The effect of weather on migrating bee-eaters studied by radio-telemetry and numeric atmospheric model

Despite being the subject of comprehensive basic and applied research, current understanding of the response of migrating birds to environmental factors is still hampered by our capacity to discern key interactions of migrating birds with their environment at appropriate small and short scales over which a bird senses, and responds to, environmental variation. To overcome this major limitation, we employed radio-telemetry systems and numeric atmospheric modeling to study the migration of the European Bee-eater (Merops apiaster) over Southern Israel. The Regional Atmospheric Modeling System (RAMS) was applied to simulate the meteorological conditions encountered by the birds at 0.25 km and 1 min resolution. We found that bird take-off from stopover sites was associated with temperature peak at the between-day scale, and occurred during increasing temperature trend at the within-day scale. Moreover, soaring birds took-off at significantly higher temperatures compared to flapping birds. During cross-country migration, flight-mode was found primarily to depend on the atmosphere’s turbulence kinetic energy (TKE), and temperature. Soaring was executed under high TKE and temperature, and weak tailwind assistance, while flapping took place under extremely low TKE and temperature, and headwind conditions. Bird airspeed during flapping was higher than during soaring, but the corresponding ground speeds were similar due to the effect of tailwind assistance. Bird heart-rate during soaring was found to be substantially lower compared to flapping, explaining the propensity of bee-eaters to migrate by soaring. We suggest that application of numeric atmospheric modeling may help scrutinize the response of migrating birds to their environment.