Remotely-sensed tornado signatures indicate an ecological threshold for eagle nest destruction

Abstract

Bald Eagles (Haliaeetus leucocephalus) will reuse massive nests placed high in trees over multiple years, potentially exposing them to catastrophe loss during severe storms. The stochasticity of localized weather, however, has traditionally been viewed as impeding the quantitative study of such risks. In March 2015 a severe weather outbreak along the Arkansas River near Tulsa, Oklahoma caused widespread damage among a highly-concentrated and long-monitored population of nesting Bald Eagles. We conducted field surveys on the extent of nest loss and consulted weather and observer records to determine what characteristics of the storm (maximum azimuthal wind shear) or nests (years of use) might have been associated with nest destruction. We found 5 of 9 nests along a ~24km stretch of the river were destroyed during the storm, causing the death of at least 8 eaglets. Mean years of use was higher among destroyed nests (4.8) than surviving nests (4.0), though not significantly so within this limited sample. The degree of maximum azimuthal shear (i.e., wind rotation) during the storm within 800m of the nests, however, did significantly differ both in terms of maximums (15.8 vs 9.5 ms-1) and means (9.4 vs 6.6 ms-1) for destroyed versus persisting nests, respectively. Our findings suggest a threshold of tornadic wind shear beyond which Bald Eagle nests, irrespective of age, could be prone to catastrophe. Such insights are key to developing accurate models of population persistence, especially in light of potential shifts in severe weather patterns under various climate change scenarios.