Humaninduced Losses

In some of the in-flight incidents mentioned above, human artefacts may have increased the losses at night, because on dark or misty nights birds were attracted to lights, resulting in collisions (Swenk 1922, Roth 1976). Such casualties have long been known from coastal and offshore lighthouses on nights of poor visibility, with low cloud or rain (e.g. Gatke 1895). In the years 1886-1939, some 500-8000 birds per year were reported as killed at lighthouses around Denmark, a total of 33 800 in autumn and 20 700 in spring (Hansen 1954). Relative to their numbers, some species were killed more often than others: for example Brambling Fringilla montifringilla more than Chaffinch F. coelebs, and Jack Snipe Lymnocryptes minimus more than Common Snipe Gallinago gallinago. At lighthouses and other structures, rails seemed especially vulnerable. Such losses almost ceased where continuous beam lights were replaced by flashing on-off lights in the late twentieth century.

Even greater losses have been associated with the tall, illuminated masts used for radio, television and mobile phone transmission, especially in North America. Along with tall buildings and ceilometers (light beams for measuring cloud height that attract birds which then collide with nearby buildings), these masts kill many migrant birds (mainly by collision), especially those flying at night (e.g. Brewer & Ellis 1958, Tordoff & Mengel 1956, Taylor & Anderson 1973, Weir 1976, Avery et al. 1977, 1978, Lid 1977, Crawford 1978, 1981, Kemper 1996, Kerlinger 2000). In North America in the 1970s, an estimated 1.3 million migrants were killed in this way each year (Banks 1979). By the year 2000, tower numbers had increased roughly four-fold, as had the associated death toll, reaching an estimated 4-5 million birds per year (US FWS 2002). This is likely to be an underestimate, because many birds probably die away from the point of collision, or are removed by nocturnal scavengers before they can be found by people in daylight (for effects of scavenger control on numbers of carcasses found, see Crawford & Engstrom 2001). About 350 species have been recorded as casualties, the vast majority being Nearctic-Neotropical migrants which fly at night, such as Ovenbird Seiurus aurocapillus, Tennessee Warbler Vermivora peregrina, Black-and-white Warbler Mniotilta varia, Blackpoll Warbler Dendroica striata, Prairie Warbler Dendroica discolor and Magnolia Warbler Dendroica magnolia. In one of the most detailed studies, at a television tower in Florida, 44 007 victims of 186 species were found over a 29-year period, an average of 1517 per year. More than 94% were Neotropical migrants, with Red-eyed Vireos Vireo olivaceous the most frequent (Crawford & Engstrom 2001). Higher totals in late summer/autumn than in spring could be attributed to the greater numbers of birds migrating in autumn.

Mortality has sometimes been heavy, as exemplified by the 50 000 birds of 53 species killed in one night at a ceilometer in Georgia (Johnston & Haines 1957). In general, towers taller than about 150 m kill the largest numbers of birds, and shorter towers relatively few (apart from one anomalous incident involving the deaths of 5000-10 000 Lapland Longspurs Calcarius lapponicus on the snowy night of 22 January 1998 at three 130-m towers in Western Kansas, Kerlinger 2000). Reducing the height of some towers has greatly reduced the fatalities (Crawford & Engstrom 2001), while increasing the height of other towers has greatly increased the fatalities (Kemper 1996). As at lighthouses, mass mortalities are most frequent on nights of low cloud and fog or rain, when birds fly lower than normal. At most other times, birds fly too high for masts and tall buildings to represent a hazard, and I know of no evidence that windows of low buildings kill more birds at migration times than at other times of year.

Gas flares on oilrigs also attract birds on dark foggy nights; up to several thousand per night having been killed at individual flares (Lid 1977). The usual numbers are much lower, however, estimated by Bourne (1979) at a few hundreds of birds per rig per year, a small proportion of the numbers passing. Modern wind turbines are known to kill migrants by night or by day, but information is only just beginning to emerge on the scale of these losses (which generally seem small, being estimated at a total of 33 000 birds per year in the USA, US FWS 2002). The greatest losses seem to occur at windfarms situated on narrow migration routes (with many raptors killed in southwest Spain), or near wetlands, which attract large numbers of gulls and other large birds. As these turbines continue to multiply in the years ahead, they could collectively begin to impose a heavier drain on migratory bird populations, perhaps achieving population levels effects, especially on large species with low reproductive rates. Some forms of mortality are so long established, and affect birds year-round, that it is hard to separate mortality at migration times from that at other times. This holds for mortality caused by collisions with power lines (Ferrer & Janss 1991), and for mortality inflicted by oil pollution on seabirds, although some species are clearly affected mainly during migration. An example is the Magellanic Penguin Spheniscus magellanicus, of which an estimated 41 000 are killed each year, mainly when on return migration off the Argentinian coast (Gandini et al. 1994). Another form of human-induced loss is the hunting inflicted on many species while on migration, and discussed for Palaearctic-Afrotropical migrants in Chapter 24.

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