Our Wildlife-Friendly Gardens Are Killing More Birds

I grew up in a rural bird-friendly house. But this house wasn’t as bird-friendly as it first appeared because in fact, it was a bird-killer.

Even now, I can recall my dread as I investigated the source of the most recent “thud” on one of the many windows. Sometimes, a few feathers fluttered in the breeze, anchored to the glass, marking the collision point. If I was lucky, I would find nothing (indicating that the bird survived and flew away) or maybe I’d discover the stunned — but still living — bird, which might recover after spending half an hour or so in a dark paper bag. But more often than not, I’d retrieve a limp feathery body, still warm, in my trembling hands.

But my childhood house wasn’t unique: it has been estimated up to one billion birds die in North America each year as a result of bird-window collisions (ref). Although urban high-rise buildings are usually the biggest individual offenders, there are vast numbers of suburban homes everywhere across North America — homes just like the one I grew up in. This means that even a few bird deaths per dwelling add up fast.

Which raises some questions: are more bird mortalities occurring in skyscraper-filled urban environments or in rural residential areas? Are wildlife-friendly yards that are filled with native plants and tall trees less or more deadly to wild birds? Previous work hinted that bird feeders might actually increase bird-window collisions (ref), so the obvious follow-up question to that work became; “Does the distance of feeders from windows affect the number of collisions?”

These questions inspired Justine Kummer, who recently earned her master’s degree from the University of Alberta, to investigate further. She and her colleagues decided that a citizen science project would be the best way to explore these issues. Since her team was dealing with the public, this particular study involved a lot of careful advance planning and a lot of public outreach.

“Unlike some other projects, I didn’t spend my time collecting data; I spent it trying to recruit homeowners and educate the public about the issue,” said Ms. Kummer in a press release.

Raw data were collected by citizen scientists

Ms. Kummer and her team designed a dedicated website especially for this study and launched it in 2013 so volunteer citizen scientists could report their data online. More than 1,300 citizen scientists participated in the study, most of whom resided inside Canada, although some participants lived abroad. Participants lived in a variety of dwellings as well as apartments. The researchers documented very specific information about the windows in each dwelling. For the purposes of this study, all housing was designated as either urban or rural based on Statistics Canada classifications.

Ms. Kummer and her team documented how many bird feeders or other “bird attractors,” such as bird baths or bird houses, were located within 10 meters (33 feet) of the dwelling. Participants daily tracked bird collisions around their dwellings for 18 months and reported any evidence of birds colliding with the windows, and any dead birds were photographed so they could be identified to species.

Ms. Kummer and her team analyzed more than 34,000 days’ worth of collision data from the Canadian province of Alberta. These analyses revealed a seasonal pattern for bird-window collisions, which is to be expected since there are fewer birds around in winter (figure 1):

Rural houses had a consistently higher risk of a bird collision than urban houses in all seasons; collision risk during autumn was between 1.2 to 1.4 times greater than that of the next closest season.

Different dwelling types presented different risks for bird collisions, too. Single-attached houses had the highest risk (2.3 times greater) than apartments, which had the lowest risk.

When yards were compared, the amount of development was a better indicator of bird collisions than was the dominant type of landscape. For example, dwellings surrounded by undeveloped landscapes were 2.2 times more likely to have a bird collision than houses with developed landscapes, and tall trees further increased collision risk by 4.3 times over yards with no trees or shrubs.

And so, getting back to the question that originally inspired this study, how did the presence of a bird feeder affect bird collision rates? There were no surprises here since a bird feeder was strongly associated with increased bird-window collisions (figure 2):

Overall, dwellings with a bird feeder were 6.0 times greater risk of bird collisions than those without a feeder — which is to be expected since a bird feeder means there are more birds around to begin with.

Out of Alberta’s 421 bird species, 53 were represented in the data, mostly common urban species. Risk assessed for different avian families revealed collisions were most common for Chickadees (8.78% of birds collided), Waxwings (7.07%), Thrushes (4.93%), Sparrows (4.71%) and Corvids (3.96%).

One thing that I found interesting was that some of these species (specifically, waxwings) do not rely much on traditional bird feeders that dispense seeds. This could be because homeowners who have bird feeders are more likely to cultivate native vegetation that attracts these birds.

How to make your yard more bird-friendly

“Although each typical residential dwelling only causes one or a few bird–window collisions per year, the enormous number of these buildings means we are killing far more birds in our collective backyards than are dying at large office buildings and skyscrapers,” said Scott Loss, an Assistant Professor of Natural Resources at Oklahoma State University, who was not part of this study.

“Kummer et al. provide an excellent example of how the power of citizen scientists can be harnessed to address this major conservation issue,” said Professor Loss, who was lead author of a landmark 2014 review of bird-building collisions (ref).

Although it is not known if bird-window collisions are causing the decline of any vulnerable species, it’s obviously of concern to the citizen scientists who participated in this study. Further, these homeowners typically were dedicated to encouraging birds in their yards by keeping bird feeders and by selectively cultivating native plant species and engaging in other wildlife-friendly practices.

So what can a bird-loving homeowner do?

“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”, said Ms Kummer. As a result of her research, Ms Kummer’s has a few recommendations:

• Place feeders within one meter of windows — birds will still use the feeder but most can’t build enough momentum to sustain serious injury if they hit the window as they leave the feeder (alternatively, place feeders more than 10 meters away from a window. At this distance, birds are more likely to recognize the reflected image as part of a house)
• Move indoor houseplants and flowers away from windows so they cannot be seen from outside and mistaken for shelter or food
• Close indoor curtains/blinds to cut down the reflection of vegetation in the window
• Generously apply UV decals to windows (humans cannot see these, but birds can)

And here’s a few more of my recommendations for making dwellings safer for birds:

• Place exterior screens over windows
• Install an outdoor retractible sun blind over windows that can be lowered at night and when you are not home
• Do not leave lights on at night unless blinds are closed over the windows in the lit room
• Install a motion sensor on all outdoor lights
• Install outdoor lights that are designed to shine towards the ground

Source:

Justine A. Kummer, Erin M. Bayne, and Craig S. Machtans (2016). Use of citizen science to identify factors affecting bird–window collision risk at houses, The Condor: Ornithological Applications, 118:624–639 | doi:10.1650/CONDOR-16-26.1

Also cited:

Scott R. Loss, Tom Will, Sara S. Loss and Peter P. Marra (2014). Bird–building collisions in the United States: Estimates of annual mortality and species vulnerability, The Condor: Ornithological Applications, 116(1):8-23 | doi:10.1650/CONDOR-13-090.1

Justine A. Kummer and Erin M. Bayne (2015). Bird feeders and their effects on bird-window collisions at residential houses, Avian Conservation and Ecology 10(2):6 | doi:10.5751/ACE-00787-100206