It's bird migration season in Wisconsin, and scientists are noticing unsettling changes.
Ornithologist Bill Mueller is among them. As the director of the Western Great Lake Bird and Bat Observatory north of Port Washington, he has been observing a drop in numbers over recent years.
Mueller coordinates counts of bird species that nest and migrate through Wisconsin. While he celebrates every song bird and water fowl he sees, Mueller is hearing and seeing fewer.
"Mid-May signals peak migration. When there’s a low-cloud ceiling and you go to a place where there isn't a lot of road noise, the birds are 500 meters above the ground and they're all talking to one another. They're not singing. They're just giving contact calls. But you can hear a flock of 100,000 birds," Mueller says.
Hundreds of thousands of birds fly over southeastern Wisconsin during some peak nights of migration.
That might sound like a huge number, yet Mueller says bird populations are declining among species throughout North America and around the world.
"A third of [North America] avifauna is in decline, and globally one out of eight species is threatened or endangered," Mueller says.
At least eight species of aerial insectivores — birds that snatch their insect prey while in flight — are sharply declining, says Mueller. Those being impacted include the purple martin and the common nighthawk.
One type of bird many Wisconsinites may be used to seeing is the Canada goose. Historically, these large geese migrate through Wisconsin in the spring. Mueller says some that found themselves in urban settings, such as Milwaukee, have become sedentary and so acclimated that they only migrate if winter conditions are extreme.
"The geese are grazers, so that's why they gravitate to grassy parks and golf courses. Later afternoons they mosey down to open water where they sleep in open water where they are safer from predators," Mueller explains.
And many Canada geese, Bill Mueller says, are not only surviving, they're thriving.
But the yellow warbler may be facing the challenge of not finding food when it needs it.
These are among the birds whose migration is timed with the emergence of leaf-eating insects. Mueller says yellow warblers, for example, need that food after their long migration.
"They need to find something to eat when they get here, otherwise they're not going to make it and they especially won't be successful in raising any young," Mueller says.
He says scientists are studying whether the changing climate will cause caterpillars to emerge sooner, leaving birds like the yellow warbler hungry when they show up in Wisconsin at their usual time.
"Is a mismatch happening between the timing of the emergence of these leaf-eating insects and the arrival of these spring migrant birds? And if it's mismatched enough, that could have a serious impact on their survival," Mueller says.
Mueller says the interwoven ecosystem is complicated. So too are the factors that impact its balance. But there are a few things you can do to help protect birds.
He suggests people focus on removing threats to birds and other creatures. So, he says:
- Don't overuse pesticides
- Make your home bird-safe
- Keep pet cats inside
Mueller says preserving and protecting wildlife habitat tops the list.
And if we eliminate human-driven threats, he says some bird species might rebound.
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