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Abstract Among closely related species, host phylogenetic relationships are typically a stronger predictor of gut microbiome composition than environmental variation. However, the relative impact of genetic admixture caused by hybridization versus environmental variation on gut microbiome communities is poorly understood. To explore this knowledge gap, we used fecal metabarcoding to characterize chickadee gut microbiomes along a hybrid zone transect in natural environments and after transfer to a common, controlled environment. We collected fecal samples from nestling black‐capped (Poecile atricapillus), Carolina (Poecile carolinensis), and hybrid chickadees immediately after removal from their nests and twice after entering captivity and experiencing common diet and environmental conditions. To characterize gut microbiome communities, we extracted fecal DNA and sequenced the V3–V4 region of the 16S rRNA gene using Illumina MiSeq. Overall,FirmicutesandProteobacteriawere the major microbial phyla present across host species groups. Our analysis of alpha diversity showed that the transition to common environmental conditions significantly impacted host gut microbiome richness. In contrast, the change in host environment did not impact the community composition (i.e., beta diversity) of the gut microbiomes. Although not statistically significant, host ancestry may influence the microbiome composition more than host environment. Additional analyses suggest chloroplast 16S rRNA sequences accurately characterized host diets in captivity. In certain cases, 16S rRNA sequences can provide reliable characterization of habitat and dietary variation in wild birds. Although environment more strongly shapes microbiome richness and evenness, host ancestry may have a greater influence on the specific microbes present in the microbiome. 

ABSTRACT Hybridization occurs when different species mate and produce offspring. Although hybridization can have negative consequences for cognitive performance, the mechanisms underlying those effects are still poorly understood. A fundamental physiological process found in all animals studied to date that could be disrupted in hybrids is sleep. Given that mechanisms that occur within the brain during sleep may help maintain optimal cognitive performance, here we outline the potential impacts of hybridization on sleep and cognition. We suggest that sleep loss caused by hybridization could lead to negative impacts for neural and molecular mechanisms (e.g. neurogenesis, synaptic plasticity, and brain gene expression) associated with cognition, which may help explain some of the cognitive deficiency recently observed in hybrid birds. However, we acknowledge that these mechanisms may instead be directly impacted by hybridization, which in turn could also disrupt sleep with similar negative consequences for cognition. Limitations in sleep processes apparent in hybrids might influence hybrid fitness and therefore act as a post‐zygotic isolating barrier. 

Abstract Hybrid zones can be used to identify traits that maintain reproductive isolation and contribute to speciation. Cognitive traits may serve as post-mating reproductive isolating barriers, reducing the fitness of hybrids if, for example, misexpression occurs in hybrids and disrupts important neurological mechanisms. We tested this hypothesis in a hybrid zone between two subspecies of Swainson’s thrushes (Catharus ustulatus) using two cognitive tests—an associative learning spatial test and neophobia test. We included comparisons across the sexes and seasons (spring migration and winter), testing if hybrid females performed worse than males (as per Haldane’s rule) and if birds (regardless of ancestry or sex) performed better during migration, when they are building navigational maps and encountering new environments. We documented reduced cognitive abilities in hybrids, but this result was limited to males and winter. Hybrid females did not perform worse than males in either season. Although season was a significant predictor of performance, contrary to our prediction, all birds learned faster during the winter. The hypothesis that cognitive traits could serve as post-mating isolating barriers is relatively new; this is one of the first tests in a natural hybrid zone and non-food-caching species. We also provide one of the first comparisons of cognitive abilities between seasons. Future neurostructural and neurophysiological work should be used to examine mechanisms underlying our behavioral observations.