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Portugal describes the unique morphology and habits of the secretary bird, which kills its prey with lightning-quick kicks to the head.

Animals are characterised, in part, by their use of voluntary movement, which is used to explore and exploit resources from their surrounding environment. Movement can therefore benefit animals, but will cost them their energetic reserves. Thus, adaptations for faster movements with negligible increases in energy expenditure will likely evolve via natural selection. Individual and social-level mechanisms have been shown to optimise this speed/energetic trade-off. Nevertheless, studies of social-level traits typically ignore individual variation, which is a cornerstone principle in evolutionary ecology. Furthermore, how individual phenotype interacts with the phenotypic composition of the group to govern the cost of transport may have been entirely overlooked. We investigate speed and the energetic consequences of individual-level phenotypic differences using body mass (both natural and artificially manipulated with additional weights) of homing pigeons (Columba livia) (N =16 birds; N = 193 useable flight trajectories). We then turn to social level phenomena, and manipulate the composition of pigeon groups by body mass (N= 12 birds in four treatments; N = 192 useable flight trajectories) and leadership rank (N = 30 birds in three groups, N = 286 useable flight trajectories) following earlier leadership identification flights (N = 33 birds, N = 306 useable flight trajectories). “Natural” body mass was predictive of flying speed in solo flights, but not in groups of greater mass by composition; “artificial” mass loading had no impact on speed in solo fliers, and was not tested in groups. Groups of leader phenotypes, showed faster speeds, and greater cohesion than follower phenotype groups, both in terms of flock spread, but also in consistency of positioning within the flock (“flock stasis”) across the flight. Flock stasis was further analysed across all other group flights. Its positive impact on speed was found to be consistent across all experimental treatments. Therefore, predicting flock stasis may be critical to understanding optimal phenotypic compositions of birds, and thus the social evolution of birds which fly together. We provide evidence that greater stasis may be driven by phenotypic compositions (i.e. groups of leaders, and homogeneous mass groups) and also discuss the implications of stasis for different flocking structures (e.g. V-formations) and human crowd control.

Summary The “selfish herd” hypothesis 1 provides a potential mechanism to explain a ubiquitous phenomenon in nature: that of non-kin aggregations. Individuals in selfish herds are thought to benefit by reducing their own risk at the expense of conspecifics by attracting toward their neighbors’ positions 1 , 2 or central locations in the aggregation. 3 , 4 , 5 Alternatively, increased alignment with their neighbors’ orientation could reduce the chance of predation through information sharing 6 , 7 , 8 or collective escape. 6 Using both small and large flocks of homing pigeons (Columba livia; n = 8–10 or n = 27–34 individuals) tagged with 5-Hz GPS loggers and a GPS-tagged, remote-controlled model peregrine falcon (Falco peregrinus), we tested whether individuals increase their use of attraction over alignment when under perceived threat. We conducted n = 27 flights in treatment conditions, chased by the robotic “predator,” and n = 16 flights in control conditions (not chased). Despite responding strongly to the RobotFalcon—by turning away from its flight direction—individuals in treatment flocks demonstrated no increased attraction compared with control flocks, and this result held across both flock sizes. We suggest that mutualistic alignment is more advantageous than selfish attraction in groups with a high coincidence of individual and collective interests (adaptive hypothesis). However, we also explore alternative explanations, such as high cognitive demand under threat and collision avoidance (mechanistic hypotheses). We conclude that selfish herd may not be an appropriate paradigm for understanding the function of highly synchronous collective motion, as observed in bird flocks and perhaps also fish shoals and highly aligned mammal aggregations, such as moving herds.

Climate affects all aspects of biology. Physiological traits play a key role in mediating these effects, because they define the fundamental niche of each organism.Climate change is likely to shift environmental conditions away from physiological optima. The consequences for species are significant: they must alter their physiology through plasticity or adaptation, move, or decline to extinction. The ability to understand and predict such organismal responses to global change is, however, only as good as the geographical coverage of the data on which these predictions are based.Geographical biases in the state of physiological knowledge have been identified, but it has not been determined if these geographical biases are likely to limit our capacity to predict the outcomes of global change. We show that current knowledge of physiological traits is representative of only a limited range of the climates in which terrestrial animals will be required to operate, because data for animals from only a limited range of global climates have been incorporated in existing compilations.We conclude that geographical bias in existing datasets limits our capacity to predict organismal responses in the vast areas of the planet that are unstudied, and that this geographical bias is a much greater source of uncertainty than the difference between the current climate and the projected future climate. Addressing this bias is urgent to understand where impacts will be most profound, and where the need for intervention is most pressing.A free Plain Language Summary can be found within the Supporting Information of this article.

Dominance hierarchies are a social dynamic common in many species, which help balance the costs and benefits of social living. Which individuals in a group occupy high ranking positions is influenced by a multitude of different intrinsic and/or extrinsic factors. While homing pigeons (Columba livia) have been a model system for their navigational abilities and flight dynamics, less is known about their dominance hierarchies and general social behaviour when not in flight. Here we document the dominance hierarchies in two closed populations of homing pigeons, and investigate the physiological factors associated with dominance rank, including body size, metabolic rate, personality, and iridescent plumage % reflectance. Measurements of body size and resting metabolic rate all positively correlated with dominance rank in accordance with the performance model of energetic management. Iridescent plumage % reflectance was negatively correlated with rank, suggesting a potential cost of dominance. Attempts to alter hierarchy structure through manipulations of body mass and feather iridescence were ineffective which hierarchy dynamics remaining stable throughout the perturbations and across measurement sessions.

Steven Portugal introduces the behavioral and aerodynamic underpinnings of aerial flocking in birds.

Funder: National Science Foundation; Id: http://dx.doi.org/10.13039/100000001, Funder: Tanzanian Commission for Science and Technology (COSTECH), Funder: NERC, Funder: Max-Planck-Gesellschaft; Id: http://dx.doi.org/10.13039/501100004189, Funder: Ministry of Education; Id: http://dx.doi.org/10.13039/501100002701, Funder: Tanzania Wildlife Research Institute (TAWIRI), Funder: University of Cape Town; Id: http://dx.doi.org/10.13039/501100007112, Funder: German Academic Exchange Service (DAAD), Movement of the embryo is essential for musculoskeletal development in vertebrates, yet little is known about whether, and why, species vary. Avian brood parasites exhibit feats of strength in early life as adaptations to exploit the hosts that rear them. We hypothesized that an increase in embryonic movement could allow brood parasites to develop the required musculature for these demands. We measured embryo movement across incubation for multiple brood-parasitic and non-parasitic bird species. Using a phylogenetically controlled analysis, we found that brood parasites exhibited significantly increased muscular movement during incubation compared to non-parasites. This suggests that increased embryo movement may facilitate the development of the stronger musculoskeletal system required for the demanding tasks undertaken by young brood parasites.

Complex and at times extreme environments have pushed many bird species to develop unique eggshell surface properties to protect the embryo from external threats. Because microbes are usually transmitted into eggs by moisture, some species have evolved hydrophobic shell surfaces that resist water absorption, while also regulating heat loss and the exchange of gases. Here, we investigate the relationship between the wettability of eggshells from 441 bird species and their life-history traits. We measured the initial contact angle between sessile water droplets and the shell surface, and how far the droplet spread. Using phylogenetic comparative methods, we show that body mass, annual temperature and eggshell maculation primarily explained variance in water contact angle across eggshells. Species nesting in warm climates were more likely to exhibit highly hydrophobic eggshells than those nesting in cold climates, potentially to reduce microbial colonization. In non-passerines, immaculate eggs were found to have more hydrophobic surfaces than maculate eggshells. Droplets spread more quickly on eggshells incubated in open nests compared to domed nests, likely to decrease heat transfer from the egg. Here, we identify clear adaptations of eggshell wettability across a diverse range of nesting environments, driven by the need to retain heat and prevent microbial adhesion.

The avian eggshell is a bio-ceramic structure that protects the embryo. It is composed almost entirely of calcium carbonate and a small amount of organic material. An optimal amount of calcium carbonate in the eggshell is essential for the embryo's development, yet how the ratio of calcium carbonate to organic matter varies between species has not been investigated. Calcium is a limiting resource for most birds, so its investment in their eggs should be optimized for a bird's life history. We measured the relative calcium carbonate content of eggshells in 222 bird species and tested hypotheses for how this trait has evolved with the life-history strategies of these species and other traits of their respective egg physiologies. We found that (i) eggshell calcium carbonate content was positively correlated with species having thinner eggshells and smaller than expected eggs relative to incubating parental mass, (ii) species with small mean clutch sizes had lower calcium carbonate content in their eggshells, and (iii) for species with larger clutch sizes, eggshell calcium carbonate content was negatively correlated with their mean lifespan. The pattern of lower eggshell calcium carbonate in longer lived, larger clutched birds suggests that calcium provision to the eggshell has long-term costs for the individual.

The position of the Moon in relation to the Earth and the Sun gives rise to several predictable cycles, and natural changes in nighttime light intensity are known to cause alterations to physiological processes and behaviors in many animals. The limited research undertaken to date on the physiological responses of animals to the lunar illumination has exclusively focused on the synodic lunar cycle (full moon to full moon, or moon phase) but the moon's orbit—its distance from the Earth—may also be relevant. Every month, the moon moves from apogee, its most distant point from Earth—and then to perigee, its closest point to Earth. Here, we studied wild barnacle geese (Branta leucopsis) to investigate the influence of multiple interacting lunar cycles on the physiology of diurnally active animals. Our study, which uses biologging technology to continually monitor body temperature and heart rate for an entire annual cycle, asks whether there is evidence for a physiological response to natural cycles in lunar brightness in wild birds, particularly “supermoon” phenomena, where perigee coincides with a full moon. There was a three‐way interaction between lunar phase, lunar distance, and cloud cover as predictors of nighttime mean body temperature, such that body temperature was highest on clear nights when the full moon coincided with perigee moon. Our study is the first to report the physiological responses of wild birds to “supermoon” events; the wild geese responded to the combination of two independent lunar cycles, by significantly increasing their body temperature at night. That wild birds respond to natural fluctuations in nighttime ambient light levels support the documented responses of many species to anthropogenic sources of artificial light, that birds seem unable to override. As most biological systems are arguably organized foremost by light, this suggests that any interactions between lunar cycles and local weather conditions could have significant impacts on the energy budgets of birds.

Organisms vary widely in size, from microbes weighing 0.1 pg to trees weighing thousands of megagrams — a 1021-fold range similar to the difference in mass between an elephant and the Earth. Mass has a pervasive influence on biological processes, but the effect is usually non-proportional; for example, a tenfold increase in mass is typically accompanied by just a four- to sevenfold increase in metabolic rate. Understanding the cause of allometric scaling has been a long-standing problem in biology. Here, we examine the evolution of metabolic allometry in animals by linking microevolutionary processes to macroevolutionary patterns. We show that the genetic correlation between mass and metabolic rate is strong and positive in insects, birds and mammals. We then use these data to simulate the macroevolution of mass and metabolic rate, and show that the interspecific relationship between these traits in animals is consistent with evolution under persistent multivariate selection on mass and metabolic rate over long periods of time. The authors identify a strong positive relationship between mass and metabolic rate among insects, fish, amphibians, reptiles, birds and mammals, and show that the genetic and interspecific correlations between these traits are consistent with a pattern of multivariate selection.

Remaining cohesive on the move can be beneficial for animal groups. As such, animal groups have evolved coordination mechanisms such as leadership to resolve navigational conflicts of interest. Consistent "leaders" may have an intrinsic advantage over "followers" which compromise on their preferred route to retain cohesion, which highlights the question of the inter-individual variation (phenotype) that can predict leadership. Studies in both birds and fish have revealed that intrinsically faster individuals can lead movements, and leading movements propagate from the front edge of the flock/shoal. However, these experiments are generally conducted in relatively "familiar" environments, where the degree of compromise between the "leaders" and "followers" is low. We suggested that inter-individual differences in route efficiency, while not explanatory of leadership from familiar locations, may emerge as predictors of leadership from unfamiliar locations. We tested this prediction - and the potential impact of multiple other behavioral, morphological and "in-flight" phenotypes on leadership - using two groups of homing pigeons (Columba livia) (N = 16), a classic model species of leadership. We recorded N = 966 unique GPS trajectories from birds in (i) solo and familiar, and (ii) solo and unfamiliar contexts to measure solo speed and solo route efficiency; and (iii) group and familiar, and (iv) group and unfamiliar contexts to assess group leadership. Pigeon leadership hierarchies were similar across environmental context (i.e., familiarity). However, we found that no covariates could consistently predict leadership score in either context.

The flight speeds that animals should adopt to minimise energy expenditure in different scenarios can be predicted by the curve of power against speed. However, relatively few studies have examined how animals respond to the landscape below them, which could affect speed through modifications in climb rate and perceived predation risk. We equipped homing pigeons(Columba livia)with high frequency loggers to examine how flight speed varies in relation to topography and land cover, predicting that these parameters may have a substantial impact on flight speed and hence cost. Pigeons showed mixed evidence for an energy saving strategy, as they minimised climb rates by starting their ascent ahead of hills, but selected rapid speeds in their ascents. Birds did not modify their speed substantially in relation to land cover, but higher speeds were observed during descending flight, highlighting the importance to consider the rate of change in altitude before estimating power use from speed. Finally, we document an unexpected variability in speed and altitude over fine scales; a source of substantial inefficiency. We suggest this may a form of protean behaviour adopted to reduce predation risk when flocking is not an option, and that such a strategy could be widespread.

Gillnet bycatch accounts for over 400,000 bird mortalities worldwide every year, affecting a wide variety of species, especially those birds that dive when foraging. Technological solutions to improve gillnet visibility or deter birds from approaching nets, such as LED lights, are essential for aiding diving birds to perceive nets as a hazard. Designing such solutions requires obtaining visual and behavioural ecology information from species to assess their ability to see the warning devices, and to examine their behavioural responses to them. Seaducks, particularly Long-tailed Ducks Clangula hyemalis, have high bycatch mortality rates. We examined the visual fields of four Long-tailed Ducks to understand their three-dimensional view around the head. The visual field characteristics of this species indicate a reliance on visual guidance for foraging associated with their capture of varied, mobile prey in their generalist diet. We subsequently conducted dive tank trials to test the effectiveness of 12 different LED treatments as visual deterrents to the underwater foraging behaviour of 8 Long-tailed Ducks. During each trial, ducks were offered food rewards from a specific underwater location in a dive tank, having the choice of whether to take the food or not. At the same time, they were exposed to either one LED light or the control (no light) to determine whether the presence of each light affected the foraging success rate of dives compared to the control. Exposure of ducks to all 13 treatment combinations was randomised over the trial period. White lights with an increasing flash rate were shown to have a significant positive effect on foraging success, and likely acted as a visual attractant, rather than as a deterrent. No light treatment significantly reduced the foraging success of ducks. LED lights did not inhibit the feeding of Long-tailed Ducks. Such lights may be ineffective as underwater visual deterrents when deployed on gillnets, while white flashing lights may make foraging sites more attractive to Long-tailed Ducks.

Dominance hierarchies confer benefits to group members by decreasing the incidences of physical conflict, but may result in certain lower ranked individuals consistently missing out on access to resources. Here, we report a linear dominance hierarchy remaining stable over time in a closed population of birds. We show that this stability can be disrupted, however, by the artificial mass loading of birds that typically comprise the bottom 50% of the hierarchy. Mass loading causes these low-ranked birds to immediately become more aggressive and rise-up the dominance hierarchy; however, this effect was only evident in males and was absent in females. Removal of the artificial mass causes the hierarchy to return to its previous structure. This interruption of a stable hierarchy implies a strong direct link between body mass and social behaviour and suggests that an individual's personality can be altered by the artificial manipulation of body mass.

The benefits of dominance are well known and numerous, including first access to resources such as food, mates and nesting sites. Less well studied are the potential costs associated with being dominant. Here, the movement of two flocks of domestic homing pigeons (Columba livia) – measured via accelerometry loggers – was recorded over a period of two weeks. Movement was then used to calculate each individual’s daily overall dynamic body acceleration (ODBA, G), which can be used as a proxy for energy expenditure. The dominance hierarchy of the two flocks was determined via group-level antagonistic interactions, and demonstrated a significantly linear structure. The most dominant bird within each flock was found to move significantly more than conspecifics – on average, c.39% greater than the individual with the next highest degree of movement – indicating a possible cost to possessing the top rank within a hierarchy. Despite the dominance hierarchy being linear, mean daily total ODBA did not reflect a linear nature, with no pattern observed between rank and ODBA, once the top ranked individuals had been accounted for. This suggest that energy expenditure may be more reflective of a despotic hierarchy. These results show the potential for the future use of accelerometery as a tool to study the fusion of energetics and behaviour.Subject CategorybehaviourSubject Areasbehaviour, physiology

Embryo survival in birds depends on a controlled transfer of water vapour and respiratory gases through the eggshell, and this exchange is critically sensitive to the surrounding physical environment. As birds breed in most terrestrial habitats worldwide, we proposed that variation in eggshell conductance has evolved to optimize embryonic development under different breeding conditions. This is the first study to take a broad-scale macro-ecological view of avian eggshell conductance, encompassing all key avian taxonomic groups, to assess how life history and climate influence the evolution of this trait. Using whole eggs spanning a wide phylogenetic diversity of birds, we determine that body mass, temperature seasonality and whether both parents attend the nest are the main determinants of eggshell conductance. Birds breeding at high latitudes, where seasonal temperature fluctuations are greatest, will benefit from lower eggshell conductance to combat temporary periods of suspended embryo growth and prevent dehydration during prolonged incubation. The nest microclimate is more consistent in species where parents take turns incubating their clutch, resulting in lower eggshell conductance. This study highlights the remarkable functional qualities of eggshells and their importance for embryo survival in extreme climates.

The visual fields of Aegypiinae vultures have been shown to be adapted primarily to meet two key perceptual challenges of their obligate carrion-feeding behaviour: scanning the ground and preventing the sun’s image falling upon the retina. However, field observations have shown that foraging White-headed Vultures (Trigonoceps occipitalis) are not exclusively carrion-feeders; they are also facultative predators of live prey. Such feeding is likely to present perceptual challenges that are additional to those posed by carrion-feeding. Binocularity is the key component of all visual fields and in birds it is thought to function primarily in the accurate placement and time of contact of the talons and bill, especially in the location and seizure of food items. We determined visual fields in White-headed Vultures and two species of carrion-eating Gyps vultures, and show that the visual field of White-headed Vultures have more similarities with those of predatory raptors (e.g. Accipitrid hawks), compared with the taxonomically more closely related Gyps vultures. We found that maximum binocular field width in White-headed vultures (30°) is significantly wider than Gyps vultures (20°). The broader binocular fields in White-headed Vultures probably facilitate accurate placement and timing of the talons when capturing evasive live prey.

Dominant individuals within animal groups will frequently place themselves in the most beneficial position for maximal protection against predation. Higher perches are generally associated with reduced predation risk in birds, so we predicted that dominant birds will preferentially place themselves on higher perches. We tested this by determining the dominance hierarchy in two populations of captive birds (Homing Pigeons Columba livia and Great Cormorants Phalacrocorax carbo), and relating rank within the dominance hierarchy to observed perch height preferences. We found that perch choice was significantly repeatable in pigeons, and that more dominant individuals of both species selected higher perches. As well as facilitating early detection of and escape from potential predators, higher perches are also likely to facilitate the display of aggression to other group members.

Closely related tropical bird species often occupy mutually exclusive elevational ranges, but the mechanisms generating and maintaining this pattern remain poorly understood. One hypothesis is that replacement species are segregated by interference competition (e.g. territorial aggression), but the extent to which competition combines with other key factors such as specialization to distinct habitats remains little studied. Using vegetation surveys and reciprocal playback experiments, we explored the dynamics of interspecific aggression between two Nightingale‐Thrushes Catharus sp. in Central America. We show that lower‐elevation Black‐headed Nightingale‐Thrushes Catharus mexicanus are aggressive towards higher‐elevation Ruddy‐capped Nightingale‐Thrushes Catharus frantzii where they meet at contact zones. However, interspecific aggressive responses were weak and unidirectional, and the two species were associated with different habitats. We conclude that the contact zone is maintained and located primarily by habitat selection, and is probably reinforced by interspecific aggression. This has important implications for understanding how montane species will respond to climate change because the pace and extent of range shifts will not depend solely on habitat shifts or interspecific competition, but instead on interactions between these two factors.

Flapping flight is the most energetically demanding form of sustained forwards locomotion that vertebrates perform. Flock dynamics therefore have significant implications for energy expenditure. Despite this, no studies have quantified the biomechanical consequences of flying in a cluster flock or pair relative to flying solo. Here, we compared the flight characteristics of homing pigeons (Columba livia) flying solo and in pairs released from a site 7 km from home, using high-precision 5 Hz global positioning system (GPS) and 200 Hz tri-axial accelerometer bio-loggers. As expected, paired individuals benefitted from improved homing route accuracy, which reduced flight distance by 7% and time by 9%. However, realising these navigational gains involved substantial changes in flight kinematics and energetics. Both individuals in a pair increased their wingbeat frequency by 18% by decreasing the duration of their upstroke. This sharp increase in wingbeat frequency caused just a 3% increase in airspeed but reduced the oscillatory displacement of the body by 22%, which we hypothesise relates to an increased requirement for visual stability and manoeuvrability when flying in a flock or pair. The combination of the increase in airspeed and a higher wingbeat frequency would result in a minimum 2.2% increase in the total aerodynamic power requirements if the wingbeats were fully optimised. Overall, the enhanced navigational performance will offset any additional energetic costs as long as the metabolic power requirements are not increased above 9%. Our results demonstrate that the increases in wingbeat frequency when flying together have previously been underestimated by an order of magnitude and force reinterpretation of their mechanistic origin. We show that, for pigeons flying in pairs, two heads are better than one but keeping a steady head necessitates energetically costly kinematics., Flocking costs may have been underestimated by an order of magnitude: a study of homing pigeons shows that they flap 20% faster when flying in pairs relative to flying solo to keep their heads steady and reap the ultimate benefits of flocking.

Obligate brood-parasitic cheats have fascinated natural historians since ancient times. Passing on the costs of parental care to others occurs widely in birds, insects and fish, and often exerts selection pressure on hosts that in turn evolve defences. Brood parasites have therefore provided an illuminating system for researching coevolution. Nevertheless, much remains unknown about how ecology and evolutionary history constrain or facilitate brood parasitism, or the mechanisms that shape or respond to selection. In this special issue, we bring together examples from across the animal kingdom to illustrate the diverse ways in which recent research is addressing these gaps. This special issue also considers how research on brood parasitism may benefit from, and in turn inform, related fields such as social evolution and immunity. Here, we argue that progress in our understanding of coevolution would benefit from the increased integration of ideas across taxonomic boundaries and across Tinbergen’s Four Questions: mechanism, ontogeny, function and phylogeny of brood parasitism. We also encourage renewed vigour in uncovering the natural history of the majority of the world's brood parasites that remain little-known. Indeed, it seems very likely that some of nature’s brood parasites remain entirely unknown, because otherwise we are left with a puzzle: if parental care is so costly, why is brood parasitism not more common? This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.

The evolution of group living transformed the history of animal life on earth, yielding substantial selective benefits. Yet, without overcoming fundamental challenges such as how to coordinate movements with conspecifics, animals cannot maintain cohesion, and coordination is thus a prerequisite for the evolution of sociality in nonstationary animals. Although it has been considered that animal groups must coordinate the timing and direction of movements, coordinating speed is also essential to prevent the group from splitting. We investigated speed consensus in homing pigeon, Columba livia, flocks using high-resolution GPS. Despite observable differences in average solo speed (which was positively correlated with bird mass) compromises of up to 6% from the preferred solo speed were made to reach consensus in flocks. These results match theory which suggests that groups fly at an intermediate of solo speeds, which suggests speed averaging. By virtue of minimizing extreme compromises, speed averaging can maximize selective benefits across the group, suggesting shared consensus for group speed could be ubiquitous across taxa. Nevertheless, despite group-wide advantages, contemporary flight models have suggested unequal energetic costs in favour of individuals with intermediate body mass/preferred speed (hence the ‘Goldilocks principle’).

Flapping flight is the most energetically demanding form of sustained forwards locomotion that vertebrates perform. Flock dynamics therefore have significant implications for energy expenditure. Despite this, no studies have quantified the biomechanical consequences of flying in a cluster flock relative to flying solo. Here, we compared the flight characteristics of homing pigeons (Columba livia) flying solo and in pairs, using high-precision 5 Hz GPS and 200 Hz tri-axial accelerometer biologgers. Paired flight increased route accuracy by ~7%, but, was accompanied by an increase in wingbeat frequency of ~18%. As expected, paired individuals benefitted from improved homing route accuracy, which reduced flight distance by ~7% and time by ~9%. However, realising these navigational gains involved substantial changes in flight kinematics and energetics. Both individuals in a pair increased their wingbeat frequency byc.18%, by decreasing the duration of their upstroke. This sharp increase in wingbeat frequency caused just a 3% increase in airspeed, but reduced the oscillatory displacement of the body by ~22%, which we hypothesise relates to an increased requirement for visual stability and manoeuvrability when flocking. Overall, the shorter flight distances and increased wingbeat frequency in a pair resulted in a net increase in the aerodynamic cost of returning home, which we estimate was ~14%. Our results demonstrate that flocking costs have been underestimated by an order of magnitude and force reinterpretation of their mechanistic origin. We show that, for pigeons, two heads are better than one, but keeping a steady head necessitates energetically costly kinematics.

Waterfowl undergo an annual simultaneous flight-feather moult that renders them flightless for the duration of the regrowth of the flight feathers. In the wild, this period of flightlessness could restrict the capacity of moulting birds to forage and escape predation. Selection might therefore favour a short moult, but feather growth is constrained and presumably energetically demanding. We therefore tested the hypothesis that for birds that undergo a simultaneous flight-feather moult, this would be the period in the annual cycle with the highest minimum daily heart rates, reflecting these increased energetic demands. Implantable heart rate data loggers were used to record year-round heart rate in six wild barnacle geese ( Branta leucopsis ), a species that undergoes a simultaneous flight-feather moult. The mean minimum daily heart rate was calculated for each individual bird over an 11-month period, and the annual cycle was divided into seasons based on the life-history of the birds. Mean minimum daily heart rate varied significantly between seasons and was significantly elevated during wing moult, to 200 ± 32 beats min −1 , compared to all other seasons of the annual cycle, including both the spring and autumn migrations. The increase in minimum daily heart rate during moult is likely due to feather synthesis, thermoregulation and the reallocation of minerals and protein.

Avian embryos are a commonly used model system for developmental studies, but monitoring of physiological parameters such as heart rate (HR) and movement in ovo poses a challenge to researchers. These are also increasingly common research objectives for ecological and embryo behavior studies in oviparous species. We therefore explored the validity of a new digital egg-monitoring system for the noninvasive monitoring of these parameters. We tested the relationship between frequency-of-movement values gathered by digital monitoring and those gathered by the current standard method, which is comparatively invasive and requires egg windowing, and demonstrated that the digital monitoring method effectively distinguishes individual movements but cannot reliably monitor HR in actively motile embryos. We therefore provide recommendations for the appropriate use of this technique for avian physiologists. We also applied the digital monitoring method to reveal how frequency of movement varies throughout prenatal ontogeny in the chicken and showed that commonly used protocols in developmental studies can themselves alter motility; egg windowing and application of light modulate frequency of movement. Recent work has revealed the importance of embryo motility in regulating gene expression and cellular activity during developmental processes. Together with our data, this highlights the value of noninvasive monitoring methods and the importance of controlling for altered embryo motility/behavior in developmental studies.

Energy management models provide theories and predictions for how animals manage their energy budgets within their energetic constraints, in terms of their resting metabolic rate (RMR) and daily energy expenditure (DEE). Thus, uncovering what associations exist between DEE and RMR is key to testing these models. Accordingly, there is considerable interest in the relationship between DEE and RMR at both inter- and intraspecific levels. Interpretation of the evidence for particular energy management models is enhanced by also considering the energy spent specifically on costly activities (activity energy expenditure [AEE] = DEE - RMR). However, to date there have been few intraspecific studies investigating such patterns. Our aim was to determine whether there is a generality of intraspecific relationships among RMR, DEE, and AEE using long-term data sets for bird and mammal species. For mammals, we use minimum heart rate (fH), mean fH, and activity fH as qualitative proxies for RMR, DEE, and AEE, respectively. For the birds, we take advantage of calibration equations to convert fH into rate of oxygen consumption in order to provide quantitative proxies for RMR, DEE, and AEE. For all 11 species, the DEE proxy was significantly positively correlated with the RMR proxy. There was also evidence of a significant positive correlation between AEE and RMR in all four mammal species but only in some of the bird species. Our results indicate there is no universal rule for birds and mammals governing the relationships among RMR, AEE, and DEE. Furthermore, they suggest that birds tend to have a different strategy for managing their energy budgets from those of mammals and that there are also differences in strategy between bird species. Future work in laboratory settings or highly controlled field settings can tease out the environmental and physiological processes contributing to variation in energy management strategies exhibited by different species.

No single hypothesis is likely to explain the diversity in eggshell coloration and patterning across birds, suggesting that eggshell appearance is most likely to have evolved to fulfill many nonexclusive functions. By controlling for nonindependent phylogenetic associations between related species, we describe this diversity using museum eggshells of 71 British breeding passerine species to examine how eggshell pigment composition and concentrations vary with phylogeny and with life-history and nesting ecology traits. Across species, concentrations of biliverdin and protoporphyrin, the two main pigments found in eggshells, were strongly and positively correlated, and both pigments strongly covaried with phylogenetic relatedness. Controlling for phylogeny, cavity-nesting species laid eggs with lower protoporphyrin concentrations in the shell, while higher biliverdin concentrations were associated with thicker eggshells for species of all nest types. Overall, these relationships between eggshell pigment concentrations and the biology of passerines are similar to those previously found in nonpasserine eggs, and imply that phylogenetic dependence must be considered across the class in further explanations of the functional significance of avian eggshell coloration. Pravděpodobně neexistuje jediná hypotéza vysvětlující rozdílností ve zbarvení a vzorování skořápek u ptáků, což naznačuje, že vzhled skořápek se s největší pravděpodobností vyvinul pro plnění mnoha funkcí. Prostřednictvím kontroly nezávislých fylogenetických asociací mezi příbuznými druhy jsme popsali tyto rozdíly za použití muzejních skořápek od 71 druhů pěvců Britského chovu k prozkoumání toho, jak kolísá složení a koncentrace pigmentu skořápek s fylogenezí a s životním cyklem a ekologickými rysy hnízdění. U všech druhů koncentrace biliverdinu a protoporfyrinu, dvou hlavních barviv ve skořápkách, byly silně a pozitivně korelovány a oba pigmenty silně kolísaly s fylogenetickou příbuzností. Kontrolou fylogeneze bylo zjištěno, druhy hnízdící v dutinách kladou vejce s nižší koncentrací protoporfyrinu ve skořápce, zatímco vyšší koncentrace biliverdinu byly asociovány s tlustšími skořápkami u druhů všech hnízdících typů. Tyto vztahy mezi koncentrací pigmentu skořápek a biologií pěvců jsou podobné těm, které byly již dříve nalezeny u vajec nepěvců, což naznačuje, že fylogenetické závislosti musí být hodnoceny napříč skupinami u příštích výkladů funkční významnosti zbarvení ptačích skořápek.

Birds’ eggshells are primarily composed of calcite, an abundant polymorph of calcium carbonate (CaCO3). However, the eggshells of some species are coated with spherules of vaterite, a rare and thermodynamically unstable polymorph of CaCO3, the function of which remains unknown. Here we experimentally test the mechanical and physiological effects of the vaterite coating on eggshells of the Greater Ani Crotophaga major, a tropical cuckoo. Vaterite removal did not affect vapour conductance rates across the eggshell, indicating that the vaterite coating does not influence gas exchange during embryonic development. By contrast, nanoindentation revealed that the hardness and elasticity of vaterite is similar to that of calcite, and white light interferometry showed that the vaterite layer increased the total thickness of the shell cuticle by up to 10%. Furthermore, calculations of contact mechanics found that when two eggs come into contact, the depth of the surface deformation caused by the contact is far less than the thickness of the vaterite coating. These results suggest the layer of vaterite spherules may act as a ‘shock absorber’ for the underlying calcite shell, protecting it from mechanical damage caused by collision with other eggs in the nest and reducing the risk of eggshell fracture during incubation.

One conspicuous feature of several larger bird species is their annual migration in V-shaped or echelon formation. When birds are flying in these formations, energy savings can be achieved by using the aerodynamic up-wash produced by the preceding bird. As the leading bird in a formation cannot profit from this up-wash, a social dilemma arises around the question of who is going to fly in front? To investigate how this dilemma is solved, we studied the flight behavior of a flock of juvenile Northern bald ibis (Geronticus eremita) during a human-guided autumn migration. We could show that the amount of time a bird is leading a formation is strongly correlated with the time it can itself profit from flying in the wake of another bird. On the dyadic level, birds match the time they spend in the wake of each other by frequent pairwise switches of the leading position. Taken together, these results suggest that bald ibis cooperate by directly taking turns in leading a formation. On the proximate level, we propose that it is mainly the high number of iterations and the immediacy of reciprocation opportunities that favor direct reciprocation. Finally, we found evidence that the animals' propensity to reciprocate in leading has a substantial influence on the size and cohesion of the flight formations.

Mechanisms of avian navigation have received considerable attention, but whether different navigational strategies are accompanied by different flight characteristics is unknown. Managing energy expenditure is critical for survival; therefore, understanding how flight characteristics, and hence energy allocation, potentially change with birds' familiarity with a navigational task could provide key insights into the costs of orientation. We addressed this question by examining changes in the wingbeat characteristics and airspeed of homing pigeons (Columba livia) as they learned a homing task. Twenty-one pigeons were released 20 times individually either 3.85 or 7.06 km from home. Birds were equipped with 5 Hz GPS trackers and 200 Hz tri-axial accelerometers. We found that, as the birds' route efficiency increased during the first six releases, their median peak-to-peak dorsal body (DB) acceleration and median DB amplitude also increased. This, in turn, led to higher airspeeds, suggesting that birds fly slower when traversing unfamiliar terrain. By contrast, after route efficiency stabilised, birds exhibited increasing wingbeat frequencies, which did not result in further increases in speed. Overall, higher wind support was also associated with lower wingbeat frequencies and increased DB amplitude. Our study suggests that the cost of early flights from an unfamiliar location may be higher than subsequent flights because of both inefficient routes (increased distance) and lower airspeeds (increased time). Furthermore, the results indicate, for the first time, that birds modulate their wingbeat characteristics as a function of navigational knowledge, and suggest that flight characteristics may be used as ‘signatures’ of birds' route familiarity.