Review of Avian Mortality Due to Collisions with Manmade Structures

INTRODUCTION Birds have undoubtedly been colliding with manmade structures ever since humans started building things skyward. The earliest documented instances of collision mortality in this country are from the late 1800's (Coues 1876, Merriam 1885) and the problems continue unabated today. The total avian mortality due to collisions with manmade obstacles is probably greater every year as buildings, towers, chimneys, overhead power lines, and other structures are erected in ever-increasing numbers. Weir (1976) provides an excellent review of bird migration, weather, and collision mortality associated with various types of structures. The biological significance of collision mortality to any species is unknown. There are too many other factors, such as total population size, natural mortality levels, and other human related influences, for which insufficient data exist to be able to put the collision mortality factor in proper perspective. Recognizing these limitations, Banks (1979) recently analyzed avian mortality, including collisions, related to human activities and estimated that about 4.75 million (0.05%) of the 10 billion annual bird deaths result from collisions with manmade structures. In this paper, I will discuss each of the major sources of avian collision mortality and briefly review any pertinent investigations recently completed, currently in progress, or planned for the near future. Where relevant, comparisons with mortality estimates of Banks (1979) and others will be made. In addition, measures that have been developed to reduce the amount of various types of collision mortality will be mentioned. Basically, this is a review, and the information presented herein was obtained from the published literature and through personal communications with investigators in this field. Most of the available reports and articles were previously compiled in an annotated bibliography (Avery et al. 1978a). For some of the mortality categories, certain analyses were made to obtain an indication of the magnitude of the annual losses attributable to these sources. These analyses are described in the appropriate sections of the discussion. TV and Radio Towers Since the mid 1950's, bird losses at TV and radio towers have attracted much attention , especially throughout the Eastern United States. These structures and their supporting guy wires comprise the largest single category among the available reports and articles dealing with collision mortality (Avery et al. 1978a). The annual losses due to collisions with towers have been estimated at 1.0 to 1.25 million birds (Aldrich et al. 1966, Banks 1979) which represents approximately 0.013 to 0.016 percent of …