This column is the last of three leading up to Earth Week in which I compare 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. The proximity of Canada to Maine makes the Canadian findings relevant to our state.

The effect of wind turbines on bird mortality, described in the previous column, is always a hot-button topic. Not surprisingly I received emails on the one hand dismissing wind turbines as a significant risk to birds. 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.

As a longtime opponent to wind turbines on mountain ridges, I am heartened the wind industry acknowledges that turbines can pose a significant threat to birds and bats. A consortium of wind power companies in Norway is experimenting with putting either 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.

Yes, there is uncertainty about how many birds die from wind turbine collisions. But even the highest reasonable estimates of that bird mortality pale in comparison to two other sources of bird deaths.

The second-most important cause is collisions with building windows. Craig Machtans and colleagues used field data and modeling to estimate that about 25 million birds are killed annually in Canada from window collisions. One might think that tall buildings would be the deadliest structures, but only about 1 percent of collision-related deaths occur at this type of building. Collisions with house windows are responsible for 90 percent of the mortality. The remaining deaths occur from collisions at low-rise commercial or institutional buildings.

The importance of houses stems from the fact there are far more houses than other types of buildings.

The reasons for these collisions are twofold. Sometimes birds strike clear glass when a breezeway or other narrow glassed-in structure is in their flight path. The birds think they can fly directly through the transparent glass. At other times they strike reflective glass when they are trying to reach vegetation or the sky reflected in the glass.

The authors’ model predicts the number of birds killed at houses ranges between 0.3 and 16 per house each year. Not surprisingly, houses with bird feeders have more window kills.

I wrote a column in 2009 that offered suggestions for reducing window collisions at your home. That article can be read at http://bit.ly/1i3c51w

The most important source of human-related bird mortality is almost certainly cats. Peter Blancher in his article estimates that between 100 and 350 million birds per year are killed by cats. The majority of this predation stems from feral cats (cats that live their entire lives apart from humans). Canadians own about 8.5 million pet cats. The feral cat population lies somewhere between 1.4 to 4.2 million additional cats.

Blancher finds that between 2 and 7 percent of birds in Canada die each year from cat predation. When you realize that many birds live to be several years old and some live much longer than that, a 2-7 percent reduction per year is huge.

About 70 percent of pet cats spend some time outdoors and hence contribute to bird mortality. Blancher’s work suggests urban cats account for about one-sixth of cat-related bird deaths. Feral cats make up roughly 25 percent of Canadian cats but cause 59 percent of the cat-related bird deaths. The remaining bird mortality comes from pet cats in rural areas.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

whwilson@colby.edu