Hummingbird blood traits track oxygen availability across space and time

Methods: Increasing feasibility of total blood cell count analysis in field studies: Effects of plastic tubes and storage duration

Total red (RBC) and white (WBC) blood cell counts are highly informative haematological parameters, holding substantial potential for advancing ecological and evolutionary research and conservation efforts. Yet, their use in studies on wild bird populations is rare as blood samples need to be stored in fragile glass flasks and analysed on the day of collection, which poses significant challenges in field conditions. Here, we aimed to make total blood cell counts more accessible for field studies by assessing their reliability in blood samples collected in more durable plastic tubes and stored for varying periods of time. We used the zebra finch (Taeniopygia castanotis) as a model and aliquoted the collected blood samples into plastic tubes and glass flasks with Natt-Herrick's staining solution. When analysed on the day of collection, total RBC and WBC counts from the plastic tubes and glass flasks showed no difference and were highly repeatable. A subsequent repeated analysis of samples stored at 4-8 °C in plastic tubes for 18 months showed moderate repeatabilities of resultant blood cell counts that did not decrease over time. Together with the absence of linear trends in the results, consistent repeatabilities indicate that the total blood cell count information is effectively preserved in the long term. Overall, our study shows that blood samples can be collected in plastic tubes with Natt-Herrick's solution for analysis of total RBC and WBC counts. Moreover, long-term sample storage can be a viable option when immediate cell count analysis is impracticable, such as in field studies requiring sampling in remote locations. However, the moderate repeatability of cell counts in stored samples must be considered, particularly in within-population studies where relatively low biological variation is expected, which may require high analytical precision. These findings may facilitate more extensive application of total blood cell counts in wild bird research and conservation.

Resilience and robustness: from sub-organismal responses to communities

Coping with challenges is essential to life on earth. Determining the processes that generate resilience and robustness to disturbance across levels of biological organization is increasingly important as the pace of global change accelerates; however, to date, multiscale models have primarily focused on population to ecosystem scales. In this opinion article we combine conceptual models from different fields to develop a unified a framework of resilience and robustness that explicitly links sub-organismal responses with higher-level outcomes. This framework predicts that interactions among sub-organismal response components - including their temporal dynamics and the plasticity of homeostatic regulatory networks - are key drivers of current and future resilience.

The catecholamine response to graded high-altitude flight in yellow-rumped warblers (Setophaga coronata)

Chronic exposure to low oxygen (hypoxia) leads to amplification of the hypoxic chemoreflex, increasing breathing and sympathetic nervous system (SNS) activation. Prolonged SNS activation redistributes blood to hypoxia-sensitive tissues away from muscles. Recent tracking studies have shown that migratory songbirds can fly 5,000 m or higher above sea level, leading us to hypothesize that migratory birds may have a blunted hypoxic chemoreflex to maintain blood flow to muscles during migratory flight at high altitudes. To test this hypothesis, we used a hypobaric wind tunnel and measured circulating plasma catecholamines after maximal altitude flight, flight at 75% of maximal altitude, flight at ground level (∼250 m), and after rest at 75% of maximal altitude and ground level in migratory myrtle yellow-rumped warblers (Setophaga coronata). Yellow-rumped warblers were capable of flying above 4,000 m simulated altitude above sea level (average maximum altitude of ∼3,600 m) and would maintain flights at 75% of individual maximum altitudes (∼2,700 m). Circulating dopamine and noradrenaline were similar between resting and flight conditions at ground level and with exposure to 75% of maximal altitude, whereas adrenaline significantly increased with flight, but did not change further with flight at 75% of maximal altitude. In contrast, both adrenaline and noradrenaline concentrations increased after maximum altitude flights compared with 75% and ground-level flights. Our findings show that exercise increases plasma adrenaline in migratory songbirds and suggest that warblers flying at high altitudes below their maximum altitude may be minimally hypoxic, allowing them to maintain oxygen transport to flight muscles.NEW & NOTEWORTHY Yellow-rumped warblers, a small songbird that conducts migratory flights, were found to fly to altitudes above 4,000 m above sea level in simulated flights using a hypobaric wind tunnel. Circulating adrenaline suggests that warblers flying at 75% of individual maximum altitudes are not experiencing arterial hypoxia, allowing them to maintain aerobically demanding migratory flight at high altitudes.

On the Physiology of High-altitude Flight and Altitudinal Migration in Birds

Many bird species fly at high altitudes for short periods and/or shift seasonally in altitude during migration, but little is known about the physiology of these behaviors. Transient high-altitude flight, or short-term flight at extreme altitudes, is a strategy used by lowland-native birds, often in the absence of topographic barriers. Altitudinal migration, or seasonal roundtrip movement in altitude between the breeding and non-breeding seasons, is a form of migration that occurs as a regular part of the annual cycle and results in periods of seasonal residency at high altitudes. Despite their nuanced differences, these two behaviors share a common challenge: exposure to reduced oxygen environments during at least part of the migratory journey. In this perspective piece, we compare what is known about the physiology of oxygen transport during transient high-altitude flight and altitudinal migration by highlighting case studies and recent conceptual advances from work on captive and wild birds. We aim to open avenues for integrative research on the ecology, evolution, and physiology of high-flying and mountain-climbing birds.

Elevational and Seasonal Patterns of Plant-Hummingbird Interactions in a High Tropical Mountain

Tropical mountain ecosystems harbor diverse biological communities, making them valuable models for exploring the factors that shape ecological interactions along environmental gradients. We investigated the spatial and temporal drivers of plant-hummingbird interaction networks across three forest types (pine-oak, fir, and subalpine) along a tropical high mountain gradient in western Mexico (2400 to 3700 m.a.s.l.). We measured species abundance, diversity, morphology, and interaction frequencies. Plant diversity metrics significantly declined in the highest elevation subalpine forest, whereas hummingbird diversity remained consistent across elevations. Interaction networks were similarly nested across elevations, but they were more specialized in the subalpine forest, where lower plant species richness and higher floral abundance led to greater resource partitioning among hummingbirds. Plant-hummingbird networks were larger and less specialized during the dry season, driven by greater species diversity and abundance. Species turnover explained network variation along the elevational gradient, while interaction rewiring and the arrival of migratory hummingbirds explained changes between seasons. Phenological overlap was the most important driver of the observed variation in interaction frequencies across elevations and seasons. Flower abundance had a minor influence on interaction frequencies at low- and mid-elevation networks, and hummingbird abundance was significant for dry- and rainy-season networks. Morphological matching was significant in the low-elevation forest and in the dry season. Plant phylogenetic relatedness had negligible effects on interaction patterns, but hummingbird phylogeny influenced feeding preferences in high-elevation and rainy-season networks. Our findings highlight the role of species turnover, interaction rewiring, and phenological overlap in structuring plant-hummingbird networks, with specific effects of abundance, morphology, and phylogeny varying with elevation and season. High-elevation ecosystems play a crucial role as reservoirs of floral resources for both resident and migratory hummingbirds during resource-scarce periods, emphasizing their importance in maintaining biodiversity in tropical mountain gradients.

Extreme elevational migration spurred cryptic speciation in giant hummingbirds

The ecoevolutionary drivers of species niche expansion or contraction are critical for biodiversity but challenging to infer. Niche expansion may be promoted by local adaptation or constrained by physiological performance trade-offs. For birds, evolutionary shifts in migratory behavior permit the broadening of the climatic niche by expansion into varied, seasonal environments. Broader niches can be short-lived if diversifying selection and geography promote speciation and niche subdivision across climatic gradients. To illuminate niche breadth dynamics, we can ask how "outlier" species defy constraints. Of the 363 hummingbird species, the giant hummingbird (Patagona gigas) has the broadest climatic niche by a large margin. To test the roles of migratory behavior, performance trade-offs, and genetic structure in maintaining its exceptional niche breadth, we studied its movements, respiratory traits, and population genomics. Satellite and light-level geolocator tracks revealed an >8,300-km loop migration over the Central Andean Plateau. This migration included a 3-wk, ~4,100-m ascent punctuated by upward bursts and pauses, resembling the acclimatization routines of human mountain climbers, and accompanied by surging blood-hemoglobin concentrations. Extreme migration was accompanied by deep genomic divergence from high-elevation resident populations, with decisive postzygotic barriers to gene flow. The two forms occur side-by-side but differ almost imperceptibly in size, plumage, and respiratory traits. The high-elevation resident taxon is the world's largest hummingbird, a previously undiscovered species that we describe and name here. The giant hummingbirds demonstrate evolutionary limits on niche breadth: when the ancestral niche expanded due to evolution (or loss) of an extreme migratory behavior, speciation followed.

Illuminating the Mysteries of the Smallest Birds: Hummingbird Population Health, Disease Ecology, and Genomics

Hummingbirds share biologically distinctive traits: sustained hovering flight, the smallest bird body size, and high metabolic rates fueled partially by nectar feeding that provides pollination to plant species. Being insectivorous and sometimes serving as prey to larger birds, they fulfill additional important ecological roles. Hummingbird species evolved and radiated into nearly every habitat in the Americas, with a core of species diversity in South America. Population declines of some of their species are increasing their risk of extinction. Threats to population health and genetic diversity are just beginning to be identified, including diseases and hazards caused by humans. We review the disciplines of population health, disease ecology, and genomics as they relate to hummingbirds. We appraise knowledge gaps, causes of morbidity and mortality including disease, and threats to population viability. Finally, we highlight areas of research need and provide ideas for future studies aimed at facilitating hummingbird conservation.

Evolutionary trade-off between innate and acquired immune defences in birds

BACKGROUND

The development, maintenance, and use of immune defences are costly. Therefore, animals face trade-offs in terms of resource allocation within their immune system and between their immune system and other physiological processes. To maximize fitness, evolution may favour investment in one immunological defence or subsystem over another in a way that matches a species broader life history strategy. Here, we used phylogenetically-informed comparative analyses to test for relationships between two immunological components. Natural antibodies and complement were used as proxies for the innate branch; structural complexity of the major histocompatibility complex (MHC) region was used for the acquired branch.

RESULTS

We found a negative association between the levels of natural antibodies (i.e., haemagglutination titre) and the total MHC gene copy number across the avian phylogeny, both at the species and family level. The family-level analysis indicated that this association was apparent for both MHC-I and MHC-II, when copy numbers within these two MHC regions were analysed separately. The association remained significant after controlling for basic life history components and for ecological traits commonly linked to pathogen exposure.

CONCLUSION

Our results provide the first phylogenetically robust evidence for an evolutionary trade-off within the avian immune system, with a more developed acquired immune system (i.e., more complex MHC architecture) in more derived bird lineages (e.g., passerines) being accompanied by an apparent downregulation of the innate immune system.