Wildlife-friendly yards may be deadly to birds

Collisions with windows are a serious source of mortality for birds: hundreds of millions die from window strikes each year in the U.S. alone. Most attention to this problem has focused on high-rise buildings, because each individual building of this type can kill a great many birds.

But because there are so many residential dwellings, even a few collisions per home means that collectively these structures are responsible for a huge number of bird deaths. Yet researchers don’t know why one house has more collisions than another, let alone how to prevent them.

A new study suggests that that yards that provide better overall bird habitat also lead to increased risk of window strikes— a result that brings home (quite literally) how our efforts to share our habitat with wildlife sometimes have unintended consequences.

In the study, researchers recruited residents of Alberta, Canada to participate in a citizen-science effort dubbed the Birds and Windows Project. Participants walked the perimeter of their home or apartment building daily and looked for evidence of bird-window collisions such as dead or injured birds, feathers, or blood on windows.

The researchers also collected information about the characteristics of each home and yard, such as whether it was in an urban or rural area, the surrounding vegetation, square footage, number of windows, and the number and location of bird feeders in the yard.

Several past studies have looked at factors that increase the risk of bird strikes, but this is the first to consider four spatial scales simultaneously: neighborhood, yard, house, and window.

The participants documented 930 collisions and 102 bird fatalities over the course of 34,144 person-days of monitoring. Window collisions affected 53 species of birds, all common urban dwellers.

The most important factors increasing a given house’s collision risk are the presence of bird feeders in the yard; tall trees and mature vegetation surrounding the house; and location in a more rural area, the researchers reported last week in The Condor: Ornithological Applications.

In other words, factors that attract birds to yards also increase collision rates. This makes sense from a probability point of view: more birds in a yard means more chances of colliding with a nearby pane of glass.

The researchers also found that windows with a reflective coating like low-E and UV glass windows have an increased risk of bird strikes. This means that these types of windows save energy, but are bad for birds—an example of the way sustainability goals can sometimes conflict with one another.

Still, it could be that at a population level, better overall bird habitat cancels out the increased risk of bird strikes, a question that will require further studies to answer.

And anyway, most people want to attract birds to their yards, not turn them away. “As homeowners don’t want to reduce the number of birds in their yards, I think the next step will be to determine the best window deterrents they can use at their homes,” says study leader Justine Kummer of the University of Alberta.

That means researchers need to investigate how to reduce collisions at the window level, such as by adding films or decals that make the window visible to the bird as an obstacle. Few studies of these products have been conducted so far, but future work will need to evaluate them both as a deterrent to birds and as aesthetically pleasing to homeowners, the researchers say. – Sarah DeWeerdt | 2 August 2016

Source: Kummer J.A. et al. “Use of citizen science to identify factors affecting bird-window collision risk at houses.” The Condor: Ornithological Applications DOI: 10.1650/CONDOR-16-26.1

Header image: A Northern Flicker felled by a window strike at a home in Seattle. Credit: Sarah DeWeerdt.

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