In 2013, the ornithological journal Avian Conservation and Ecology (ACE) published a special issue documenting human-related sources of bird mortality in Canada. Since Maine abuts Canada, the Canadian research has broad applicability to Maine. I will devote the next three posts to a discussion of some of the papers as Earth Day approaches.
A variety of human or human-enabled impacts result in significant bird deaths or injury. These impacts are usually additive to natural deaths. In other words, the human-related factors caused the demise of birds that would have otherwise survived. The value of the work published in the ACE special issue is that the relative importance of the various human impacts is evaluated. These rankings will allow conservationists and environmental managers to most effectively target their efforts and resources to reduce bird mortality stemming from human direct and indirect effects.
In Canada, several billion birds of over 400 species nest each year. These birds breed in a broad range of habitats, each of which may have its own particular human-related threats.
Sébastien Rioux and colleagues examined the importance of collisions with transmission lines on bird mortality. Getting accurate estimates of this impact is difficult because of the dearth of studies and the certain underestimate of mortality because some of the birds killed by collisions are scavenged by foxes and other animals before they can be found and counted by researchers.
The high level of uncertainty is evident in a recent study in the United States where estimates of bird deaths from line collisions ranged from hundreds of thousands to 175 million. Rioux and colleagues presented their most realistic estimates for Canadian bird mortality from these collisions as 2.5 million to 26.5 million birds per year. At the high end, those deaths kill about 0.7% of Canadian birds each year.
Some groups of birds are particularly susceptible to colliding with transmission lines, particularly during migration. Birds at most risk include grebes, waterfowl, shorebirds and cranes. Some waterfowl species are increasing in population size in Canada. While regrettable, line collisions do not seem to be limiting population growth of those waterfowl. But a collision-related death for an endangered Whooping Crane is a major blow.
[Originally published on March 30, 2014)