It pays to follow the leader: Metabolic cost of flight is lower for trailing birds in small groups.

Many bird species commonly aggregate in flocks for reasons ranging from predator defense to navigation. Available evidence suggests that certain types of flocks-the V and echelon formations of large birds-may provide a benefit that reduces the aerodynamic cost of flight, whereas cluster flocks typical of smaller birds may increase flight costs. However, metabolic flight costs have not been directly measured in any of these group flight contexts [Zhang and Lauder, J. Exp. Biol. 226, jeb245617 (2023)]. Here, we measured the energetic benefits of flight in small groups of two or three birds and the requirements for realizing those benefits, using metabolic energy expenditure and flight position measurements from European Starlings flying in a wind tunnel. The starlings continuously varied their relative position during flights but adopted a V formation motif on average, with a modal spanwise and streamwise spacing of [0.81, 0.91] wingspans. As measured via CO2 production, flight costs for follower birds were significantly reduced compared to their individual solo flight benchmarks. However, followers with more positional variability with respect to leaders did less well, even increasing their costs above solo flight. Thus, we directly demonstrate energetic costs and benefits for group flight followers in an experimental context amenable to further investigation of the underlying aerodynamics, wake interactions, and bird characteristics that produce these metabolic effects.

Revisiting Perdeck's massive avian migration experiments debunks alternative social interpretations.

Whether avian migrants can adapt to their changing world depends on the relative importance of genetic and environmental variation for the timing and direction of migration. In the classic series of field experiments on avian migration, A. C. Perdeck discovered that translocated juveniles failed to reach goal areas, whereas translocated adults performed 'true-goal navigation'. His translocations of > 14 000 common starlings (Sturnus vulgaris) suggested that genetic mechanisms guide juveniles into a population-specific direction, i.e. 'vector navigation'. However, alternative explanations involving social learning after release in juveniles could not be excluded. By adding historical data from translocation sites, data that was unavailable in Perdeck's days, and by integrated analyses including the original data, we could not explain juvenile migrations from possible social information upon release. Despite their highly social behaviour, our findings are consistent with the idea that juvenile starlings follow inherited information and independently reach their winter quarters. Similar to more solitarily migrating songbirds, starlings would require genetic change to adjust the migration route in response to global change.

Choosing fast and simply: Construction of preferences by starlings through parallel option valuation.

The integration of normative and descriptive analyses of decision processes in humans struggles with the fact that measuring preferences by different procedures yields different rankings and that humans appear irrationally impulsive (namely, show maladaptive preference for immediacy). Failure of procedure invariance has led to the widespread hypothesis that preferences are constructed "on the spot" by cognitive evaluations performed at choice time, implying that choices should take extra time in order to perform the necessary comparisons. We examine this issue in experiments with starlings (Sturnus vulgaris) and show that integrating normative and descriptive arguments is possible and may help reinterpreting human decision results. Our main findings are that (1) ranking alternatives through direct rating (response time) accurately predicts preference in choice, overcoming failures of procedure invariance; (2) preference is not constructed at choice time nor does it involve extra time (we show that the opposite is true); and (3) starlings' choices are not irrationally impulsive but are instead directly interpretable in terms of profitability ranking. Like all nonhuman research, our protocols examine decisions by experience rather than by description, and hence support the conjecture that irrationalities that prevail in research with humans may not be observed in decisions by experience protocols.

Variation in oxidative status, but not structural and physiological development, associated with changing ontogenetic environments.

Despite the potential for temporally dependent relationships between trait values and fitness (e.g. as juveniles approach life-stage transitions such as fledging), how developmental stage affects canalization (a measure of robustness to environmental variation) of morphological and physiological traits is rarely considered. To test the sensitivity of morphological and physiological traits to environmental variation in two developmental stages, we manipulated brood size at hatch in European starlings (Sturnus vulgaris) and cross-fostered chicks between enlarged and reduced broods approaching fledging. We measured body size (mass, tarsus, wing length) and physiological state (aerobic capacity, oxidative status) at asymptotic mass on day 15, then cross-fostered chicks between 'high' and 'low' quality environments and assessed the same traits again on day 20, after 5 days of pre-fledging mass recession. Chicks in reduced broods were heavier at asymptotic mass and had lower reactive oxygen metabolites than enlarged broods, whereas structural size, aerobic capacity and antioxidant capacity were unaffected by experimental brood size. The observed canalization of structural and physiological traits during early development was maintained after cross-fostering, during late development. However, in contrast to early development, antioxidant capacity approaching fledging appeared sensitive to environmental conditions, as trajectories varied by cross-fostering treatment. Elevated reactive oxygen metabolites observed after early development in enlarged brood chicks were maintained after cross-fostering, suggesting that canalized development in low-quality environments could produce oxidative costs that carry over between life stages, even when conditions improve. These data reveal trait-specific relationships between environmental conditions and development, and highlight how natal environment effects may vary by developmental stage.

Spike Train Coactivity Encodes Learned Natural Stimulus Invariances in Songbird Auditory Cortex.

The capacity for sensory systems to encode relevant information that is invariant to many stimulus changes is central to normal, real-world, cognitive function. This invariance is thought to be reflected in the complex spatiotemporal activity patterns of neural populations, but our understanding of population-level representational invariance remains coarse. Applied topology is a promising tool to discover invariant structure in large datasets. Here, we use topological techniques to characterize and compare the spatiotemporal pattern of coactive spiking within populations of simultaneously recorded neurons in the secondary auditory region caudal medial neostriatum of European starlings (Sturnus vulgaris). We show that the pattern of population spike train coactivity carries stimulus-specific structure that is not reducible to that of individual neurons. We then introduce a topology-based similarity measure for population coactivity that is sensitive to invariant stimulus structure and show that this measure captures invariant neural representations tied to the learned relationships between natural vocalizations. This demonstrates one mechanism whereby emergent stimulus properties can be encoded in population activity, and shows the potential of applied topology for understanding invariant representations in neural populations.SIGNIFICANCE STATEMENT Information in neural populations is carried by the temporal patterns of spikes. We applied novel mathematical tools from the field of algebraic topology to quantify the structure of these temporal patterns. We found that, in a secondary auditory region of a songbird, these patterns reflected invariant information about a learned stimulus relationship. These results demonstrate that topology provides a novel approach for characterizing neural responses that is sensitive to invariant relationships that are critical for the perception of natural stimuli.

Plasticity in social behaviour varies with reproductive status in an avian cooperative breeder.

Cooperatively breeding vertebrates are common in unpredictable environments where the costs and benefits of providing offspring care fluctuate temporally. To balance these fitness outcomes, individuals of cooperatively breeding species often exhibit behavioural plasticity according to environmental conditions. Although individual variation in cooperative behaviours is well-studied, less is known about variation in plasticity of social behaviour. Here, we examine the fitness benefits, plasticity and repeatability of nest guarding behaviour in cooperatively breeding superb starlings (Lamprotornis superbus). After demonstrating that the cumulative nest guarding performed at a nest by all breeders and helpers combined is a significant predictor of reproductive success, we model breeder and helper behavioural reaction norms to test the hypothesis that individuals invest more in guarding in favourable seasons with high rainfall. Variation in nest guarding behaviour across seasons differed for individuals of different reproductive status: breeders showed plastic nest guarding behaviour in response to rainfall, whereas helpers did not. Similarly, we found that individual breeders show repeatability and consistency in their nest guarding behaviour while individual helpers did not. Thus, individuals with the potential to gain direct fitness benefits exhibit greater plasticity and individual-level repeatability in cooperative behaviour.

The effects of training, acute exercise and dietary fatty acid composition on muscle lipid oxidative capacity in European starlings.

Migratory birds undergo seasonal changes to muscle biochemistry. Nonetheless, it is unclear to what extent these changes are attributable to the exercise of flight itself versus endogenous changes. Using starlings (Sturnus vulgaris) flying in a wind tunnel, we tested the effects of exercise training, a single bout of flight and dietary lipid composition on pectoralis muscle oxidative enzymes and lipid transporters. Starlings were either unexercised or trained over 2 weeks to fly in a wind tunnel and sampled either immediately following a long flight at the end of this training or after 2 days recovery from this flight. Additionally, they were divided into dietary groups that differed in dietary fatty acid composition (high polyunsaturates versus high monounsaturates) and amount of dietary antioxidant. Trained starlings had elevated (19%) carnitine palmitoyl transferase and elevated (11%) hydroxyacyl-CoA dehydrogenase in pectoralis muscle compared with unexercised controls, but training alone had little effect on lipid transporters. Immediately following a long wind-tunnel flight, starling pectoralis had upregulated lipid transporter mRNA (heart-type fatty acid binding protein, H-FABP, 4.7-fold; fatty acid translocase, 1.9-fold; plasma membrane fatty acid binding protein, 1.6-fold), and upregulated H-FABP protein (68%). Dietary fatty acid composition and the amount of dietary antioxidants had no effect on muscle catabolic enzymes or lipid transporter expression. Our results demonstrate that birds undergo rapid upregulation of catabolic capacity that largely becomes available during flight itself, with minor effects due to training. These effects likely combine with endogenous seasonal changes to create the migratory phenotype observed in the wild.

Changes in Body Temperature of Small Mammals and Birds in a Few Minutes Range as Reflection of Environmental Influences.

The study examined the changes in intraperitoneal body temperature of laboratory mice, Jungar hamsters, European greenfinch Chloris chloris, and starlings. In a few minutes range, these changes significantly correlated not only between the animals of the same species, but also between the different classes such as birds and mammals, which were isolated from each other and maintained under different illumination regimen. This phenomenon indicates some external influence(s) on the central mechanisms of the thermal control system not related to illumination regiment. In 80% cases, the phases of most pronounced rhythms of body temperature oscillating with the periods of 8-9 and 12-13 min coincided with those of geomagnetic field within the accuracy of ±1 min. However, the amplitude of body temperature oscillations did not depend on the amplitude of geomagnetic field (GMF) oscillations. Synchronicity of the changes in body temperature and GMF was observed at the amplitude of GMF oscillation of 0.4 nT, which is extremely low value. In contrast, there was no reaction of body temperature to greater (6-10 nT) but irregular and abrupt perturbations of GMF.

Survival Benefits of Group Living in a Fluctuating Environment.

Group living is predicted to arise only when the fitness benefits outweigh the costs of sociality. Group-living species-including cooperatively breeding and family-living birds and mammals-occur most frequently in environments where climatic conditions fluctuate unpredictably from year to year. The fitness consequences of group living are thus expected to vary with changing environmental conditions, though few studies have examined this possibility. We examined whether living in large social groups improves adult survivorship in cooperatively breeding superb starlings (Lamprotornis superbus). We also tested the hypothesis that larger groups buffer against harsh conditions by increasing survivorship most under periods of low rainfall. We found that group size was positively correlated with adult survival but in a sex-specific manner: female survival increased with group size across all environmental conditions, whereas male survival increased with group size only in wet years. Together with previous work in this system, our results suggest that larger groups confer survival benefits by reducing predation, rather than by improving access to food or buffering against physiological stress. Although group living does not appear to buffer against harsh conditions in adult starlings living in a fluctuating environment, living in larger groups does confer a survival advantage.

Flight training in a migratory bird drives metabolic gene expression in the flight muscle but not liver, and dietary fat quality influences select genes.

Training and diet are hypothesized to directly stimulate key molecular pathways that mediate animal performance, and flight training, dietary fats, and dietary antioxidants are likely important in modulating molecular metabolism in migratory birds. This study experimentally investigated how long-distance flight training, as well as diet composition, affected the expression of key metabolic genes in the pectoralis muscle and the liver of European starlings (Sturnus vulgaris, n = 95). Starlings were fed diets composed of either a high or low polyunsaturated fatty acid (PUFA; 18:2n-6) and supplemented with or without a water-soluble antioxidant, and one-half of these birds were flight trained in a wind-tunnel while the rest were untrained. We measured the expression of 7 (liver) or 10 (pectoralis) key metabolic genes in flight-trained and untrained birds. Fifty percent of genes involved in mitochondrial metabolism and fat utilization were upregulated by flight training in the pectoralis (P < 0.05), whereas flight training increased the expression of only one gene responsible for fatty acid hydrolysis [lipoprotein lipase (LPL)] in the liver (P = 0.04). Dietary PUFA influenced the gene expression of LPL and fat transporter fatty acid translocase (CD36) in the pectoralis and one metabolic transcription factor [peroxisome proliferator-activated receptor (PPAR)-α (PPARα)] in the liver, whereas dietary antioxidants had no effect on the metabolic genes measured in this study. Flight training initiated a simpler causal network between PPARγ coactivators, PPARs, and metabolic genes involved in mitochondrial metabolism and fat storage in the pectoralis. Molecular metabolism is modulated by flight training and dietary fat quality in a migratory songbird, indicating that these environmental factors will affect the migratory performance of birds in the wild.

In vivo online monitoring of testosterone-induced neuroplasticity in a female songbird.

Adult neuroplasticity in the song control system of seasonal songbirds is largely driven by photoperiod-induced increases in testosterone. Prior studies of the relationships between testosterone, song performance and neuroplasticity used invasive techniques, which prevent analyzing the dynamic changes over time and often focus on pre-defined regions-of-interest instead of examining the entire brain. Here, we combined (i) in vivo diffusion tensor imaging (DTI) to assess structural neuroplasticity with (ii) repeated monitoring of song and (iii) measures of plasma testosterone concentrations in thirteen female photosensitive starlings (Sturnus vulgaris) who received a testosterone implant for 3 weeks. We observed fast (days) and slower (weeks) effects of testosterone on song behavior and structural neuroplasticity and determined how these effects correlate on a within-subject level, which suggested separate contributions of the song motor and anterior forebrain pathways in the development of song performance. Specifically, the increase in testosterone correlated with a rapid increase of song rate and RA volume, and with changes in Area X microstructure. After implant removal, these variables rapidly reverted to baseline levels. In contrast, the more gradual improvement of song quality was positively correlated with the fractional anisotropy values (DTI metric sensitive to white matter changes) of the HVC-RA tract and of the lamina mesopallialis, which contains fibers connecting the song control nuclei. Thus, we confirmed many of the previously reported testosterone-induced effects, like the increase in song control nuclei volume, but identified for the first time a more global picture of the spatio-temporal changes in brain plasticity.

The Phenomenon of Synchronous Food Intake in Starlings (Sturnus vulgaris) under the Conditions of Isolation from Each Other.

The dynamics of body temperature was used to determine the food intake synchronization in starlings located at a distance of 105 km from each other. Thermal accumulators were implanted intraperitoneally to the birds; the birds had free access to food or were kept under the conditions of food deprivation. It was found that food intake moments significantly more often coincided with an accuracy of up to a minute even in the birds separated by a considerable distance. The difference in time of sunrise of a few minutes, as well as the conditions of constant lighting did not affect the synchronism of food intake. On the basis of the data obtained, it can be assumed that there is a certain external factor, probably of an electromagnetic nature, which synchronizes the functional activity of the body, including food intake.

Harsh conditions during early development influence telomere length in an altricial passerine: Links with oxidative stress and corticosteroids.

Stress during early development can induce substantial long-term effects in organisms. In the case of birds, despite growth compensations, nestlings reared under harsh conditions typically show reduced survival chances in adulthood. It has been proposed that environmental early-life stressors could affect longevity via effects on telomere length, possibly mediated through oxidative stress. However, the link between these processes is not clear. In this study, we experimentally manipulated brood size in spotless starlings (Sturnus unicolor) to test the causal relationship between early stress, oxidative and corticosterone-mediated stress and telomere shortening. Our results show that experimentally enlarged brood sizes led to a reduction in morphometric development on nestlings, the effect being stronger for females than males. Additionally, basal corticosterone levels increased with increasing brood size in female nestlings. Neither plasma antioxidant status nor malondialdehyde levels (a marker of lipid peroxidation) were affected by experimental brood size, although the levels of a key intracellular antioxidant (glutathione) decreased with increasing brood size. We found that the treatment showed a quadratic effect on nestling telomere lengths: these were shortened either by increases or by decreases in the original brood size. Our study provides experimental evidence for a link between developmental stress and telomere length, but does not support a direct causal link of this reduction with corticosterone or oxidative stress. We suggest that future studies should focus on how telomere length responds to additional markers of allostatic load.

Co-localization of mu-opioid and dopamine D1 receptors in the medial preoptic area and bed nucleus of the stria terminalis across seasonal states in male European starlings.

In seasonally breeding animals, changes in photoperiod and sex-steroid hormones may modify sexual behavior in part by altering the activity of neuromodulators, including opioids and dopamine. In rats and birds, activation of mu-opioid receptors (MOR) and dopamine D1 receptors in the medial preoptic area (mPOA) often have opposing effects on sexual behavior, yet mechanisms by which the mPOA integrates these opposing effects to modulate behavior remain unknown. Here, we used male European starlings (Sturnus vulgaris) to provide insight into the hypothesis that MOR and D1 receptors modify sexual behavior seasonally by altering activity in the same neurons in the mPOA. To do this, using fluorescent immunohistochemistry, we examined the extent to which MOR and D1 receptors co-localize in mPOA neurons and the degree to which photoperiod and the sex-steroid hormone testosterone alter co-localization. We found that MOR and D1 receptors co-localize throughout the mPOA and the bed nucleus of the stria terminalis, a region also implicated in the control of sexual behavior. Numbers of single and co-labeled MOR and D1 receptor labeled cells were higher in the rostral mPOA in photosensitive males (a condition observed just prior to the breeding season) compared to photosensitive males treated with testosterone (breeding season condition). In the caudal mPOA co-localization of MOR and D1 receptors was highest in photosensitive males compared to photorefractory males (a post-breeding season condition). Seasonal shifts in the degree to which neurons in the mPOA integrate signaling from opioids and dopamine may underlie seasonal changes in the production of sexual behavior.

Oxidative status and telomere length are related to somatic and physiological maturation in chicks of European starlings (Sturnus vulgaris).

Telomere length can be considered as an indicator of an organism's somatic state, long telomeres reflecting higher energy investment in self-maintenance. Early-life is a period of intense investment in somatic growth and in physiological maturation but how this is reflected in telomere length remains unclear. Using European starling chicks we tested: (i) how telomere length measured at asymptotic mass is related to proxies of somatic growth and physiological maturity in 17-day-old nestlings; (ii) how telomere length measured at 17 days then predicts the changes in somatic and physiological maturity occurring in fledglings (between 17 and 21 days); (iii) how growth and telomere length co-vary when chicks are under experimentally good (fed) growth conditions. Depending on environmental conditions, our data suggest links between somatic growth, physiological maturation and body maintenance parameters (positive with oxidative stress and negative with telomere length) in nestlings. Telomere length measured at day 17 predicted a subsequent change in physiological maturation variables observed in fledglings, but only in second-brood chicks: chicks with shorter telomeres had a higher pre-fledging rate of increase in haematocrit and haemoglobin content and a greater decrease in reticulocyte count. Finally, food supplementation of chicks did not change telomere length compared with that in control siblings. Our results suggest that physiological maturation prior to fledging may occur at the expense of telomere length but only when environmental conditions are sub-optimal.

Unique evolution of vitamin A as an external pigment in tropical starlings.

Pigments are largely responsible for the appearance of organisms. Most biological pigments derive from the metabolism of shikimic acid (melanins), mevalonic acid (carotenoids) or levulinic acid (porphyrins), which thus generate the observed diversity of external phenotypes. Starlings are generally dark birds despite iridescence in feathers, but 10% of species have evolved plumage pigmentation comprising bright colors that are known to be produced only by carotenoids. However, using micro-Raman spectroscopy, we have discovered that the bright yellow plumage coloration of one of these species, the Afrotropical golden-breasted starling Cosmopsarus regius, is not produced by carotenoids, but by vitamin A (all-trans-retinol). This is the first organism reported to deposit significant amounts of vitamin A in its integument and use it as a body pigment. Phylogenetic reconstructions reveal that the retinol-based pigmentation of the golden-breasted starling has independently appeared in the starling family from dark ancestors. Our study thus unveils a unique evolution of a new class of external pigments consisting of retinoids.

Developmental history and stress responsiveness are related to response inhibition, but not judgement bias, in a cohort of European starlings (Sturnus vulgaris).

Judgement bias tasks are designed to provide markers of affective states. A recent study of European starlings (Sturnus vulgaris) demonstrated modest familial effects on judgement bias performance, and found that adverse early experience and developmental telomere attrition (an integrative marker of biological age) both affected judgement bias. Other research has shown that corticosterone levels affect judgement bias. Here, we investigated judgement bias using a modified Go/No Go task in a new cohort of starlings (n = 31) hand-reared under different early-life conditions. We also measured baseline corticosterone and the corticosterone response to acute stress in the same individuals. We found evidence for familial effects on judgement bias, of a similar magnitude to the previous study. We found no evidence that developmental treatments or developmental telomere attrition were related to judgement bias per se. We did, however, find that birds that experienced the most benign developmental conditions, and birds with the greatest developmental telomere attrition, were significantly faster to probe the learned unrewarded stimulus. We also found that the birds whose corticosterone levels were faster to return towards baseline after an acute stressor were slower to probe the learned unrewarded stimulus. Our results illustrate the potential complexities of relationships between early-life experience, stress and affectively mediated decision making. For judgement bias tasks, they demonstrate the importance of clearly distinguishing factors that affect patterns of responding to the learned stimuli (i.e. response inhibition in the case of the Go/No Go design) from factors that influence judgements under ambiguity.

Physics-based simulations of aerial attacks by peregrine falcons reveal that stooping at high speed maximizes catch success against agile prey.

The peregrine falcon Falco peregrinus is renowned for attacking its prey from high altitude in a fast controlled dive called a stoop. Many other raptors employ a similar mode of attack, but the functional benefits of stooping remain obscure. Here we investigate whether, when, and why stooping promotes catch success, using a three-dimensional, agent-based modeling approach to simulate attacks of falcons on aerial prey. We simulate avian flapping and gliding flight using an analytical quasi-steady model of the aerodynamic forces and moments, parametrized by empirical measurements of flight morphology. The model-birds' flight control inputs are commanded by their guidance system, comprising a phenomenological model of its vision, guidance, and control. To intercept its prey, model-falcons use the same guidance law as missiles (pure proportional navigation); this assumption is corroborated by empirical data on peregrine falcons hunting lures. We parametrically vary the falcon's starting position relative to its prey, together with the feedback gain of its guidance loop, under differing assumptions regarding its errors and delay in vision and control, and for three different patterns of prey motion. We find that, when the prey maneuvers erratically, high-altitude stoops increase catch success compared to low-altitude attacks, but only if the falcon's guidance law is appropriately tuned, and only given a high degree of precision in vision and control. Remarkably, the optimal tuning of the guidance law in our simulations coincides closely with what has been observed empirically in peregrines. High-altitude stoops are shown to be beneficial because their high airspeed enables production of higher aerodynamic forces for maneuvering, and facilitates higher roll agility as the wings are tucked, each of which is essential to catching maneuvering prey at realistic response delays.

Calling rhythm as a predictor of the outcome of vocal interactions: flight departure in pale-winged starling pairs.

Vocal communication plays an important role in the regulation of social interactions and the coordination of activities in many animal species. Synchrony is an essential part of the establishment and maintenance of pair bonds, but few reports have investigated decision-making at the pair level. We investigated temporal characteristics of call exchanges in pale-winged starlings (Onychognathus nabouroup) that could predict whether one, two, or neither members of a pair would take off. Our analysis of these interactions revealed that the overall rhythm of a call exchange, as well as the acceleration towards the end of an interaction, were significantly associated with the type of behavioural outcome. Faster rhythms were associated with higher probabilities that both birds would fly away. Our results confirm the findings of previous studies showing that higher rates of alarm calls indicate imminent departure and highlight the relationship between temporal features of vocal interactions and their outcome.

Songbirds use spectral shape, not pitch, for sound pattern recognition.

Humans easily recognize "transposed" musical melodies shifted up or down in log frequency. Surprisingly, songbirds seem to lack this capacity, although they can learn to recognize human melodies and use complex acoustic sequences for communication. Decades of research have led to the widespread belief that songbirds, unlike humans, are strongly biased to use absolute pitch (AP) in melody recognition. This work relies almost exclusively on acoustically simple stimuli that may belie sensitivities to more complex spectral features. Here, we investigate melody recognition in a species of songbird, the European Starling (Sturnus vulgaris), using tone sequences that vary in both pitch and timbre. We find that small manipulations altering either pitch or timbre independently can drive melody recognition to chance, suggesting that both percepts are poor descriptors of the perceptual cues used by birds for this task. Instead we show that melody recognition can generalize even in the absence of pitch, as long as the spectral shapes of the constituent tones are preserved. These results challenge conventional views regarding the use of pitch cues in nonhuman auditory sequence recognition.