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State of the Birds Report

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Climate Change

Birds are at grave risk from habitat
changes caused by climate change,
including inundated nesting areas and
altered food supplies.
Photo by Gerrit Vyn

The U.S. has warmed by an average of 1 degree Fahrenheit during the last century, primarily because of greenhouse gas emissions. Our nation is also 5–10% wetter on average now than historically, though most of this can be attributed to severe weather events, which can damage habitats without alleviating drought. Most estimates suggest that without action, the U.S. will warm by another 5–9 degrees over the next century and the sea level will rise by more than 1.5 feet.

Climate change already has influenced the abundance, distribution, and timing of migration and breeding for many bird species. A recent study by the National Audubon Society showed that more than half of the birds commonly found on the Christmas Bird Count are wintering farther north now than 40 years ago. American Robins are now arriving approximately 14 days earlier than they did in 1981 on their breeding grounds in the Colorado Rocky Mountains. Tree Swallows have advanced their breeding date by up to nine days earlier from 1959 to 1994. Red-winged Blackbirds, Eastern Bluebirds, and eastern populations of Song Sparrows now lay their eggs earlier because spring temperatures are warmer. A great concern is that the earlier arrival of migrating birds may be out of sync with food availability.

In addition to these effects on migration and breeding, birds are at grave risk from habitat changes caused by climate change, especially in arctic tundra, alpine meadows, sea ice and glaciers, coastal wetlands, marine atolls, and ocean ecosystems. Many specialized birds live in these habitats, including Ivory Gulls that scavenge polar bear kills on floating sea ice, rosy-finches that depend on high altitude meadows, rails and saltmarsh sparrows that depend on brackish coastal areas, and Kittlitz’s Murrelets that appear to depend on glaciers. These species may face severe conservation challenges in the coming decades. Sea level rise will inundate islands, jeopardizing nesting birds. The potential spread of mosquito-borne avian malaria to highland refugia for Hawaiian honeycreepers is also a serious concern.

Climate change can affect the survival and reproduction of many bird species. Changes in prey distribution and abundance, reduced productivity, shrinking habitats, and competition and stresses from increasing populations will present a great challenge to birds on land and at sea.

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