Inclement weather forces stopovers and prevents migratory progress for obligate soaring migrants

Abstract

Background Migrating birds experience weather conditions that change with time, which affect their decision to stop or resume migration. Soaring migrants are especially sensitive to changing weather conditions because they rely on the availability of environmental updrafts to subsidize flight. The timescale that local weather conditions change over is on the order of hours, while stopovers are studied at the daily scale, creating a temporal mismatch. Methods We used GPS satellite tracking data from four migratory Turkey Vulture (Cathartes aura) populations, paired with local weather data, to determine if the decision to stopover by migrating Turkey Vultures was in response to changing local weather conditions. We analyzed 174 migrations of 34 individuals from 2006 to 2019 and identified 589 stopovers based on variance of first passage times. We also investigated if the extent of movement activity correlated with average weather conditions experienced during a stopover, and report general patterns of stopover use by Turkey Vultures between seasons and across populations. Results Stopover duration ranged from 2 h to more than 11 days, with 51 % of stopovers lasting < 24 h. Turkey Vultures began stopovers immediately in response to changes in weather variables that did not favor thermal soaring (e.g., increasing precipitation fraction and decreasing thermal updraft velocity) and their departure from stopovers was associated with improvements in weather that favored thermal development. During stopovers, proportion of activity was negatively associated with precipitation but was positively associated with temperature and thermal updraft velocity. Conclusions The rapid response of migrating Turkey Vultures to changing weather conditions indicates weather-avoidance is one of the major functions of their stopover use. During stopovers, however, the positive relationship between proportion of movement activity and conditions that promote thermal development suggests not all stopovers are used for weather-avoidance. Our results show that birds are capable of responding rapidly to their environment; therefore, for studies interested in external drivers of weather-related stopovers, it is essential that stopovers be identified at fine temporal scales.

Welcome aboard: are birds migrating across the Mediterranean Sea using ships as stopovers during adverse weather conditions?

“…2019, Mallon et al . 2021) and secondly, the importance of energy reserves, hence pointing to the physiological condition of individuals ( cf . Fusani et al .…”

“…2020, Mallon et al . 2021). The median length of stopover in our sample was approximately 42 min, which is much less than the mean of 9 and 41 h calculated on island sites for fat and lean birds, respectively (e.g.…”

“…Indeed, there are two major lines of evidence -first, the effect of weather on stopovers, especially during autumn migration (cf. Brust et al 2019, Mallon et al 2021) and secondly, the importance of energy reserves, hence pointing to the physiological condition of individuals (cf. Fusani et al 2009, Lupi et al 2016.…”

“…Fusani et al 2009, Aamidor et al 2011, Smith & McWilliams 2014, for physiological recovery (Fuchs et al 2006, Ferretti et al 2021, to wait for the passage of inclement weather (e.g. Smith & McWilliams 2014) or for the arrival of optimal flight conditions, such as the rise of thermal updrafts strong enough to sustain the flight of soaring birds (Vansteelant et al 2015, Shamoun-Baranes et al 2017, Mallon et al 2021.…”

“…Migratory stopovers are the prime example of the flexible behaviour used by migratory individuals, be they large soaring Accipitrids or small broad-front passerines, to adapt to the changing conditions encountered en route or to cross barriers (Schmaljohann et al 2007, Mallon et al 2021, Roques et al 2021. Stopovers have multiple functions that change depending on the ecological and physiological needs of individuals, are related to the life history of species and are also contextdependent (reviewed in Linscott & Senner 2021, Schmaljohann et al 2022.…”