Birds miss spring greening because of climate change
The early bird gets the worm. But what happens when the worm is even earlier than the bird?
In that case, some familiar backyard bird species don’t get enough worms to feed their chicks.
That's the conclusion of a recent study of changes in spring “green-up” dates across North America and the arrival dates of spring migratory bird species in those areas.
Biologist Stephen Mayor, at the Florida Museum of Natural History of the University of Florida, and nine colleagues from other institutions reported their findings online this month in the journal Scientific Advances.
The research team used satellite images to detect the date that vegetation in sample areas began to green up at the start of the growing season.
They also used first sightings of the year of common migratory birds at specific locations as reported by eBird, a citizen-science website. The dates allowed the biologists to calculate when each species arrived at its breeding grounds.
The team compared plant green-up and bird arrival data from 2001 to 2012 across North America. Green-up date and arrival time of 48 bird species studied usually moved in the same direction. In areas where plants began to leaf out earlier, migratory birds also arrived earlier.
But the plants came to life even earlier than the arrival of most birds species.
The research team found a growing gap between the budding of plants in an area and arrival date of birds that depend on insects that eat those plants. That gap grew by about half a day per year across all species studied.
Many North American birds overwinter in Central and South America. They begin their migration to their breeding grounds in response to seasonal shifts in day length, a signal unaffected by climate change. Plants and the insects that eat them, however, respond to increased warming and decreased snow cover.
These are local, environmental signals affected by climate change.
The researchers identified eight Eastern migratory birds with extreme gaps between green-up and subsequent insect flush and the birds' arrival. By the time the birds made it to their breeding grounds, built nests and had chicks to feed, the peak availability of plant-eating insect larvae had passed.
These species include some of our most iconic spring migrants in the eastern United States, including great crested flycatchers, indigo buntings, scarlet tanagers, rose-breasted grosbeaks, eastern woodpewees, yellow-billed cuckoos, northern parulas and blue-winged warblers.
I have spotted each of those species in my yard or on campus in the past three weeks. Indeed, a great crested flycatcher was calling in my backyard as I wrote this column.
The physical science of climate change is clear. If we continue to mine and burn fossil fuels, we will increase greenhouse gases in our atmosphere. Mean global temperatures will rise, as we are currently observing.
The biology of climate change is an order of magnitude more complex. Like some migratory birds, maybe we’ll arrive at a climate change solution too late for them to survive.