Long‐distance migration has evolved in many organisms moving through different media and using various modes of locomotion and transport. Migration continues to evolve or become suppressed as shown by ongoing dynamic and rapid changes of migration patterns. This great evolutionary flexibility may seem surprising for such a complex attribute as migration. Even if migration in most cases has evolved basically as a strategy to maximise fitness in a seasonal environment, its occurrence and extent depend on a multitude of factors. We give a brief overview of different factors (e.g. physical, geographical, historical, ecological) likely to facilitate and/or constrain the evolution of long‐distance migration and discuss how they are likely to affect migration. The basic driving forces for migration are ecological and biogeographic factors like seasonality, spatiotemporal distributions of resources, habitats, predation and competition. The benefit of increased resource availability will be balanced by costs associated with the migratory process in terms of time (incl. losses of prior occupancy advantages), energy and mortality (incl. increased exposure to parasites). Furthermore, migration requires genetic instructions (allowing substantial room for learning in some of the traits) about timing, duration and distance of migration as well as about behavioural and physiological adaptations (fuelling, organ flexibility, locomotion, use of environmental transport etc) and control of orientation and navigation. To what degree these costs and requirements put constraints on migration often depends on body size according to different scaling relationships. From this exposé it is clear that research on migration warrants a multitude of techniques and approaches for a complete as possible understanding of a very complex evolutionary syndrome. In addition, we also present examples of migratory distances in a variety of taxons. In recent years new techniques, especially satellite radio telemetry, provide new information of unprecedented accuracy about journeys of individual animals, allowing re‐evaluation of migration, locomotion and navigation theories.

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Recent shifts in phenology in response to climate change are well established but often poorly understood. Many animals integrate climate change across a spatially and temporally dispersed annual life cycle, and effects are modulated by ecological interactions, evolutionary change and endogenous control mechanisms. Here we assess and discuss key statements emerging from the rapidly developing study of changing spring phenology in migratory birds. These well-studied organisms have been instrumental for understanding climate-change effects, but research is developing rapidly and there is a need to attack the big issues rather than risking affirmative science. Although we agree poorly on the support for most claims, agreement regarding the knowledge basis enables consensus regarding broad patterns and likely causes. Empirical data needed for disentangling mechanisms are still scarce, and consequences at a population level and on community composition remain unclear. With increasing knowledge, the overall support ('consensus view') for a claim increased and between-researcher variability in support ('expert opinions') decreased, indicating the importance of assessing and communicating the knowledge basis. A proper integration across biological disciplines seems essential for the field's transition from affirming patterns to understanding mechanisms and making robust predictions regarding future consequences of shifting phenologies.

The problems in understanding bird flight aerodynamics A complete, correct and/or detailed understanding of the aerodynamic mechanisms of importance in bird flight is complicated immensely by a number of factors. The most basic problem is that flight speeds are sufficiently slow (typical values for the mean forward speed, U, may range from 1-20·m·s -1 ) and the length scales are sufficiently small (mean chord, c, ranging from 1-10·cm), that the effects of viscosity are not ordinarily negligible. This fact can be written more formally by calculating a characteristic value for the dimensionless Reynolds number,where ν is the kinematic viscosity. For U=10·m·s -1 and c=5·cm, Re≈3×10 4 . This is an extremely inconvenient number. It lies well below typical values of 10 6 for small planes where viscous effects can safely be presumed to be restricted to thin, attached boundary layers, and it lies well above characteristic values of 10 2 where the flow over the body and in any wake is laminar and well-organised. On the contrary, even at moderate angles of attack, the flow over well-designed aerofoils veers notoriously between separated and nonseparated states, with dramatic differences in mean and instantaneous force coefficients as a result. Over and above treatments found in standard aerodynamics texts, one must also account for the fact that in animal flight the wings themselves are moving relative to the body, and furthermore that they are not rotating steadily like a propeller, but are beating up and down, accelerating and decelerating with each cycle. To this one adds the effects of flexible wing surfaces that not only have complex geometric descriptions, but also significantly change their shape during the wing beat cycle. Although it is straightforward to compile long lists of complicating factors, it is not clear which of them are important, and why and when. 2313The doi:10.1242/jeb.00423 In view of the complexity of the wing-beat kinematics and geometry, an important class of theoretical models for analysis and prediction of bird flight performance entirely, or almost entirely, ignores the action of the wing itself and considers only the resulting motions in the air behind the bird. These motions can also be complicated, but some success has previously been recorded in detecting and measuring relatively simple wake structures that can sometimes account for required quantities used to estimate aerodynamic power consumption. To date, all bird wakes, measured or presumed, seem to fall into one of two classes: the closedloop, discrete vortex model at low flight speeds, and the constant-circulation, continuous vortex model at moderate to high speeds. Here, novel and accurate quantitative measurements of velocity fields in vertical planes aligned with the freestream are used to investigate the wake structure of a thrush nightingale over its entire range of natural flight speeds. At most flight speeds, the wake cannot be categorised as one of the two standard types, but has an intermediate structure, with approxima...

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Staying aloft when hovering and flying slowly is demanding. According to quasi-steady-state aerodynamic theory, slow-flying vertebrates should not be able to generate enough lift to remain aloft. Therefore, unsteady aerodynamic mechanisms to enhance lift production have been proposed. Using digital particle image velocimetry, we showed that a small nectar-feeding bat is able to increase lift by as much as 40% using attached leading-edge vortices (LEVs) during slow forward flight, resulting in a maximum lift coefficient of 4.8. The airflow passing over the LEV reattaches behind the LEV smoothly to the wing, despite the exceptionally large local angles of attack and wing camber. Our results show that the use of unsteady aerodynamic mechanisms in flapping flight is not limited to insects but is also used by larger and heavier animals.

Gliding birds continually change the shape and size of their wings, presumably to exploit the profound effect of wing morphology on aerodynamic performance. That birds should adjust wing sweep to suit glide speed has been predicted qualitatively by analytical glide models, which extrapolated the wing's performance envelope from aerodynamic theory. Here we describe the aerodynamic and structural performance of actual swift wings, as measured in a wind tunnel, and on this basis build a semi-empirical glide model. By measuring inside and outside swifts' behavioural envelope, we show that choosing the most suitable sweep can halve sink speed or triple turning rate. Extended wings are superior for slow glides and turns; swept wings are superior for fast glides and turns. This superiority is due to better aerodynamic performance-with the exception of fast turns. Swept wings are less effective at generating lift while turning at high speeds, but can bear the extreme loads. Finally, our glide model predicts that cost-effective gliding occurs at speeds of 8-10 m s(-1), whereas agility-related figures of merit peak at 15-25 m s(-1). In fact, swifts spend the night ('roost') in flight at 8-10 m s(-1) (ref. 11), thus our model can explain this choice for a resting behaviour. Morphing not only adjusts birds' wing performance to the task at hand, but could also control the flight of future aircraft.

ACKNOWLEDGMENTSThis study was conducted under the auspices of the Bats and Wind Energy Cooperative. We wish to thank the American Wind Energy Association (AWEA), Bat Conservation International (BCI), the National Renewable Energy Laboratory-Department of Energy (NREL), and the U.S. Fish and Wildlife Service (USFWS) for partnering to form the Bats and Wind Energy Cooperative (BWEC). Tom Gray (AWEA), Alex Hoar (USFWS), Bob Thresher (NREL), and Merlin Tuttle (BCI) provided oversight for the BWEC the project.We wish to thank the U.S. Fish and Wildlife Service, donors to BCI, the National Renewable Energy Lab, and Iberdrola Renewables for funding the curtailment study at the Casselman Wind Project in 2008. We are indebted to Holly McCready, Jeff Miller, Erica LaMore, Mario Desilva, Brian Farless, Paula Shover, Ryan Claire, and Ann Zurbriggen for their dedication in the field and collecting and managing the data throughout the study. We thank Iberdrola Renewables employees, in particular Andy Linehan, Sam Enfield, Jason Bell, Scott McDonald, Dave DeCaro, and Chris Long, for their support and efforts to make this study happen and run smoothly. Zac Wilson (BCI) conducted all GIS analysis for the study. Finally, we appreciate the support and hospitality of the private landowners that graciously allowed access to their lands for this study; they should be commended for supporting proactive research for solving wildlife and wind energy issues. EXECUTIVE SUMMARYWe implemented the first U.S.-based experiment on the effectiveness of changing turbine cut-in speed on reducing bat fatality at wind turbines at the Casselman Wind Project in Somerset County, Pennsylvania. Our objectives were to 1) determine the difference in bat fatalities at turbines with different cut-in-speeds relative to fully operational turbines, and 2) determine the economic costs of the experiment and estimated costs for the entire project area under different curtailment prescriptions and timeframes.Twelve turbines of the 23 turbines at the site were randomly selected for the experiment and we employed three treatments at each turbine with four replicates on each night of the experiment: 1) fully operational, 2) cut-in speed at 5.0 m/s (C5 turbines), and 3) cut-in speed at 6.5 m/s (C6 turbines). We used a completely randomized design and treatments were randomly assigned to turbines each night of the experiment, with the night when treatments were applied being the experimental unit. We conducted daily searches at the 12 turbines from 26 July to 10 October 2008. During this same period, we also conducted daily searches at 10 different turbines that were part of a complementary study to determine if activity data collected prior to construction with acoustic detectors can be used to predict post-construction fatalities, and to meet permitting requirements of the Pennsylvania Game Commission's (PGC) voluntary agreement for wind energy (herein referred to as "PGC" turbines). These 10 turbines formed an alternative 'control' to the curtailed turbines. We perfo...