identifying these patterns is a key for understanding the mechanisms that shaped symbiosis between GM and its hosts. Therefore, both GM-host species specificity and GM-host co-divergence have been investigated by numerous studies. However, most of them neglected a possibility that different groups of bacteria within GM can vary in the tightness of their association with the host. Consequently, unlike most of these studies, we aimed to directly address how the strength of GM-host species specificity and GM-host co-divergence vary across different GM clades. We decomposed GM communities of 52 passerine species (394 individuals), characterized by 16S rRNA amplicon sequence variant (ASV) profiles, into monophyletic Binned Taxonomic units (BTUs). Subsequently, we analyzed strength of host species specificity and correlation with host phylogeny separately for resulting BTUs. We found that most BTUs exhibited significant host-species specificity in their composition. Notably, BTUs exhibiting high host-species specificity comprised bacterial taxa known to impact host's physiology and immune system. However, BTUs rarely displayed significant co-divergence with host phylogeny, suggesting that passerine GM evolution is not shaped primarily through a shared evolutionary history between the host and its gut microbes.

Feather growth and quality across passerines is explained by breeding rather than moulting latitude.

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.

Telomere length is highly repeatable and shorter in individuals with more elaborate sexual ornamentation in a short-lived passerine.

Quantifying an individual's state as a fitness proxy has proven challenging, but accumulating evidence suggests that telomere length and attrition may indicate individual somatic state and success at self-maintenance, respectively. Sexual ornamentation is also thought to signal phenotypic quality, but links between telomeres and sexual ornamentation have been little explored. To address this issue, we examined whether telomere length and dynamics are predicted by the expression of a sexually selected ornament, the length of the outermost tail feathers (streamers), using longitudinal data from a population of European barn swallows (Hirundo rustica). In 139 adult individuals, each measured twice, we further assessed associations of telomere length with age, sex, breeding status and survival. Telomere length showed high individual repeatability (R = .97) across years while shortening with age in both sexes. Telomere length and dynamics were not significantly associated with survival to the next year, remaining lifespan or reproduction status (comparing breeding and nonbreeding yearlings). Tail streamer length, a sexually selected trait in barn swallows, was negatively associated with telomere length, independent of sex. Thus, telomere length may reflect the costs of carrying an elaborated sexual ornament, although ornament size did not significantly predict telomere shortening. In conclusion, telomere length in adult barn swallows is a highly consistent trait that shows a negative relationship with sexual ornamentation, suggesting a trade-off between sexual ornamentation and telomere length.