By Steve Howard
Where have the birds gone?
One species has declined by 52% over the last twenty years. Another has declined 54% in the same time frame. Another has dropped 29%. Another 28%.
These are not species of insects from a foreign continent. They are small insect eating birds in Ontario.
According to The Atlas of the Breeding Birds of Ontario, the population of aerial insectivores, or small insect-eating birds have steadily decline in Ontario over the last twenty years. Why these birds are declining and what can be done to reverse the trend is a question that continues to baffle scientists.
Mike Cadman, who is with the Canadian Wildlife Services and helped coordinate the Atlas, has spent the years since research finished in 2005 looking over the data. Some trends jumped out at him.
One the most obvious culprits of declining bird population is human interference. For an example of how harmful the human footprint has been, Cadman points to the Carolinian region of Canada, which is southernmost tip of Canada with Grand Bend marking the northwest of the area and Toronto the northeast. It’s the area which has seen the worst decline in Ontario since the first study was finished in 1986.
“It’s by far the most densely populated part of Canada, the heaviest ecological footprint on the land is in this part of Canada,” said Cadman. “You would expect this would be the area where birds would be doing the poorest and it really seems to be true.”
But what are humans doing that is harming bird populations? That question is much more difficult to answer. Cadman says the use of land for farming and industrialization may have something to do with it. As humans plow field, and takeover grasslands and wetlands, Cadman says there is simply less room for birds.
Another theory is pesticide use. Pesticides are used to kill insects which would lower the amount of food for the insect-eating birds, which seem to be suffering the most. The problem with that theory is that pesticides would have a stronger effect on birds of prey who would take in pesticides from smaller birds and the insects they’d eaten. Cadman points out larger birds would suffer the most from pesticides and they seem to be doing fine.
“It’s mainly used as an insecticide so (small birds) would indeed get some and it would have affected them, but not as strongly as the birds at the top of the chain. Not nearly as directly.”
Like Cadman, Silke Nebel has also been studying the decline of birds. Nebel, a Postdoctoral Fellow at the University of Western Ontario’s avian research facility, is using data from the North American Breeding Bird Survey, a study which has monitored bird populations in North America over the last 40 years.
Nebel’s data covers a wider area than Cadman’s and goes back further. Still, as Cadman suggested, it seems that if you are talking about a decline in bird population, you are talking about insect-eating birds.
Nebel’s data points out several patterns in bird decline, one of which is that decline is the steepest in the north-eastern part of North America. While this seems to support a thesis that human interference has something to do with a decrease in bird population, Nebel’s research also shows bird populations are declining quickest in northern Canada, while most of the human population has stayed south. Still, Nebel’s discovered during research that acidity from industry in southern Ontario is pushed north which causes acid rain in areas where birds are on the decline. Nebel believes the acid rain may be harming birds indirectly by killing insects which are sensitive to acidity.
Nebel’s research showed her two other trends which may give scientists a better idea why birds are disappearing.
The first is timing.
“In the mid-eighties, declines really started to go downhill,” said Nebel. “It could be something that’s a time lag. So maybe nothing happened in 1985, but the process had started ten years before.”
Nebel’s research also shows birds that travel the furthest are declining the quickest. Nebel says birds which are travelling to South America for the winter are doing better than birds that stay in the continent. Again this may come back to pesticides. Several types of pesticides which have been banned in North America are still used in South America. Nebel says insect-eaters who winter in the far south may be getting harmed by the pesticides, either because it’s killing their food or because they may be soaking up enough pesticide to harm them or birds back home.
When the data is all pulled together, Nebel comes up with many of the same theories as Cadman.
“My personal favorite is pesticide use,” said Nebel. Like Cadman, she also warns it’s just a theory. Her data can’t tell her anything for sure. “Like with anything, it’s probably a number of factors interacting. It’s unlikely that it’s one single thing. Everything is connected.”
Another possibility is the most obvious one.
“One thing that all these species have in common is that they’re all feeding on insects,” said Nebel. “What you really would want to know is have the insects declined over the last 40 years. And that can’t be answered.”
While there are plenty of studies on individual types of insects, Nebel says researchers haven’t done studies that track insect populations over long periods of time.
Katie Marshall, a biology PhD candidate at UWO, has been studying how insects survive winter. While she doesn’t know how insect populations have changed over the last 40 years, she has been studying how climate change is affecting how insects spend their winter, which may have an impact on birds.
Marshall has discovered in her research that although the winters are warmer overall, this has actually meant a colder winter for insects who spend their winters under the ‘litter layer’, which is a layer of leaves, grass and snow which covers the ground.
“A lot of insects spend the winter at the ground level,” said Marshall. “If you spend your winters under the litter layer, generally in winter’s past, you had a nice big blanket of snow on top of you. You would expect the ground to be at about zero degrees Celsius for most of the winter.”
Global warming may have made winter more tolerable above the litter layer, but for the insects below, their blanket of snow is now melting, making it colder. Marshall says more freezing and thawing cycles, means more trouble for insects. Marshall is still conducting research, but has found some results.
“It seems with caterpillars that an increase numbers of freezing and thawing cycles actually cause a lot of mortality, it seems they can’t repair any damage that happens as a result,” said Marshall.
Not all insects are the same. Fruit flies on the other hand seem to be able to better survive the winter thanks to the cycles. But when they awake from the winter, Marshall says they seemed to have less energy for reproducing, which would lower the amount of offspring and the population for the summer.
Marshall also suggests insects may be coming out of diapauses, their version of hibernation, at different times of the year than they used to because of global warming.
“This is where it’s really important for birds,” said Marshall. “The timing of when you come out can be dependent on temperature or it can be dependent on light. If it’s dependent on temperature, if it’s really warm in the spring, you might come out too early. Maybe the birds haven’t migrated back from Mexico yet.”
This mismatch in timing is something which worried Cadman and Nebel as well. If the lifecycle of insects changes, birds may come back at a different time than their food.
While it’s tempting to draw conclusions on bird populations based on Marshall’s research, she warns just because two sets of data seem to be related, doesn’t mean they are.
The increase in air temperature is linked to the number of pirates. As pirates have declined air temperature has actually gone up,” laughs Marshall. “Just because two things are correlated with each other, it’s really hard to say one thing causes the other.”
In spite of the decline of certain bird populations, Cadman sees reason for optimism.
“In terms of the results of the project, and the things that I see happening overall in Ontario and Canada, I’m reasonably optimistic,” said Cadman. “The eastern North America is reforesting, much of southern Ontario is reforesting, returning to a more natural state. That’s obviously very good for the birds.”
Cadman thinks it’s important for humans to start thinking differently about how they are interacting with the animals around them.
“Where we are having an effect, we’re having a strong effect. We need to be managing are activities there more carefully than we have been to this point.”