Each year, more than five million songbirds fly through Chicago as they migrate. But as the climate warms, the timing of those trips are changing.

For more than 30 years, the Field Museum in Chicago has been monitoring migration patterns by studying the bodies of birds that collide with buildings.

Based on collision numbers and dates, they’ve found that some flocks pass through Chicago about a week earlier in the spring than they did several decades ago.

Douglas Stotz is a conservation ecologist at the museum. He says that climate change is the clear cause. Warmer temperatures mean that some birds may not fly as far south to find a wintering ground, so it does not take them as long to get back to Chicago in the spring.

And warmer temperatures are causing some birds – like many white-throated sparrows – to stay in Chicago rather than flying south for the winter. Stotz says it’s important to pay attention to these changes in a bird’s migration patterns.

Stotz: “They are an indicator of what’s happening to the ecosystem as a whole, to the climate as a whole.”

For example, if birds do not arrive in time to eat certain insects that damage trees, the trees will suffer.

Stotz: “It’s sort of scary to think how much bigger these effects are going to become in the future if we don’t come to terms with climate change.”

Reporting credit: Sarah Kennedy/ChavoBart Digital Media.