The Northern Andes, acting both as barrier and corridor, exert strong influence in Neotropical biodiversity. Still, the diversification of foothill taxa inhabiting ecotonal zones between lowland rainforest and montane environments in this region remains poorly understood. We investigated evolutionary patterns in Eutoxeres aquila, a hermit hummingbird distributed across Central America, the Chocó, and both Andean slopes. Using mitochondrial DNA (ND2, cytb), morphometrics, resistance surface analyses, and ecological niche models, we examined the roles of topography, connectivity, and climatic oscillations in shaping lineage divergence. Phylogenetic analyses of 59 individuals revealed two major E. aquila clades: (i) eastern slope–upper Magdalena and middle Magdalena–Tamá, and (ii) western slope–Chocó–Antioquia and Panama. Divergence from E. condamini occurred ~4.0 Mya, while crown diversification began ~1.6 Mya, coinciding with Pleistocene climate cycles. The Panama lineage diverged ~0.6 Mya, suggesting post-Isthmus dispersal. Morphometric analyses of 66 specimens showed significant bill shape differentiation among regions, broadly consistent with genetic structure. Ecological niche projections indicated expanded connectivity during the Mid-Holocene and LGM, but contraction and fragmentation during the LIG, with ~227,500 km² of long-term climatic stability concentrated mostly in Amazonia and the Chocó. These results highlight recurrent isolation and reconnection of mountain environments as key drivers of foothill diversification and emphasize the importance of ecotonal taxa for understanding Andean evolutionary processes.

Effective conservation of wildlife is often hindered by poor understanding of where focal species and their habitats occur across large landscapes. Advancements in remote sensing have enabled researchers to improve detection of focal species (e.g., autonomous recording units; ARUs) and the characterization of their habitats (e.g., light detection and ranging [LiDAR]), thus mitigating these issues. Research into Ruffed Grouse (Bonasa umbellus), a declining forest game bird, stands to benefit from these technologies given the species’ low detectability and preference for particular forest structure conditions that are difficult to capture using imagery-based remotely sensed data. Herein, we investigated regional occurrence of Ruffed Grouse across Pennsylvania via the use of a multi-year passive acoustic monitoring dataset, fine scale LiDAR-derived forest structure metrics, and a suite of other forest and landscape variables to predict state-wide Ruffed Grouse occurrence probability and identify areas in need of targeted habitat management. Our analyses indicated that, well-connected, high-elevation hardwood forests with some conifers and well-developed understories (e.g., timber harvests) were predicted to have the highest probability of grouse occurrence. Likewise, similar forests with open understories (e.g., mature forests within similar landscape contexts) were predicted to be the most promising for future management. ARUs proved to be effective at building a large detection dataset, which when paired with the superior forest structure data provided by LiDAR, allowed us make predictions about Ruffed Grouse across Pennsylvania with greater confidence than ever before.

Experimental studies in humans and laboratory species have shown that the decline of the immune system with age (immunosenescence) and the accumulation of oxidative damage to macromolecules are two key contributors to the onset and progression of the ageing process. Although laboratory models have provided important insights, the physiological basis of ageing in natural populations remains comparatively understudied, constraining our mechanistic understanding of the ageing process. The complexity of age-related physiological changes increases further in long-lived species, which appear to possess unique adaptations reducing immunosenescence and oxidative damage. However, studies investigating the underlying physiological mechanisms in long-lived birds have yielded contrasting results. In this study, we compared four markers of oxidative status and eight immune markers between younger and older breeders of a long-lived seabird, the Scopoli’s shearwater (Calonectris diomedea), to identify potential physiological signatures of ageing. Regardless of sex, older individuals exhibited higher levels of blood antioxidant enzymes, natural antibodies, and lymphocytes compared to younger birds, while levels of DNA damage and cellular effectors of innate immunity did not differ between age classes. These findings suggest that older shearwaters may upregulate antioxidant enzyme activity, possibly to cope with increased basal production of reactive oxygen species, in line with the oxidative stress theory of ageing. Alternatively, the higher antioxidant levels of older birds might reflect selective mortality of birds with reduced protection. In contrast to the oxidative status, the observed immune patterns do not support the immunosenescence hypothesis

Migratory birds face considerable temporal and energetic constraints during spring migration. To balance these demands and maximize breeding success, migratory birds may optimize spring migration by adopting a migration strategy that primarily favors either minimizing time or energy spent on migration. We used GPS telemetry to quantify interspecific, interindividual, and annual variation in spring migration strategies of 3 species of waterfowl that share a common wintering area in southern New England, USA, but differed in distance traveled between wintering and breeding areas, breeding latitude, breeding strategy, and breeding range size. Specifically, we characterized spring migration strategies by quantifying ordinal spring migration initiation date, ordinal spring migration completion date, migration duration, number of stopovers, average stopover duration, proportion of migration time spent in stopover, and a stopover to travel ratio. American Black Ducks (Anas rubripes; n = 20) demonstrated the most time-minimizing spring migration strategy whereas Atlantic Brant (Branta bernicla hrota; n = 20) and Greater Scaup (Aythya marila; n = 9) demonstrated more energy-minimizing spring migration strategies. Atlantic Brant and Greater Scaup also had less intraspecific variation in metrics of spring migration strategy than American Black Ducks, particularly those associated with stopover behavior, highlighting the importance of key stopover sites for long-distance migrating species. We found limited evidence of carryover effects of winter movement and space use patterns on spring migration behavior across all 3 species. Our findings demonstrate that sympatric-wintering waterfowl adopt distinct spring migration strategies along the continuum from time- to energy-minimization and have differing extents of intraspecific variation in migration strategy, which together have important conservation implications.

Birds can show patterns of sexual size dimorphism as early as during the nestling stage. This raises the question of how the faster growing sex might reconcile the energetic and nutritional needs of a faster growth rate with resource allocation to other important life functions, such as the development of innate immune function. Innate immunity represents the main line of defence against diseases, and while some innate immune defences are already present at hatching, substantial development occurs throughout the nestling stage. Hence, this development may compete for resource allocation with growth, potentially affecting nestlings in a sex-specific way in species showing sexual size dimorphism at early age. However, little is known about how sex might shape life-history strategies early into the life cycle. In this two-year study, we molecularly determined the sex of Great tit (Parus major) nestlings. We measured morphometrics (mass, wing and tarsus) and carried out innate immunity assays (Hemolysis-hemagglutination assay, Bacteria Killing Assay, and Haptoglobin assay). We then compared size, mass and immune function among sexes shortly before fledging, likely reflecting the outcome of relative resource allocation during ontogeny. We also carried out a brood size manipulation experiment to simulate resource limitation in the nest. We found that male nestlings grew to a larger size at day 14 than their female siblings. However, we also found some indication that males developed a better immune defense than females albeit their faster growth. Thus, males manage to invest more heavily in both growth rate and immune defence, probably depending on males being dominant to females in the competition for parental feeding, resulting in higher resource acquisition.

Avian haemosporidian parasites are globally widespread with a broad repertoire of hosts. When infected, the host can either reduce the parasite burden (resistance) and/or limit the severity of parasitaemia (tolerance). Oxidative stress is known to play a pivotal role in the host’s resistance and tolerance as well as its detrimental endpoints. The rationale behind this paradox lies in the dual role of reactive oxygen species (ROS): they can have both beneficial and detrimental effects for the host, while being largely harmful to the parasite. Thus, it is in the parasite’s interest to maintain a reduced environment within the host’s cell, whereas the host needs a fine-tuned balance between generating ROS to eliminate the parasites and maintaining sufficient antioxidant levels to protect its own tissues. This dynamic is what we refer to as the host-parasite oxidative arms-race. In this study, Eurasian siskins (Spinus spinus) were experimentally infected with Plasmodium ashfordi to investigate how the fundamental antioxidant system – the glutathione redox-system – responds to infection over time compared to control birds. By combining physiological measures and gene expression data of key glutathione related genes from both the parasite and the host at different time points, we provide evidence of this oxidative arms race. The gene expression data show that the parasite actively maintains reduced intracellular environment and eliminates ROS through high expression of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione synthetase. In contrast, the host upregulates glutathione S-transferases (GSTs) and glutathione peroxidases (GPX), which reduce the physiologically active levels of the key antioxidant, glutathione. Although, the parasite seems to win the race in terms of the oxidative state of the cell, the marked decrease in parasitaemia from day 21 (45%) to 31 (15%) suggests that the host’s strategy by lowering the physiological glutathione levels is sufficient to defeat the parasite.

Avian tracking studies have proliferated due to the miniaturization of tags and the opportunities for massive data collection facilitated by an extensive satellite and cellular infrastructure. However, assessments of the effects of tags on the behaviour and survival of birds are rarely conducted and disseminated – raising animal welfare concerns, risking project failure, and hindering the optimization of tagging methods within the ornithological community. Here we quantified the effects of tags on banded dotterels (Anarhynchus bicinctus) – a threatened small-bodied (median 59 g) partially migratory shorebird native to New Zealand and a priority species for conservation planning on Austral flyways. We deployed ten 1.2 g archival GPS tags and ten 1.8–2 g Argos satellite tags on breeding dotterels in Kaikōura, New Zealand. Including leg bands and silicone-tubing leg-loop harness, tag deployments constituted 2.7–4.3% of an average individual’s mass (or 1.9–3.4% based on the tag alone). Both tag types documented the curiously mixed winter strategies characteristic of banded dotterels: migrants flew north to the upper North Island or south to inland Canterbury, while other individuals stayed resident in Kaikōura. Compared to a control group of 74 untagged dotterels, neither tag technology had adverse effects on subsequent breeding outcomes, annual apparent survival, behaviour, or body condition, but Argos satellite tags provided data over a longer period than archival GPS tags. One possible reason for the absence of adverse effects could be that banded dotterels (and other Charadriinae species) primarily rely on ground-based locomotion, characterized mainly by walking and running – movements that are less hindered by the added mass of auxiliary attachments. Our findings support the ‘3% rule’ (i.e., using tag weight alone as a guideline), but we suggest that tag deployment limits could be refined by considering both the species’ ability to carry additional weight and its primary mode of locomotion.

In double-nesting species with uniparental incubation, such as the rock partridge (Alectoris graeca saxatilis), the male incubates one clutch and the female the other clutch. A previous study has shown no difference in hatching and predation rates on rock partridge nests incubated by each sex. Therefore we hypothesised that the incubation rhythms would be similar between the sexes. To test this hypothesis, incubation rhythms of 40 nests incubated by females and 41 by males of Alpine rock partridge were monitored in the French Alps between 2011 and 2020. Temperature data loggers were placed, one inside the nest and the other outside, to measure ambient temperatures. The timing and duration of incubation recesses were quantified for each monitored bird by analysing the temperature curves. Statistical modelling was used to assess the effects of incubating bird sex, minimum ambient temperature, incubation day, clutch size and recess rank on incubation parameters. The daily timing of recesses, total recess duration, number of recesses and nest temperature are similar between the sexes. Birds made an average of 2.5 recesses per day, averaging 75 min each. On average, they spend 87.9% of their time incubating eggs. Recesses occurred throughout the day, with a peak 15-30 min after sunrise and a second, less pronounced maximum, before sunset. Nocturnal recesses were rare caused by disturbance of the incubating bird. Modelling showed only slight differences in mean recess duration between sexes throughout incubation. Other modelling results showed that both sexes adapted in a similar way their rhythms to incubation progress and to daily minimum temperature. Low temperatures might force parents to spend more time foraging outside the nest to meet energy requirements. Rock partridge nest attendance was lower than that of grouse species, a difference probably related to the rock partridge’s lower body reserves compared to grouse.

DNA methylation, which can change within-individuals over time and regulate gene expression, is important to many aspects of avian biology. It is particularly important in avian responses to various stressors associated with introductions, such as infection and environmental changes. However, it remains unclear whether native and invasive bird species differ in their epigenetic responses to stress, and how DNA methylation may contribute to the success of invasive species because of the limited availability of longitudinal epigenetic studies. To address this knowledge gap, we used epiRADseq to investigate changes in DNA methylation within-individual house sparrows (Passer domesticus) over an eight hour period in response to simulated infection. We compare wild-caught house sparrows from introduced populations with those from native populations, assessing the number of genomic locations that exhibit changes, the magnitude of those changes, and the variance among individuals. Our results show that individuals from introduced populations experience more widespread changes in DNA methylation, with greater magnitude and higher variance, compared to their counterparts from native populations. Together, these findings suggest that DNA methylation plays a significant role in an individual’s response to infection. They also indicate that individuals from introduced populations may exhibit distinct epigenetic responses compared to their native counterparts, consistent with the concept of epigenetic buffering.

Phenotypic plasticity is a major mechanism whereby organisms adjust their traits to changes in environmental conditions. In the context of range expansions, plasticity is especially important, as plastic changes in traits can lead to rapid adaptation. For these reasons, there has been growing interest in the role of molecular epigenetic processes in range expansions. One epigenetic process in particular, DNA methylation, enables organisms to adjust gene expression contingent on the environment, which suggests it may play a role in some invasions. Nevertheless, we know little about how methylation is regulated in wildlife, especially expression of the enzymes responsible for altering methyl marks on the genome. The most important forms of these enzymes in vertebrates are DNA methyltransferase 1, which largely maintains existing methyl marks, DNA methyltransferase 3, which creates most de novo methyl marks, and TET2, which is a major demethylator of CpG motifs, genomic regions where most methyl marks occur. In this study, we compared expression of these genes in three tissues (i.e., gut, liver, and spleen) of house sparrows (Passer domesticus) from 9 locations. Some sparrow populations derived from the native range (i.e., Israel, Netherlands, Norway, Spain, and Vietnam) whereas others were introduced <150 years ago (i.e., Australia, Canada, New Zealand, Senegal). Our hypothesis was that non-native birds and/or birds from sites with comparatively unpredictable climates would express more of all three genes. We found, however, that DNMT expression differences, while extensive, were reversed of predictions: all three genes were expressed more in sparrows from the native range and from areas with more predictable temperatures. Surprisingly, gene expression was also strongly correlated among populations and within-individuals. Our results reveal no simple role for these enzymes in range expansions, but the appreciable among and within-population variation in these enzymes warrants more detailed investigations.

“‘latex Wading birds have declined globally with pronounced declines in Western Europe. Multiple species are now IUCN Red listed, with northern lapwing (Vanellus vanellus) near threatened and declining. Declines have been attributed to poor breeding success due to high rates of predation on nests and chicks, by avian and mammalian predators. In Britain badgers (Meles meles) have increased their range and abundance, and can occur at high densities, with potential for acute local predation impacts on vulnerable wader populations. However, factors affecting rates of badger predations on wader nests remain unexplored. Using data from six years of lapwing nest monitoring at a key breeding site in North-East Scotland, we aimed to identify the factors associated with badger nest predation. The overall probability of badger predation was above 0.1 when mean daily temperature was below 4 °C in the 7 days before a nest’s completion, dropping close to zero when above 10 °C. Predation on lapwing clutches also increased with earthworm availability, and inter-annual effects were observed matching variations in temperature, whereby intense badger predation in 2021 coincided with unseasonably cold temperatures. This highlights the potential for weather forecasting to be used to deploy pre-emptive management strategies to mitigate badger predation impacts on lapwing nests.

As species interactions influence species distributions, attempts to work backwards from observed species distributions to infer the effect of a potential interaction have been enticing. The “natural removal experiment,” an approach that tests for patterns consistent with competition by comparing a species’ habitat relationships in sympatry and allopatry with a potential competitor, has held promise, but has often been limited by the scales of data required. The recent expansion of community science-based datasets invites renewed opportunity for investigation using this approach. We revitalize the natural removal experiment by applying it to ask whether the distribution of the Chestnut-backed Chickadee (Poecile rufescens; CBCH) is consistent with competition with the Black-capped Chickadee (Poecile atricapillus; BCCH) in urban areas. Using data from the community science project eBird, we compared relationships of CBCH relative abundance to urban- and forest-related variables in individual urban-centres and across regions of allopatry and sympatry with the BCCH. As predicted under competition, we found that in allopatry, the CBCH adopted habitat relationships similar to the less forested, more urban habitats of the BCCH at both the urban-centre and regional scale. By applying predictive modeling to “imagine” either the absence or ubiquity of BCCH across the studied range of CBCH, we found that sympatry with the BCCH suppressed CBCH abundance in urban areas, and that in their absence CBCH abundance increased, consistent with competitive release. These lines of evidence suggest that the observed distribution of CBCH is consistent with that expected under competition with the BCCH in urban areas. Expanding beyond this case study, we discuss important considerations in the application of this approach, which presents a significant opportunity for researchers to harness community science to conduct more powerful investigations of species interactions extending to broad spatial scales.

Avian haemosporidians are a diverse group of parasites that infect birds worldwide and have been a major focus of research for decades. Yet, few studies have identified the drivers of infection at the intraspecific host level. We aimed to study the drivers of prevalence and diversity of haemosporidian parasites infecting a common North American songbird species, the American Robin (Turdus migratorius), which breeds across most of the continent. We found little seasonal variation in haemosporidian prevalence in robins, although prevalence of Leucocytozoon increases with breeding latitude in this host species. We also detected a significantly positive relationship between the robin breeding latitude and coinfection with different haemosporidian parasite lineages. Additionally, robins infected with Plasmodium had a significantly better body condition than those of uninfected robins, which could be due to migratory culling. When matched against known haemosporidian lineages, our results suggest that robins harbor a higher diversity of haemosporidian parasites than previously known. The results of this study suggest that comparisons of common, widespread bird species such as robins across their range could help unveil novel aspects of the haemosporidian–host relationship and how such a relationship may change under current and future rapid environmental change.

Although the negative impact of human-induced environmental effects on bird populations has been widely demonstrated, the question of whether adaptive adjustments may potentially arise as a result of unforeseen challenges is still unclear. Despite their obvious pervasive effect, human-induced challenges may activate, under certain circumstances, physiological and behavioural compensatory mechanisms that allow organisms to cope better with an altered and distressful environment. In this viewpoint, we highlight that understanding such compensatory responses (or the lack of them) requires adopting an ontogenetic and transgenerational perspective, as well as a multidisciplinary approach that integrates physiology, ageing biology, molecular processes and behaviour. Given the outstanding capacity for plasticity during development, we focus on how early-life (human-induced) experiences potentially shape, even prenatally, specific physiological and molecular processes (i.e., protection against oxidative damage and telomere maintenance mechanisms), and lifelong reproductive strategies (i.e., maternal allocation into eggs), which may in turn activate physiological and behavioural adjustments across generations. To test whether such adjustments in the developmental trajectory allow individuals to make “the best of a bad situation” or even increase their performance or that of their offspring in human-altered environments, we call for studies using a lifelong approach and that explore transgenerational effects. We thus propose experimental designs that could help the advancement in the field.

Ecologically-relevant factors such as exercise and diet quality can directly influence how multifaceted physiological systems work; however, little is known about how such factors directly and interactively affect key components of the antioxidant system in multiple tissues of migratory songbirds. We tested 3 main hypotheses across three tissues in European Starlings fed diets with more or less antioxidants (anthocyanins) and long-chain polyunsaturated fats (18:2n6) while being flight-trained in a wind tunnel. Stimulatory effect of flight: Flight-training stimulated the antioxidant system in that a) plasma oxidative damage was reduced during a given acute flight, and b) antioxidant capacity and oxidative damage in plasma and tissues of flight-trained birds were similar to that of untrained birds. Flight-trained birds that expended more energy per unit time (kJ/min) during their longest, final flight decreased the non-enzymatic component of their antioxidant system the most during the final flight. Dietary antioxidant effect: Flight-trained birds that consumed more dietary anthocyanins had similar antioxidant capacity in liver and flight-muscle compared to untrained birds, and oxidative damage was prevented in the flight-muscle and reduced in the liver of flight-trained birds compared to untrained birds. Dietary fat quality effect: Contrary to our predictions, dietary 18:2n-6 did not influence oxidative status even after flight training. We found limited evidence that circulating and tissue-level oxidative capacity and damage were tightly regulated in flight-trained starlings, in contrast to the precise regulation on gene expression and enzyme activity that were observed in companion studies. In sum, the antioxidant system of songbirds flexibly responded to changes in availability of dietary antioxidants as well as increased flight time and effort, and such condition-dependent, individual-level, tissue-specific responses to the oxidative costs of long-duration flights apparently requires recovery periods for maintaining oxidative balance during migration.

As day length increases in spring, birds prepare to migrate and breed, relying on timing mechanisms shaped by selection to match ecological conditions suitable for reproduction. As the climate changes these mechanisms will determine how successful birds will be in keeping up. In this contribution, we review studies comparing free-living and captive migratory and resident populations of a songbird, the dark-eyed junco, as they diverge in behavior and physiology during pre-breeding (spring) despite co-occurrence in the same environment. We report on differences in mechanisms related to timing including photoperiodic thresholds, endocrine profiles of testosterone and corticosterone, and gene expression. We also provide evidence for genetic differences in timing among populations based on sustained differences in timing when populations were held in a common environment. We conclude that existing variation in pre-breeding physiology likely serves to match timing of migration and reproduction to the appearance of favorable environments where breeding occurs. Greater knowledge of mechanisms responsible for variation in timing will allow better predictions of future adaptation.

In consideration of current global climate change, ecophysiological research on wild birds has increased its emphasis on approaches related to thermal tolerance. Many studies have investigated how desert specialists are adapted physiologically to the hot and xeric conditions they live in. Our aim was to test whether migratory passerines from temperate areas also have physiological adaptations to cope with heat stress and whether such adaptations may be related to habitat or migration distance. Using video recording and flow-through respirometry, we measured temperatures of panting onset (TPANT) of 113 individuals of 14 different species, exposed to increasing ambient temperature. Our study species differed in size, migration type (short-distance migrants vs. trans-Saharan migrants) and habitat preferences (woodland, farmland, reeds). We found that trans-Saharan migrants started panting at higher ambient temperatures (TA) than short-distance migrants of similar size, but no difference between species from different habitats. This finding suggests that migrants facing a desert crossing may have adaptations to decrease the risk of dehydration while maintaining body temperature below the critical range. According to this, we suggest that there may be selection on traits related to the modulation of respiratory water loss in birds that cross the Sahara Desert during migration. Flexibility in these traits will be of crucial importance in a warmer future.

Preen oil – the secretion from the uropygial gland of birds – may have diverse functions in avian reproduction: protection against eggshell bacteria, olfactory crypsis against nest predators and olfactory mate choice. To investigate such functions, we should first characterise variation in preen oil composition, but also confirm that previously described patterns are robust. Replication studies are crucial to test the reproducibility of previous findings, but are rarely undertaken in chemical ecology. Here, we conducted an almost exact replication of a previous study on the chemical composition of preen oil in a wild passerine bird, the Pied Flycatcher (Ficedula hypoleuca). We aimed to estimate the reproducibility of the previous results using larger sample sizes and following a pre-registered analysis. In addition, we explored the ontogeny of preen oil composition by comparing nestling and adult preen oil. In line with previous findings, preen oil composition was similar between breeding partners and not repeatable within individual females across breeding stages. Female preen oil changed across breeding stages more clearly than in the original study (higher richness, diversity and volatility during incubation than nestling-rearing), further refuting a role of preen oil in olfactory crypsis in this species. Unlike the original study, we found no difference in chemical profiles between sexes (nestling-rearing), casting doubt on the proposed role of preen oil as a sex semiochemical in this species. Nestling preen oil differed from adults, was more similar to adult males than to adult females, but was not more similar to parents than to non-parents. We found family chemical signatures, which, along with the breeding pair signature, suggests an influence of the nest environment on preen oil composition. Our study highlights the importance of replication and provides novel insights into the function and development of preen oil.