1972

It has long been recognised that birds are killed or injured by flying into overhead wires, but the problem has received little in the way of detailed investigation. Although birds strike telephone wires, this paper is largely concerned with electrical transmission cables. In Britain, the Central Electricity Generating Board has some 17,000 km of major (400, 275 and 132 kv) transmission lines, 5,800 km of which have been erected within the last ten years. Lines sited near estuaries, in river valleys or between bodies of water provide a particular hazard when they lie across the flight paths used by wildfowl, waders, gulls and other water birds between feeding and roosting areas. A high incidence of casualties among such birds was reported from the Teesmouth area in 1963 and 1964 (Teesmouth Bird Club per Mrs Angela Cooper in litt.) and near Carlisle in 1968 (J. Jackson in litt.).

The vulnerability of the Mute Swan (Cygnus olor) to collisions with overhead wires has been discussed by Ogilvie (1967) and Perrins and Reynolds (1967). Indeed, analysis of the ringing recoveries for this species and the Grey Heron(Ardea cinerea) indicates that overhead wires are the cause of a considerably higher proportion of deaths in these large, low-flying species than in the Black-headed Gull (Larus ridibundus) and raptors (table 1). The figures stated are limited to recoveries in which the cause of death is known. Since, however, it is fairly easy to determine whether a bird has flown into wires, the true proportion which die in this way may be somewhat lower. The significant increase since 1946 in the proportion of Black-headed Gulls killed as a result of striking overhead wires is noteworthy.

The aim of the survey at Dungeness was to record systematically the effect of overhead wires on birds in one area over a relatively long period. In particular, it provided an opportunity to assess the hazard of overhead wires to migrant passerines, and to test methods of protection.