Characteristics of coastal updrafts and flight patterns of white-tailed sea eagles

Abstract

We observed the flight behavior of white-tailed sea eagles Haliaeetus albicilla at the town of Tomamae in northern Hokkaido from 2009 to 2016, to determine how the eagles’ flight behavior was affected by updrafts created by the topography of coastal cliffs. The frequency, location, direction and speed of eagle flights were recorded by theodolite, along with weather conditions and topographic variables. Based on those variables, a statistical model of flight behavior was created. The eagles flew in the study area most frequently when the wind blew in the west-northwesterly direction (29.5%), and in the westerly direction (23.5%). The statistical model, based on the flight frequency of the eagles and the terrain variables in the 10m grid, indicated that slope degree, slope direction (east-northeast and west-northwest), number of cliffs, and presence of coastline had a positive effect on the flight frequency. Based on this model, we performed a numerical simulation of airflow in the survey area. As a result, it was determined that as the wind velocity increased, the resulting updraft generated by the coastal cliff developed farther away from the cliff. The eagles were observed flying farther out to sea when the wind blew stronger, apparently to make use of the updraft located further from the shore. The observed flight speed of eagles flying along the cliffs was less than the minimum flight speed required to maintain their altitude, as estimated by the general lift model. In this case, eagles would be expected to lose altitude and descend. However, the result of the simulation showed that the updraft at the particular position of the eagles’ flights were stronger than the estimated subsidence force. We conclude that updrafts occurring in the vicinity of the cliffs overcome the subsidence.