Limitations and mechanisms influencing the migratory performance of soaring birds

Abstract

Migration is costly in terms of time, energy and safety. Optimal migration theory suggests that individual migratory birds will choose between these three costs depending on their motivation and available resources. To test hypotheses about use of migratory strategies by large soaring birds, we used GPS telemetry to track 18 adult, 13 sub-adult and 15 juvenile Golden Eagles Aquila chrysaetos in eastern North America. Each age-class had potentially different motivations during migration. During spring, the migratory performance (defined here as the directness of migratory flight) of adults was higher than that of any other ageclasses. Adults also departed earlier and spent less time migrating. Together, these patterns suggest that adults were primarily time-limited and the other two age-classes were energylimited. However, adults that migrated the longest distances during spring also appeared to take advantage of energy-conservation strategies such as decreasing their compensation for wind drift. During autumn, birds of all age-classes were primarily energy-minimizers; they increased the length of stopovers, flew less direct routes and migrated at a slower pace than during spring. Nonetheless, birds that departed later in autumn flew more directly, indicating that time limitations may have affected their decision-making. During both seasons, juveniles had the lowest performance, sub-adults intermediate performance and adults the highest performance. Our results show age-and seasonal variation in time and energy-minimization strategies that are not necessarily exclusive of one another. Beyond time and energy, a complex suite of factors, including weather, experience and navigation ability, influences migratory performance and decision-making.

Energy-Neutral Data Collection Rate Control for IoT Animal Behavior Monitors

“…Two major operating schemes were observed from data in a recent study by Miller [7] of wildlife tracking data due to a fixed rate of data collection of 15 minutes. Firstly, available energy from the battery dropped below a minimum charge level and data collection stopped until adequate energy had been harvested and stored.…”

“…The devices' data collection rates were not adjusted during the study, which is something that must occur manually on a remote system. Fixed data collection rates are typical of wildlife behavior devices due to simplicity of operation [7]. Movement data collected by animal behavior monitors are typically desired at fixed intervals for ease of integration into statistical behavior models, but are rarely achieved due to sometimes aggressive data collection rates.…”

“…A noise component similar to the device used in [7] of the sourced PVC current was introduced to model factors such as cloud cover, partial PVC obstruction, or lack of sunlight due to foliage. Equation (2) shows noise current I Noise , between a lower I Min and upper I Max bound, where these limits represent ±20% of I(k) from Equation (1), which is similar to literature describing modeling of energy-harvesting PVCs [37].…”

“…Real fix rate and battery performance data were collected from a field-deployed behavior monitoring device on a golden eagle free-roaming in eastern North America [7]. The fix interval, battery capacity, PVC voltage, and charge rate (mA) measured at each fix acquisition were analyzed.…”

Albatross-Like Utilization of Wind Gradient for Unpowered Flight of Fixed-Wing Aircraft

“…A wind field in which the wind gradient varies with the altitude is known to contain a large amount of energy [1][2][3]. In addition, albatrosses have been observed to soar for long periods without flapping their wings [4][5][6][7]. Lord Rayleigh was perhaps the first to discuss the problem relevant to albatrosses soaring in a horizontal but non-uniform wind field [8].…”