This column is the second of three leading up to Earth Day, as I describe the human-related causes of bird mortality. Much of this information comes from a 2013 volume of the journal Avian Conservation Ecology, wherein these various sources of bird mortality are assessed for Canada.
Keith Hobson and colleagues evaluated the impact of industrial forest harvesting and management on avian mortality. These industrial forest practices have two major effects: destruction of nests and elimination of suitable habitat. Hobson calculated that between 600,000 and two million nests are destroyed each year by log cutting. Habitat destruction is harder to quantify because birds in older growth forests like Swainson’s thrushes, scarlet tanagers and Blackburnian warblers may be replaced by chestnut-sided warblers, mourning warblers and Lincoln’s sparrows in cut-over areas.
Farming practices also take a toll on bird populations. Mowing, tilling, seeding and harvesting may destroy nests or reduce suitable breeding habitat. Joerg Tews and colleagues provided estimates of mortality from agricultural activities of some grassland species in Canada. They estimated that 138,000 horned larks, 249,000 savannah sparrows and 667,000 bobolinks meet their demise from agricultural practices.
Christine Bishop and Jason Brogan summarized data on birds killed by automobile collisions on roads passing through a variety of forested and unforested habitats. Each year, 1,167 bird carcasses are found along every 62 miles of road. The authors adjusted their estimates to account for the removal of bird carcasses by foxes, American crows and other scavengers. The revised estimates suggest that nearly 3,500 birds are killed each year for every 62-mile stretch. Extrapolated to include all Canadian roads, an estimated 14 million birds die from collisions with automobiles.
Ryan Zimmerling and colleagues provided estimates of avian mortality because of collisions with wind turbines. Their figures were based on carcass searches at 43 wind farms in Canada. A carcass count, however, will be low because scavengers remove some of the dead birds before a researcher can find them, some carcasses will be overlooked by researchers and some carcasses fall beyond the search area. Applying a correction factor, Zimmerling estimated that on average eight birds per turbine are killed each year. The numbers ranged from zero to 27 birds killed by each turbine, accounting for about 23,000 bird deaths across Canada. Of note: Birds seem to be better at avoiding wind turbines than bats. Reducing bat mortality from turbine collisions is proving more difficult than reducing avian collisions.
Zimmerling and colleagues also considered the effect of habitat loss from wind farm construction and maintenance. They estimated that 5,700 nests are lost each year due to this habitat loss. With wind farms predicted to increase tenfold in the next decade, we can expect a tenfold increase in this type of mortality.
Joanne Ellis and colleagues assessed the impact of offshore gas and oil production on bird mortality. They found that between 2,700 and 46,000 birds die each year in Canadian marine waters as a part of the by-catch. In other words, diving birds get tangled in nets and drown.
These authors also assessed the impacts of oil slicks and sheens (impairing the ability of birds to fly and thermoregulate if coated with oil) and collisions with platforms and ships. The estimated mortality from these effects is modest – some 200 to 4,500 birds year.
Van Wilgenburg and colleagues provided comparable mortality estimates for land-based fuel exploration and extraction. Human-related activities leading to bird mortality include seismic exploration, habitat loss from the creation of pipelines and the mining processes themselves. Their estimates for all of these effects range from 10,000 to 41,000 birds killed each year.
In the next two columns, I’ll discuss the two most important human-related sources of bird mortality. Can you guess what they are?
Herb Wilson teaches ornithology and other biology courses at Colby College.