Understanding how diet and host phylogeny shape gut microbiota is fundamental to elucidating host-microbe interactions in extreme environments. The Qinghai-Tibet Plateau (QTP), characterized by harsh conditions, provides a natural laboratory for examining these relationships among sympatric species. Here, we investigated the dietary composition and gut microbiota of six passerine species inhabiting the QTP, comprising two endemic residents (White-rumped Snowfinch Onychostruthus taczanowskii and Ground Tit Pseudopodoces humilis), two non-endemic residents (Rock Sparrow Petronia petronia and Eurasian Tree Sparrow Passer montanus), and two non-endemic migratory species (Twite Linaria flavirostris and Black Redstart Phoenicurus ochruros), using high-throughput 18S and 16S rRNA sequencing. Our results revealed that dietary composition—dominated by Archaeplastida, Metazoa, Fungi, and the SAR supergroup—exhibited no significant interspecific variation, reflecting a high degree of trophic niche overlap. Although the overall diet was similar across species, the relative abundances of certain dietary components independently influenced specific microbial taxa. In particular, dietary Archaeplastida and Fungi showed phylogeny-independent positive correlations with 16 and 3 microbial genera, respectively, revealing fine-scale diet-microbiota associations. Evidence of phylosymbiosis was detected, as closely related species harbored more similar microbial communities driven by species-specific microbial biomarkers. Notably, our results suggested deterministic processes played a stronger role in endemic species, whereas stochastic community assembly dominated in non-endemic species, indicating distinct assembly mechanisms shaped by biogeographic history. Overall, this study reveals that while dietary similarity promotes convergent trophic niches among sympatric passerines, host phylogeny exerts a stronger influence on gut microbiota composition and assembly. These findings underline the synergistic roles of diet-microbiota interactions and phylosymbiosis dynamics as key adaptive strategies that enable birds to cope with the extreme environments of the QTP.

Avian vocal communication represents one of the most intricate forms of animal language, playing a critical role in behavioral interactions. Both peripheral and central auditory-vocal pathways are essential for precisely integrating acoustic signals, ensuring effective communication. Like humans, songbirds exhibit vocal learning behaviors supported by complex neural mechanisms. However, unlike most mammals, songbirds possess the remarkable ability to regenerate damaged auditory cells. These capabilities offer unique opportunities to explore how birds adjust their vocal behavior and auditory processing in response to dynamic environmental conditions. Recent studies have advanced our understanding of the plasticity of avian vocal communication system, yet the vocal diversity and neurophysiological mechanisms underlying vocalization and hearing have often been examined independently. A comprehensive overview of how these systems interact and adapt in birds remains lacking. To address this gap, this review synthesizes the peripheral and central features of avian vocalization and hearing, while also exploring the mechanisms that drive the remarkable plasticity of these systems. Furthermore, it explores seasonal variations in bird vocalization and hearing and adaptations to environmental noise, focusing on how hormonal, neural, and ecological factors together shape vocal behavior and auditory sensitivity. Avian vocal communication systems present an exceptional model for studying the integration of peripheral and central vocal-auditory pathways and their adaptive responses to ever-changing environments. This review underscores the dynamic interactions between avian vocal communication systems and environmental stimuli, offering new insights into broader principles of sensory processing, and neuroplasticity.

The avian hippocampus, akin to its mammalian counterpart, plays a critical role in cognitive and physiological processes despite notable structural differences. Initially thought to be less developed, recent studies over the past two decades have revealed it as a complex brain region essential for diverse functions in both laboratory and free-living birds. This review synthesizes current knowledge on the avian hippocampus' organization, functionality, and neurophysiological significance. We first examine its anatomical structure and neuronal connectivity, comparing it with the mammalian hippocampus. We then highlight how its volume, neuronal density, and neurogenesis support spatial memory and navigation, influencing behaviors such as migration, food storing, brood parasitism, and homing. Beyond spatial functions, the avian hippocampus mediates emotion and stress physiology through interactions with the endocrine system, particularly via glucocorticoid receptors. It also influences spatial memory through sex hormones, especially estradiol, with local estrogen production through aromatase activity enhancing memory and plasticity. Therefore, the avian hippocampus serves as a central neural hub, integrating sensory information with internal states to facilitate essential behaviors and responses to external environmental stimuli. This review underscores the progress made in understanding this brain structure's roles, highlighting conserved neurophysiological functions across vertebrate taxa.

Light is an essential environmental cue influencing the endogenous circadian clocks that regulate behavioral and physiological processes in animals. Despite extensive research on the circadian rhythms of avian behavior and physiology, the mechanisms by which they adapt and adjust to abnormal photoperiod conditions, such as artificial light, have yet to be fully elucidated. Here, the circadian rhythms of the body mass, activity, and core and peripheral body temperatures of the Eurasian Tree Sparrow (Passer montanus, ETS)—a diurnal passerine—were examined under three different lighting conditions: constant light (LL), constant darkness (DD), and normal light–dark cycles (LD). Our results showed that the activity, body mass, core, tarsometatarsus, and eye temperatures of the ETSs varied significantly with treatment, time, and the interaction between treatment and time; however, there was no significant change in body mass in response to the interaction between time and treatment. Under LD conditions, the ETSs exhibited significant circadian rhythms in activity and core, tarsometatarsus, and eye temperatures. Under LL conditions, only the core temperature exhibited significant rhythmicity. Under DD conditions, there were no significant circadian rhythms in activity, body mass, and eye temperature; however, the core and tarsometatarsus temperatures remained rhythmic. Our results indicate that the core body temperature rhythm of ETS is endogenous and can be maintained under abnormal photoperiods. In contrast, the activity and body surface temperature, uncoupled from the core body temperature under abnormal photoperiod conditions, strongly depend on light. These results differ from those of other diurnal avian species, suggesting a unique coping mechanism of ETS in response to an abnormal photoperiod.

Avian embryos, which develop within eggs, exhibit remarkable tolerance to extremely low temperatures. Despite being a common trait among all birds, the mechanisms underlying this cold tolerance in avian embryos remain largely unknown. To gain a better understanding of this phenomenon and the coping mechanisms involved, we reviewed the literature on severe cold tolerance in embryos of both wild and domestic birds. We found that embryos of different bird orders exhibit tolerance to severe cold during their development. In response to cold stress, embryos slow down their heartbeat rates and metabolism. In severe cold temperatures, embryos can suspend these processes, entering a torpid-like state of cardiac arrest. To compensate for these developmental delays, embryos extend their regular incubation periods. Depending on their embryonic age, embryos of all bird species can tolerate acute severe cold regimes; only a few tolerate chronic severe cold regimes. We also discussed various extrinsic and intrinsic factors that affect the tolerance of bird embryos to low temperatures before and after incubation. Cold tolerance appears to be a heritable trait shared by wild and domestic embryos of all bird classes, regardless of egg size or development (altricial/precocial). Driven by environmental variability, cold tolerance in avian embryos is an optimal physiological and ecological strategy to mitigate the adverse effects of cold conditions on their development in response to fluctuating environmental temperatures.

The hypothalamic-pituitary-gonadal (HPG) axis ubiquitously regulates seasonal reproduction, following the rhythmicity of a suite of environmental cues. Birds display prominent seasonal variations in gonad size regulated by two hypothalamic peptides, gonadotropin-releasing hormone-1 (GnRH-I) and gonadotropin-inhibiting hormone (GnIH). How multi-brooded avian species adjust GnRH-I and GnIH to regulate gonadal morphology seasonally remains largely unknown. Here, we studied the variations in the hypothalamic immunoreactivity (ir) of GnRH-I and GnIH, gonadal proliferation, and apoptosis in a typical multi-brooded species, the Eurasian Tree Sparrow (Passer montanus), across the pre-breeding (PB), first breeding (FB), second breeding (SB), pre-basic molt (PM), and wintering stages (WS). Our results showed that both sexes had higher preoptic area (POA)-GnRH-I-ir but lower paraventricular nucleus (PVN)-GnIH-ir neurons during the breeding stages (FB and SB) relative to other life-history stages, with no significant differences between two broods. The testes and ovaries were significantly larger during the breeding stages. Testicular volume increased during the two broods due to anincreased diameter of seminiferous tubules. Furthermore, there were more testicular apoptotic cells in PB and WS stages than in breeding stages, and in PB stage than in PM stage. Males had higher POA-GnRH-I expression than females during the breeding stages, but both sexes had comparable PVN-GnIH expression throughout the annual cycle. Both sexes of the sparrows may undergo a similar pattern of life-history stage-dependent variation in the hypothalamic GnRH-I, GnIH, and gonadal morphology, except that during breeding stages, males may display higher expression of POA-GnRH-I relative to females. The higher expression of POA-GnRH-I-ir in breeding male sparrows may be critical for male-dependent parental care.

Plastic waste and debris have caused substantial environmental pollution globally in the past decades, and they have been accumulated in hundreds of terrestrial and aquatic avian species. Birds are susceptible and vulnerable to external environments; therefore, they could be used to estimate the negative effects of environmental pollution. In this review, we summarize the effects of macroplastics, microplastics, and plastic-derived additives and plastic-absorbed chemicals on birds. First, macroplastics and microplastics accumulate in different tissues of various aquatic and terrestrial birds, suggesting that birds could suffer from the macroplastics and microplastics-associated contaminants in the aquatic and terrestrial environments. Second, the detrimental effects of macroplastics and microplastics, and their derived additives and absorbed chemicals on the individual survival, growth and development, reproductive output, and physiology, are summarized in different birds, as well as the known toxicological mechanisms of plastics in laboratory model mammals. Finally, we identify that human commensal birds, long-life-span birds, and model bird species could be utilized to different research objectives to evaluate plastic pollution burden and toxicological effects of chronic plastic exposure.