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Birds are laying their eggs a month earlier than normal

The antenna of an Argos satellite tag extends past the tail feathers of a female American robin as she feeds a worm to her hungry nestlings on a front porch in Cheverly, Md. (Carolyn Kaster/AP)
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Did your backyard nest produce little songbirds a bit earlier this year?

You’re not alone. It turns out birds are laying their eggs earlier than before — and eggshell evidence points the finger at our changing climate.

A new analysis published in the Journal of Animal Ecology shows that the average egg-laying dates have moved up by nearly a month for 72 species of birds in the Upper Midwest region.

The bodies of these shorebirds are actually shrinking, and global warming is the cause

A 120-year-old collection of eggshells held by Chicago’s Field Museum helped hatch an investigation by a national group of researchers. The museum houses hundreds of the shells, most of which were collected before the 1920s, along with data about the types of birds and when the eggs were laid. The scientists also used records of bird nesting observations taken in the Chicago area between 1880 and 1920 and about 1990 to 2015.

Over time, the researchers found, the average egg-laying date moved up for a variety of species in Chicago. Overall, the birds’ lay dates advanced by an average of 25.1 days, with less shift for resident species and a wider shift for short- and long-distance migrants.

The animals studied aren’t just early birds: They are sensitive to climactic shifts. The researchers found that small changes in temperature — approximated using carbon-dioxide data from over the years — affected birds’ laying patterns.

Birds’ winter habits are shifting as climate, land use change

Climate change has shifted seasonal rhythms of animals and plants, which affects everything from bird food to bird habitats and can place birds in competition with one another for insects and other food sources. The earlier and warmer springs that accompany human-caused climate change can effectively strand birds that are born earlier than their traditional food sources.

The study points not just to the urgency of human-caused climate change but also the value of historic observations to modern scientists.

Combining archival data with modern observations, the researchers write, “will provide the ability to track, understand and perhaps even predict responses to present and future human-driven environmental change.”

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