Monday October 21, 2024
High Country News —
At a glance, the male western tanager looks like a little flame, its ruby head blending seamlessly into its bright lemon-colored body. Females are less showy, a dusty yellow. The birds spend their winters in southern Central America and can be found in a variety of habitats, from the cool cloud forests of central Costa Rica to the deserts of southeastern Sonora in western Mexico. In early March, they prepare to migrate thousands of miles to the conifer forests of the Mountain West, flying through grasslands, deserts and, occasionally, suburban yards.
To fuel them on their lengthy journey, western tanagers fill up on insects and berries. Like most migrating birds, they eat constantly when they’re not in the air. But as global climate change causes spring to start earlier, birds like western tanagers are arriving at their destinations after green-up, when flowers begin blooming and insects emerge. According to a study published in early March in the journal PNAS, this kind of timing mismatch between migrants and their food sources, which is happening across North America, could have dire consequences for migratory birds’ survival. “In discussing climate change, we often focus on warming,” said Scott Loss, an associate professor at Oklahoma State University and lead author of the study. “But the length and timing of seasons — like when winter ends and spring begins — are some of the most dramatic effects of climate change.”
Loss and his colleagues used satellite imagery from 2002-2021 to calculate the average start of spring green-up along the typical migration routes of 150 North American bird species, then compared that timing to the current green-up, or the most recent year for which they had data. They found that spring is indeed beginning earlier along birds’ migration routes. The trend continued this year, when, following an unusually mild February, leaves and blooms emerged up to 14 days ahead of schedule along the West Coast, making this year’s green-up the earliest on record.