Critical Dimensions of Raptors on Electric Utility Poles

Abstract

Avian electrocutions on overhead power structures are a global conservation concern. Size is an important factor influencing whether a bird perched on an electric utility pole is at risk of electrocution, with larger species and larger individuals at greater risk. Ideally, electric poles should protect the largest species (typically Aquila or Haliaeetus species), but protection measures are expensive, making implementation a challenge when a utility's service area does not include eagles. In these cases, avian protection is sometimes omitted, leaving smaller species at risk because compromise recommendations are unavailable. Flesh-to-flesh distances are a primary determinant of electrocution risk because feathers are only slightly more conductive than air. Metacarpal-to-metacarpal dimensions are particularly important because they quantify the total horizontal distance which can be bridged by the flesh of a bird, but few studies describe metacarpal-to-metacarpal dimensions of at-risk species. Here, we report metacarpal-to-metacarpal and carpal-to-carpal dimensions of 230 raptors of 27 species undergoing rehabilitative care following injury in the wild. Carpal-to-carpal measures facilitate comparison with early efforts by the Avian Power Line Interaction Committee. Our maximum measurements for female Bald Eagles (Haliaeetus leucocephalus), Ferruginous Hawks (Buteo regalis), and Red-tailed Hawks (Buteo jamaicensis) exceeded the range previously reported. Wildlife resource managers and electric utility personnel should use metacarpal-to-metacarpal measurements when considering whether a utility pole poses electrocution risk to a particular species. Future research should include reporting these dimensions for at-risk species world-wide so retrofitting recommendations can be further defined beyond North America.