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BACKGROUND: Rapid morphological change is emerging as a consequence of climate change in many systems. It is intuitive to hypothesize that temporal morphological trends are driven by the same selective pressures that have established well-known ecogeographic patterns over spatial environmental gradients (e.g., Bergmans and Allens rules). However, mechanistic understanding of contemporary morphological shifts is lacking. RESULTS: We combine morphological data and whole genome sequencing from a four-decade dataset in the migratory bird hermit thrush (Catharus guttatus) to test whether morphological shifts over time are accompanied by genetic change. Using genome-wide association, we identify alleles associated with body size, bill length, and wing length. Shifts in morphology and concordant shifts in morphology-associated alleles over time would support a genetic basis for the observed changes in morphology over recent decades, potentially an adaptive response to climate change. In our data, bill size decreases were paralleled by genetic shifts in bill size-associated alleles. On the other hand, alleles associated with body size showed no shift in frequency over time. CONCLUSIONS: Together, our results show mixed support for evolutionary explanations of morphological response to climate change. Temporal shifts in alleles associated with bill size support the hypothesis that selection is driving temporal morphological trends. The lack of evidence for genetic shifts in body size alleles could be explained by a large role of plasticity or technical limitations associated with the likely polygenic architecture of body size, or both. Disentangling the mechanisms responsible for observed morphological response to changing environments will be vital for predicting future organismal and population responses to climate change.

Species-specific sexual traits facilitate species-assortative mating by reducing mating across species and reducing hybrid sexual attractiveness. For learned sexual traits, such as song in oscine birds, species distinctiveness can be eroded when species co-occur. Transcriptional regulatory divergence in brain regions involved in sensory learning is hypothesized to maintain species distinctiveness, but relatively few studies have compared gene expression in relevant brain regions between closely related species. Species differences in song are an important premating reproductive barrier between the collared (Ficedula albicollis) and pied flycatcher (F. hypoleuca). Here, we compare brain gene expression in adult males from each species and their naturally occurring F1 hybrids. We report overall conserved expression across species in a portion of the brain containing regions and nuclei known to be involved in song responses and learning. Further, among those genes that were differentially expressed between species, we find largely intermediate expression in hybrids. A single gene, SYT4 (synaptotagmin 4), known to be singing-associated, both was differentially expressed and has a putative upstream transcriptional regulatory factor containing fixed differences between the 2 species. Although a finer-scale investigation limited to song-specific regions may reveal further species differences, our findings provide insight into regulatory divergence in the brain between closely related species. [ABSTRACT FROM AUTHOR]

The process of decision making is a complex procedure influenced by both external and internal conditions. Songbirds provide an excellent model to investigate the neural mechanisms of decision making, because females rely on acoustic signals called songs as important stimuli in directing their mate choice. Previous experiments by our group and others have implicated secondary auditory brain sites in female evaluation of song quality, including the caudal portions of the nidopallium (NC) and mesopallium (CM). Recent pathway tracing experiments reveal a convergence of those sites onto a third region, the ventral portion of the intermediate arcopallium (AIV), suggesting that AIV may also play an important role in song evaluation and mate choice. Here we combined behavioral testing with lesion inactivation to investigate the role of AIV in song preference of female Bengalese finches (Lonchura striata domestica). Inactivation of AIV was associated with destabilization of rank ordering of song preferences. These data suggest a model in which the convergence of auditory activity in AIV plays an important role in female perception of song quality and production of courtship behaviors. Together with previous results that also demonstrate a role for the auditory areas that converge onto AIV, these findings extend the experimental tractability of this emerging animal model of sensory perception and decision making. [ABSTRACT FROM AUTHOR]

The accurate and reliable performance of learned vocalizations (e.g., speech and birdsong) modulates the efficacy of communication in humans and songbirds. Consequently, it is critical to understand the factors that regulate the performance of learned vocalizations. Across taxa, neural circuits underlying motor learning and control are replete with perineuronal nets (PNNs), and we analyzed how PNNs in vocal motor circuitry regulate the performance of learned song in zebra finches. We report that developmental increases in PNN expression in vocal circuitry are associated with developmental increases in song stereotypy. We also document that enzymatically degrading PNNs in the motor nucleus HVC acutely altered song structure (changes in syllable sequencing and production). Collectively, our data reveal a causal contribution of PNNs to the performance of learned behaviors and, given the parallels in the regulation of birdsong and speech, suggest that PNNs in motor circuitry could modulate speech performance. Manipulating perineuronal nets (PNNs) in the motor nucleus HVC of zebra finches alters the structure of vocalizations acquired during development, highlighting a causal contribution of PNNs in motor circuitry to the performance of learned behaviors. [ABSTRACT FROM AUTHOR]

Simple Summary: Colourful animals are often more conspicuous than dull animals, which increases their exposure to predators. One way to reduce predation risk could be to be more vigilant. We investigated this in two related songbird species, the colourful Gouldian finch and the less colourful long-tailed finch, at waterholes by measuring the frequency of head movements as a measure of vigilance. Both species showed a high frequency of head movements (high vigilance) when sitting in an open tree where they could be easily spotted. However, the more conspicuous Gouldian finch remained more vigilant when sitting under cover than the long-tailed finch. Moreover, juveniles and adults of the Gouldian finch had similar vigilance levels; whereas, juvenile long-tailed finches were less vigilant than their adult counterparts, possibly because of the juveniles often being in family groups. Both species showed a similar reduction in vigilance shortly after landing in the tree. The reduction in vigilance was affected by group size. This indicates initial uncertainty about the threat they might face, more so when alone. Finally, vigilance was higher at small waterholes, reflecting a higher perceived risk, as compared to larger waterholes. Conspicuousness increases the risk of predation. One strategy to reduce this risk is to increase vigilance. We investigated the frequency of head movements as a measure of vigilance at waterholes in two related songbird species that differed in their conspicuousness: the Gouldian finch and the long-tailed finch. While both species showed a high frequency of head movements (high vigilance) when sitting in an open tree, the more colourful Gouldian finch remained more vigilant than the less conspicuous long-tailed finch when sitting under cover. Moreover, juvenile Gouldian finches had similar vigilance levels than adults; whereas, juveniles of the long-tailed finch were less vigilant, possibly because of staying in the family group. Furthermore, both species showed a similar temporal course of vigilance with a fast decrease in vigilance after landing in the tree, which was affected by group size. Single individuals had much higher vigilance than groups of birds. Finally, vigilance was higher at small waterholes. The results show that conspicuousness, uncertainty when arriving at waterholes, and higher perceived risk at small waterholes increased vigilance. [ABSTRACT FROM AUTHOR]

Despite the wide use of zebra finches as an animal model to study vocal learning and production, little is known about impacts on their welfare caused by routine experimental manipulations such as changing their social context. Here we conduct a post-hoc analysis of singing rate, an indicator of positive welfare, to gain insights into stress caused by social isolation, a common experimental manipulation. We find that isolation in an unfamiliar environment reduces singing rate for several days, indicating the presence of an acute stressor. However, we find no such decrease when social isolation is caused by either removal of a social companion or by transfer to a familiar environment. Furthermore, during repeated brief periods of isolation, singing rate remains high when isolation is induced by removal of social companions, but it fails to recover from a suppressed state when isolation is induced by recurrent transfer to an unknown environment. These findings suggest that stress from social isolation is negligible compared to stress caused by environmental changes and that frequent short visits of an unfamiliar environment are detrimental rather than beneficial. Together, these insights can serve to refine experimental studies and design paradigms maximizing the birds' wellbeing and vocal output. [ABSTRACT FROM AUTHOR]

Species classified as "urban-adapters" are often assumed to thrive in cities because they are commonly found across the urbanization gradient. However, urban-living populations of many urban-adapted species have been found to have lower reproductive success relative to their rural counterparts. Sturnus vulgaris (European Starling) is a common urban-adapted species found across most of the globe. While S. vulgaris have lowered reproductive success in urban areas in their native range, less is known about how urbanization impacts reproduction in their invasive ranges. We tested for differences in reproductive investment and success across urban and rural S. vulgaris populations in Georgia, which is part of their North American invasive range. We found few differences in reproductive output for urban vs. rural S. vulgaris —clutch size, egg mass, egg volume, incubation behavior, provisioning rates, brooding behavior, and nestling wing chord were all similar across S. vulgaris populations from more urban and more rural study sites. Although urban birds produced a higher number of hatchlings and rural birds produced young in higher body condition, neither of these factors influenced reproductive success because the number of fledglings produced were similar for urban and rural breeding S. vulgaris. Overall, S. vulgaris in their invasive range performed similarly well in more urban vs. more rural habitats. Future work should explore whether urbanization affects other components of fitness in starlings (e.g. adult survival). [ABSTRACT FROM AUTHOR]

The arid belt of Central Asia constitutes a barrier for small landbirds during autumn migration. Some species that migrate from Siberia to Africa circumvent this barrier by taking a detour around the northern coast of the Caspian Sea. It becomes increasingly clear that at least some songbirds that migrate between eastern Europe and southern Asia also perform a detour along the northern edge of the desert. Trapping data from the northern fringe of the barrier collected in this study (Orenburg Region of Russia, 51° 22′ N, 61° 23′ E; September 2023) and from earlier studies performed west and east of our study site show that some songbirds that migrate from European breeding grounds towards south and south-eastern Asian wintering quarters make a direct crossing, whereas others make a detour and circumvent Central Asian deserts. Ecological differences between the species of migrants cannot explain the observed pattern. Those species that make a detour are the ones that are considered to have colonized Europe from their ancestral breeding areas east of the Ural Mountains most lastly. [ABSTRACT FROM AUTHOR]

Song divergence driven by social learning has been proposed to be a key factor driving allopatric speciation in oscine birds. Songbirds often respond more to songs deriving from their local population, suggesting the potential for acoustic divergence across populations to shape both intra- and intersexual interactions. However, many of these studies were conducted on species with simple songs and, as a result, we know comparatively little about the emergence of population differences and song discrimination in species with complex songs. We addressed this question in the pied flycatcher (Ficedula hypoleuca) by calculating the dissimilarity of songs from 2 foreign populations as well as from our study site to the local centroid. We then conducted a paired-design playback experiment where both local and foreign songs were played simultaneously. We found that pied flycatcher males showed significantly stronger responses to those songs that sounded more similar to the local population. This suggests that despite the high complexity of the pied flycatcher song, individuals are still able to discriminate across populations. Our results support the hypothesis that learned song divergence can act as a mechanism for assortative mating and allopatric speciation. [ABSTRACT FROM AUTHOR]

Aim: The Amazon rainforest is one of the most biodiverse regions on earth, but our understanding of the processes that have shaped its patterns of diversity remains incomplete. One hypothesis for Amazonian speciation is the river‐refuge hypothesis, which suggests that retraction of forests away from the periphery of Amazonia, where rivers are narrow, facilitated divergence of populations separated by wide rivers. Later re‐expansion of forests would have allowed secondary contact between these populations, and co‐occurring hybrid zones may reveal the location where expanding forests first reconnected. Here, we test whether a widespread Amazonian songbird species shows evidence of population contact zones in the eastern headwaters of the Tapajós river, hypothesised to be an area of secondary contact under the river‐refuge hypothesis. Location: Amazon and Atlantic forests of South America. Taxon: Ceratopipra rubrocapilla (Pipridae, Passeriformes). Methods: We sampled 147 C. rubrocapilla (Red‐headed Manakin) across its vast range, with 70 samples sequenced using reduced‐representation genomic sequencing and 139 sequenced at the mitochondrial gene cytochrome b. We use population genetic and phylogenetic analyses to investigate patterns of gene flow and population structure across the range, with a particular focus on samples from the headwaters of the Tapajós River to evaluate the potential role of the river‐refuge hypothesis. Results: We observe shallow population structure and evidence for widespread gene flow across the range of C. rubrocapilla. Patterns in the Tapajós headwaters match predictions of the river‐refuge hypothesis, with eastern headwater samples most similar to samples farther west on the opposite bank of the Tapajós River. Main Conclusions: The close co‐occurrence between population contact zones within C. rubrocapilla and other previously studied hybrid zones reinforces the hypothesis that the eastern Tapajós headwaters is where rainforest populations reconnected in the past, following predictions of the river‐refuge hypothesis of Amazonian speciation. [ABSTRACT FROM AUTHOR]

Beak morphology in birds is often linked to diet. In this study, we aimed to analyse whether diet or other drivers, such as allometry or species relatedness, explain the morphological beak variation of Neotropical passerines and whether there are specific predictors for beak shape variation for the two passerine suborders, Oscines and Suboscines. We analysed beak measurements (length, width, and depth) as response variables and body size, diet, and phylogenetic distance as predictor variables. After controlling for allometry which had major impact on beack shape, beak length and depth exhibited significant differences across diet categories and between suborders. Compared with Oscines, the beak shape of Suboscines is more influenced by species relatedness, probably owing to niche conservatism. Our findings suggest an adaptive convergence between the two passerine suborders in the Neotropics, driven mainly by allometric constraints rather than by the trophic niche. [ABSTRACT FROM AUTHOR]

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The article discusses the use and definitions of the terms "arena," "lek," and "court" in describing avian courtship sites. It argues that "arena" is inaccurate and inappropriate, while "lek" is the more appropriate term. The author reviews historical and contemporary literature to clarify the distinctions between these terms and emphasizes the importance of using accurate terminology in ornithology. The article provides definitions for "lek," "dispersed lek," "lekking," and "court," aiming to eliminate confusion and promote clarity in describing avian courtship behavior. [Extracted from the article]

Vegetation structural complexity surrounding nests can either provide concealment for intruders and mates or make it more difficult for hosts to recognize parasitic eggs. We investigated whether shading and vegetation aggregation increase extrapair paternity (the presence of broods with half‐siblings) and intraspecific brood parasitism (IBP, the occurrence of broods with unrelated offspring) in socially monogamous Blue‐black Grassquits Volatinia jacarina. We found that habitat shadowing was associated with increased occurrence of IBP, but found no association between the degree of shade and the presence of extrapair offspring. Our findings support the idea that habitat limits cryptic reproductive behaviours and that female grassquits may benefit from habitat shadows to parasitize conspecific nests. [ABSTRACT FROM AUTHOR]

High-throughput methods are revolutionizing our ability to classify neurons based on their transcriptome. In this issue of Neuron , Hozhabri and colleagues 1 first categorize songbird GABAergic neurons by functional role and then link these functional subtypes to molecular identity. High-throughput methods are revolutionizing our ability to classify neurons based on their transcriptome. In this issue of Neuron , Hozhabri and colleagues first categorize songbird GABAergic neurons by functional role and then link these functional subtypes to molecular identity. [ABSTRACT FROM AUTHOR]

Divergence in gametic traits can play a key role in reproductive isolation. Lifjeld et al. (2025) examined the evolution of sperm length in pairs of songbird populations at various stages along the speciation continuum. Their analyses demonstrated that sperm length diverges more rapidly in species with higher levels of female promiscuity, likely due to stabilizing selection favoring sperm cells that fit within female sperm storage structures. This divergence in sperm length may kickstart speciation in promiscuous songbirds. [ABSTRACT FROM AUTHOR]

Birdsong is a critical behaviour for reproductive success and therefore exhibits features that reach physiological performance limits. Only few examples of production constraints have been studied in detail. To address this gap in our knowledge, we ask whether singing during flight reaches respiratory and energetic limitations. Male common whitethroats, Curruca communis (formerly Sylvia communis), display three singing modes, perched stereotyped song, variable song during flight and variable song after landing. We compared temporal and frequency modulation characteristics of song for these three modes. When flying, males reduced the duration of syllables and increased silent periods between syllables, which resulted in markedly reduced sound density. These changes in temporal structure and similar minimal pause duration for all singing modes imply that ventilatory changes induced by flight drive these changes in song rhythm, while neither respiratory nor syringeal dynamics indicate limitations in oxygen delivery to vocal muscles. These findings identify song features that are potential indicators of the fitness of the singing bird. • Respiratory demands may limit temporal birdsong features. • We compared characteristics of whitethroat song (perched, in flight, after landing). • Song production during flight required respiratory adjustments. • Respiratory needs of flight caused a change in song rhythm. [ABSTRACT FROM AUTHOR]

Beaks of granivorous songbirds are adapted to dehusk seeds fast and efficiently. This is reflected in the large variety of beak shapes and sizes among species specialized in different seed types. Generally, larger beaks improve the dehusking of larger seeds by transmitting and withstanding higher bite forces. Meanwhile, smaller beaks are better suited for processing smaller seeds by allowing faster beak movements and better seed handling dexterity. These patterns are presumably the result of a trade-off between force and velocity inherent to lever systems. Since beak shape also varies among individuals of the same species, we investigated whether beak shape relates to variation in feeding performance and beak kinematics in the Domestic Canary (Serinus canaria). We analysed beak morphology of 87 individuals through both traditional size measurements and 3D-landmark analysis to capture metrics such as beak depth, length, width and curvature. We related these metrics of morphology to data on feeding performance and beak kinematics during feeding on smaller canary seeds and larger, tougher hemp seeds. We found that individuals with larger absolute beak depths were faster at dehusking the large seeds. Despite that individuals with shallow or long beaks displayed higher beak opening-closing frequencies, this did not result in a significantly shorter processing time of the smaller seeds. Our data are therefore in line with the presence of a force-velocity trade-off within a species, but without a velocity-related drawback of beak-size adaptations for increased bite force on the handling performance of a smaller and easier-to-crack seed., (© 2025. Published by The Company of Biologists.)

Methyl mercury (MeHg) is a widespread environmental contaminant that can have adverse effects on the health of vertebrate wildlife. This study focused on diets and MeHg contamination of nestling red-winged blackbirds (Agelaius phoeniceus) from a wetland in north Texas, USA. In previous research at the study site, a risk assessment model suggested that if emergent aquatic insects (i.e., odonates) were the dominant prey item in nestling diets, the health of nestling red-winged blackbirds could be negatively affected. The purpose of this study was to follow up on an earlier risk assessment and determine whether nestling red-winged blackbirds were accumulating elevated concentrations of MeHg at our study site. We had four objectives: (1) estimate the proportion of emergent aquatic insects, spiders, and terrestrial insects in diets of nestling red-winged blackbirds using a stable isotope-based dietary mixing model; (2) assess the concentrations of MeHg in emergent aquatic insects, spiders, and terrestrial insects; (3) assess the concentrations of MeHg in blood of nestling red-winged blackbirds; and (4) determine whether nestling red-winged blackbirds had MeHg concentrations that were high enough to pose a health risk. We found that nestling red-winged blackbirds had a diet dominated by terrestrial insect prey with low concentrations of MeHg and that the nestlings had low concentrations of MeHg in their blood, well below hazardous levels. The results of the study suggest that caution must be used when interpreting risk assessment models for nestling red-winged blackbirds. Because their diets can consist of varying proportions of emergent aquatic insects, spiders, and terrestrial insects, risk assessments based on estimates of diet from the literature that suggest nestlings could be at risk from Hg contamination should be followed up with studies to assess diet and/or the actual level of contamination of nestlings., (© The Author(s) 2025. Published by Oxford University Press on behalf of the Society of Environmental Toxicology and Chemistry. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Beak deformation, known as avian keratin disorder (AKD), can impair feeding and preening of birds, reducing their survival. This disorder is apparently caused by Poecivirus infection, although to date, the viral origin has been corroborated in only a few North American bird species. Considering that fruit-eating birds can track spatiotemporal variations in fruit abundance and that AKD may have a viral origin, the incidence of this disease can be expected to increase with flocking by birds. Therefore, we evaluated if austral thrushes (Turdus falcklandii) were attracted to urban areas when exotic plants offered fruits and if flocking of thrushes in urban areas increased the spread of AKD in this species in a Patagonian town. We fitted GPS loggers on some individuals with normal beaks in rural areas and found that they visit the town in fall. Through point count censuses, we recorded greater abundances of thrushes with normal and deformed beaks during fall-winter in urban sites than in rural sites. However, the abundance of birds with AKD declined more (78-87%) than that of individuals with normal beaks (44-52%) during the transition from fall-winter to spring-summer. In urban zones in our study area, fruits of exotic species ripen during fall, attracting austral thrushes from rural sites. Nevertheless, such an attraction for food resources may be an ecological trap for this species, as the increase in incidence of AKD in urban areas may drastically reduce the survival of birds during the most unfavorable period of the year., (© 2023 International Society of Zoological Sciences, Institute of Zoology/Chinese Academy of Sciences and John Wiley & Sons Australia, Ltd.)