When I arrived in northern California in 1972, I discovered that there were no Northern Mockingbirds but they are very common now. Mockingbirds expand and contract their ranges with the climate, preferring warmer ones, and they have been increasing in numbers in California since the 1940s. The House Finch, common over the U.S., has, over the past 40 years, expanded its range northward over 270 miles. The Purple Finch, found in the far west and east of the Mississippi, has expanded its range northward over 430 miles. Migratory robins in Colorado are arriving two weeks earlier than they did 20 years ago.
Audubon analyses of bird populations over the past 40 years indicate that birds of every type have moved an average of 35 miles northward during spring migration. . Similar changes have been noticed in Europe by the Royal Society for the Protection of Birds . Recent scientific studies of birds and their response to climate change are available at the Partners in Flight website . You can read an excellent overview by the American Bird Conservancy here. The International Union for the Conservation of Nature indicates that one out of eight bird species in the world is in danger of extinction, and that climate change is a major cause.
Birds migrate in response to photoperiod (daylength), not temperature, but their food supply emerges with warming temperatures, so global warming is skewing the normal rhythms that birds evolved with. Instead of arriving on their breeding grounds at the peak of insect, plant, flower, and seed abundance, birds are arriving later, at a time of less food. But birds, like other organisms, are subject to natural selection, part of the evolutionary process, so the birds that arrive the earliest or go farthest north, are now at an advantage and thus reproduce more successfully than those birds that arrive later or stay farther south. Over the years, the more successful birds become more abundant, and this is what we are beginning to observe. Birds are arriving earlier and/or migrating farther north and this response is correlated with the increasing temperatures of the earth.
Birds face other problems due to climate change. Habitats are drying up due to drought, snow fields are melting and allowing predators to more easily approach nests, trees are being destroyed by insect outbreaks as the insects are not killed in the warmer winter, more and more severe wildfires are occurring, weather patterns are changing, islands and wetlands are being inundated by rising sea levels, and sea water chemistry is changing, affecting the food webs of seabirds.
If we simply look at the data provided by changes in bird populations over the world, we have to conclude that something is happening, even if we didn’t have climate data. And the only logical conclusion for these changes in bird movements is a warming environment. Like the canary in the mine, birds are telling us something.
And in spite of a declining number of climate change deniers, the world is warming due to human activities, so say 99.7% of the world’s climate scientists.