This post is the last of three leading up to Earth Week in which I compare the human-related causes of bird mortality.\u00a0 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.\u00a0 The proximity of Canada to Maine makes the Canadian findings relevant to bird impacts in our state.<\/p>\n
The effect of wind turbines on bird mortality, described in the previous column, is always a hot-button topic.\u00a0 Not surprisingly, I received emails on the one hand dismissing wind turbines as a significant risk to birds.\u00a0 Others wrote to claim the reported deaths are vastly underestimated because the correction factors for unfound carcasses and removal by scavengers are too low and that the bird mortality studies are conducted by biologists hired by the wind power companies.<\/p>\n
As a long-term opponent to wind turbines on mountain ridges, I am at least heartened that the wind industry acknowledges wind turbines can pose a significant threat to birds and bats.\u00a0 A consortium of wind power companies in Norway is experimenting with putting black stripes or stripes that reflect ultraviolet wavelengths (birds can see those wavelengths but humans cannot) to mitigate White-tailed Eagle deaths (55 since 2005) at one site.<\/p>\n
Yes, there is uncertainty about how many birds die from wind turbine collisions.\u00a0 But even the highest reasonable estimates of that bird mortality pale in comparison to two other sources of bird deaths.<\/p>\n
The second-most important cause is collisions with building windows.\u00a0 Craig Machtans and colleagues used some field data and some modeling to estimate this source of avian mortality.\u00a0 They calculate that about 25 million birds are killed annually in Canada from window collisions.\u00a0 One might think that tall buildings would be the deadliest structures but only about 1% of collision-related deaths occur at this type of building.\u00a0 Collisions with house windows are responsible for 90% of the mortality.\u00a0 The remaining deaths occur from collisions at low-rise commercial or institutional buildings.\u00a0 The importance of houses stems from the fact there are far more houses than other types of buildings.<\/p>\n
The reasons for these collisions are two-fold.\u00a0 Sometimes birds strike clear glass when a breezeway or other narrow glassed-in structure is in their flight path.\u00a0 The birds think they can fly directly through the transparent glass.\u00a0 At other times, birds strike reflective glass when they are trying to reach vegetation or the sky reflected in the glass.<\/p>\n
The authors\u2019 model predicts the number of birds killed at houses ranges between 0.3 and 16 per house each year. \u00a0Houses with bird feeders have more window kills because of our feeding stations attract so many birds.<\/p>\n