Tracking and understanding the movements of animals in the wild is a fast-growing area of research, known as movement ecology . However, tracking small animals such as flying insects, which cannot easily carry an electronic tag, remains challenging as existing field methods are costly either in terms of equipment or tracking effort (e.g. VHF radio-tracking, scanning harmonic radar). Here we attempted to record the movements of free-flying butterflies from an unmanned aerial vehicle (UAV), maintaining a static position in the sky and recording video vertically downwards. With an appropriate flight height and image filtering algorithm, we recorded 166 flight tracks of Pieris butterflies ( P. brassicae and P. rapae ), with a median tracking length of 40 m (median flight duration 13 s), and a high temporal resolution of 30 positions per second. Average flight direction varied significantly over the course of the flying season, from a northern azimuth in June and early July, to a southern azimuth in September, a trans-generational migratory behaviour that had previously been documented by field observations or experiments in flight cages. In addition, UAV imagery unlocks the possibility to measure high-resolution flight movement patterns (e.g. path tortuosity and transverse oscillations), which will possibly help understand perceptual and locomotor mechanisms underlying spatial behaviour. We explore the technical details associated with UAV tracking methodology, and discuss its limitations, in particular the biases associated with a 2D projection of 3D flight movements, and the difficulty to distinguish between visually similar species, such as P. brassicae and P. rapae . ### Competing Interest Statement The authors have declared no competing interest.

ABSTRACTFlight is an efficient way of transport over a unit of distance, but it can be very costly over each unit of time, and reducing flight energy expenditure is a major selective pressure in birds. The common swift (Apus apus) is one of the most aerial bird species, performing most behaviours in flight: foraging, sleeping and also drinking by regularly descending to various waterbodies and skimming over the surface. An energy-saving way to perform such touch-and-go drinking would be to strive to conserve mechanical energy, by transforming potential energy to kinetic energy during the gliding descent, touching water at high speed, and regaining height with minimal muscular work. Using 3D optical tracking, we recorded 163 swift drinking trajectories, over three waterbodies near Rennes, France. Contrary to the energy conservation hypothesis, we show that swifts approaching a waterbody with a higher mechanical energy (higher height and/or speed 5 s before contact) do not reach the water at higher speeds, but do brake, i.e. dissipate mechanical energy to lose both height and speed. Braking seems to be linked with sharp turns and the use of headwind to some extent, but finer turns and postural adjustments, beyond the resolving power of our tracking data, could also be involved. We hypothesize that this surprisingly costly behaviour results from a trade-off between energy expenditure and safety, because approaching a water surface requires fine motor control, and high speed increases the risk of falling into the water, which would have serious energetic and survival costs for a swift.

Foraging is an extremely important behaviour for birds, especially during the breeding season, when they have to carry the cost of incubation and chick rearing, in addition to their own energy needs. Aerial insectivores perform most of their foraging behaviours in flight, so they have evolved various adaptations to reduce energy output whilst increasing energy input during this critical period. In this study, we recorded the 3D flight behaviours of 100 house martins (Delichon urbicum) flying near their colony during the breeding season in Rennes, France. We give a first description of the distribution of several kinematic and biomechanical variables (horizontal and vertical speed, rates of change in kinetic and potential energy, turning radius of curvature and centripetal force), compare flapping and gliding flight, and describe several strategies used by flying house martins to save energy, such as environmental energy extraction (thermal soaring) and optimisation of flight speed according to wind speed and direction. We also report an effect of temperature, solar radiation and humidity on the mean vertical speed of gliding birds, highlighting the effect of weather on the availability of external energy sources such as thermal updrafts. Finally, we compare the distribution of flight speed and vertical speed between 5 juveniles identified using magnified photographs and 20 adults recorded during the same field sessions, and we show that during flapping flight, juveniles exhibit higher, more variable airspeed than adults, suggesting that their flight behaviours are not immediately fine-tuned after leaving the nest. Aerial insectivores use various strategies to reduce the cost of foraging flight. Using an optical tracking method, we recorded the 3D flight behaviours of house martins (Delichon urbicum) flying near their colony during the breeding season. We describe the distribution of several biomechanical variables and show that house martins use external energy sources such as thermal updrafts and also adapt their airspeed to wind speed and direction, supporting the predictions on optimal cost of transport in birds. Moreover, juveniles were also recorded, and they show a greater variability in flight speed, possibly because they may not be as accurate as adults in finely adjusting their speed and altitude. Our findings add to the existing literature showing energy-saving strategies in aerial insectivores, and also study an ontogenetical aspect rarely explored.

AbstractAn effect of weather on sars-cov-2 transmission is regularly proposed as a putative cause of unexplained fluctuations of covid-19 new cases, but clear data supporting this hypothesis remains to be presented. Here I measured longitudinal time-series correlations between outdoor temperature, humidity and covid-19 reproduction number (Rt) in the 50 U.S. states (+DC). In order to mitigate the confounding influence of varying social restriction measures, the analysis spans a 5-month period during summer and autumn 2020 when restrictions were comparatively lower and more stable. I used a cross-covariance approach to account for a variable delay between infection and case report. For a delay near 11 days, most U.S. states exhibited a negative correlation between outdoor temperature and Rt, as well as between absolute humidity and Rt (mean r = −0.35). In 21 states, the correlation was strong (r < −0.5). Individual state data are presented, and associations between cold and/or dry weather episodes and short-term new case surges are proposed. After identifying potential confounding factors, I discuss 3 possible causal mechanisms that could explain a correlation between outdoor weather and indoor disease transmission: behavioral adaptations to cold weather, respiratory tract temperature, and the importing of outdoor absolute humidity to indoor spaces.

Flight is a unique adaptation at the core of many behaviours in most bird species, whether it be foraging, migration or breeding. Birds have developed a wide diversity of flight modes (e.g. flapping, gliding, soaring, hovering) which involves very specialized behaviours. A key issue when studying flight behaviours is to understand how they develop through all the ontogenetic stages of birds, from the embryo to the flying adult. This question typically involves classical debates on animal behaviour about the importance of maturation and experience. Here, we review the literature available on the development of flight behaviours in birds. First, we focus on the early period when young birds are not yet capable of flight. We discuss examples and show how endogenous processes (e.g. wing flapping in the nest, flight development timing) and environmental factors (e.g. maternal stress, nutritional stress) can influence the development of flight behaviours. Then, we review several examples showing the different processes involved in the development of flight in flight-capable juveniles (e.g. practice, trial and error learning, social learning). Despite the lack of experimental studies investigating this specific question at different developmental stages, we show that several patterns can be identified, and we anticipate that the development of new tracking techniques will allow us to study this question more thoroughly in more bird species.

Supplementary comments on A-Altricial-precocial spectrum, B-Developmental timing of flight in precocial birds, C-Fledging timing in altricial birds and D-Anatomical differences of the flight apparatus between juveniles and adults. +28 supplementary references.

Beyond the genetic links between mother and young, prenatal effects experienced by offspring can modulate their mother's behaviour and stress. Here we investigated the consequences of stress-induced prenatal maternal effects on quail chicks' behaviour and maternal behaviour. We evaluated how adoptive quail mothers (unstressed) raised chicks of females that had either been stressed (PS) or were non-stressed (NPS) during laying. We demonstrated previously that our social stress procedure during laying increases levels of egg yolk steroid and chicks' emotional reactivity during the postnatal period. We found that, compared to NPS mothers, PS mothers remained closer to their chicks, were aggressed and neglected them less, emitted more maternal vocalisations and called their chicks more when they were separated. Moreover, PS chicks made more requests on their mother. Our results show that mothers responded to chicks' behavioural changes caused by prenatal stress by adapting their maternal behaviour. As exemplified here, for the first time in birds, prenatal environment can affect postnatal maternal behaviour by changing their offspring's behaviour.

Archaeopteryx is an iconic fossil taxon with feathered wings from the Late Jurassic of Germany that occupies a crucial position for understanding the early evolution of avian flight. After over 150 years of study, its mosaic anatomy unifying characters of both non-flying dinosaurs and flying birds has remained challenging to interpret in a locomotory context. Here, we compare new data from three Archaeopteryx specimens obtained through phase-contrast synchrotron microtomography to a representative sample of archosaurs employing a diverse array of locomotory strategies. Our analyses reveal that the architecture of Archaeopteryx's wing bones consistently exhibits a combination of cross-sectional geometric properties uniquely shared with volant birds, particularly those occasionally utilising short-distance flapping. We therefore interpret that Archaeopteryx actively employed wing flapping to take to the air through a more anterodorsally posteroventrally oriented flight stroke than used by modern birds. This unexpected outcome implies that avian powered flight must have originated before the latest Jurassic.

Although the biomechanics of animal flight have been well studied in laboratory apparatus such as wind tunnels for many years, the applicability of these data to natural flight behaviour has been examined in few instances and mostly in the context of long-distance migration. Here, we used rotational stereo-videography to record the free-flight trajectories of foraging common swifts. We found that, despite their exquisite manoeuvring capabilities, the swifts only rarely performed high-acceleration turns. More surprisingly, we also found that despite feeding on tiny insects probably moving with ambient flow, the birds adjust their air speed to optimize cost of transport over distance. Finally, swifts spent only 25% of their time flapping; the majority of their time (71%) was spent in extended wing gliding, during which the average power expended for changes in speed or elevation was 0.84 W kg(-1) and not significantly different from 0. Thus, gliding swifts extracted sufficient environmental energy to pay the cost of flight during foraging.

How predators search for prey is a cornerstone question in behavioural ecology, which has yet to be investigated for animals foraging in 3D airspace. Do insectivorous birds such as swifts (Apodidae), swallows and martins (Hirundinidae) use similar strategies to those performed by terrestrial predators in 2D, or do they rely on different spatial search strategies because of some properties of the aerial open space? We addressed this question in the common swift, one of the most aerial birds, using a novel 3D optical tracking method. The analysis of fine-scale flight tracks revealed how birds distribute their presence in 3D space while foraging near their breeding colony. Common swifts concentrated the time spent per volume unit by adopting a tortuous path, and, to a much lesser extent, by decreasing their movement speed. By independently observing the birds' posture on tracking images, we were able to identify the occurrence of putative prey captures along flight tracks. We show that swifts' presence was concentrated mainly in the vicinity of prey captures, unveiling a volume-concentrated search (VCS) strategy in this aerial insectivore. This is an extension in 3D of the area-concentrated search classically described in terrestrial 2D space. VCS can (but does not necessarily) take place in thermal updrafts, where small insects can be concentrated in patches. In contrast to terrestrial and aquatic predators that can easily slow down or stop their movement in profitable places, a different speed-cost relationship underlying aerial movement prevents swifts from stopping in prey patches and explains why these birds rely mainly on movement tortuosity to perform intensive search. Our study thus shows how some physical properties of the environment can modulate the way an animal concentrates its search in profitable places. (C) 2017 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Mothers have a crucial influence on offspring development. Variations of maternal behaviour can be due to numerous parameters, for instance costs are related to the size of a brood/litter, which in turn can influence the level of mothers' investment in each offspring. Here we investigated the influence of brood size on the behaviour of Japanese quail mothers and chicks during the mothering period and on offspring development. We compared two types of broods: small broods of three chicks (N=9) and large broods of six chicks (N=9). Behavioural tests assessed chicks' social and emotional traits. Mothers of large broods emitted more maternal vocalisations at the beginning of the mothering period, but at the end they assumed more non-covering postures and trampled chicks more than mothers of small broods. Chicks in large broods huddled up more whereas chicks in small broods rested alone more frequently. Moreover, the social motivation of chicks in large broods was higher than that of chicks in small broods, although their emotional reactivity levels were similar. Our results evidence the importance of brood size for maintaining family cohesion and the influence of brood size on chicks' interactions with their siblings. We evaluated the influence of mothers and siblings on chicks' behavioural development.

Animal-Robot Interaction experiments have demonstrated their usefulness to understand the social behaviour of a growing number of animal species. In order to study the mechanisms of social influences (from parents and peers) on behavioural development, we design an experimental setup where young quail chicks,. after hatching,. continuously live with autonomous mobile robots. in mixed triadic groups of two chicks and one robot. As precocial birds are subject to imprinting, we compare groups where chicks meet the robot as their very first social partner, on their first day after hatching (R chicks), with groups where chicks meet a real conspecific first (Cchicks), and the robot later (on the second day after hatching). We measured the behavioural synchronization between chicks and robot over three days. Afterwards, we directly tested the existence of a possible social bond between animal and robot, by performing separation-reunion behavioural tests. R chicks were more synchronized with the robot in their daily feeding-resting activities than Cchicks. Moreover, R chicks emitted numerous distress calls when separated from the robot, even in the presence of another chick, whereas Cchicks emitted calls only when separated from the other chick. Whether the observed chick-robot attachment bond reflects filial, or sibling-imprinting of chicks towards the robot remains unclear, as the latter process is not fully understood in natural familial groups. Still, these results reveal the necessary initial conditions for stable, cohesive mixed groups of chicks and robots, a promising tool to experiment on the long-term dynamics of social behaviour.