Washington Post - Ben Guarino
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| Elecia Crumpton/University of Florida |
In 1962, Rachel Carson warned that pesticides, particularly DDT,
would lead to springs without birdsong, as she wrote in her book “
Silent Spring.” Carson's forecast kick-started an
environmental movement and was instrumental in the Environmental Protection Agency's decision to
ban the pesticides 10 years later, so her descriptions of deathly quiet did not come to pass.
But
the danger of a silent spring, according to ecologists who study birds,
did not evaporate with DDT. The looming threat is not chemical but
a changing climate, in which spring begins increasingly earlier — or in
rare cases, later — each year.
“The rate at which birds are falling out of sync with their environment is almost certainly unsustainable,” ecologist
Stephen J. Mayor told
The Washington Post. Mayor, a postdoctoral researcher at University of
Florida’s Florida Museum of Natural History, echoed Carson: “We can end
up with these increasingly quiet springs.”
Certain migratory songbirds can't keep pace with the shifting start of spring, Mayor and his colleagues wrote in a
Scientific Reports study
published Monday. Previous research noted that, in specific areas, some
species can adjust to an earlier spring start, such as wood thrushes
that breed sooner after arriving at Pennsylvania's
Laurel Highlands.
But the new study was the first to survey songbirds across the entire
North American continent. For 48 songbird species, the mismatch between
arrival date and the onset of spring grew by an average of half a day
per year between 2001 and 2012.
Of the species studied, nine
fared the worst, with a yawning gap between their arrival date and the
spring shift: blue-winged warblers, eastern wood-pewees, great crested
flycatchers, indigo buntings, northern parulas, rose-breasted grosbeaks,
scarlet tanagers, Townsend's warblers and yellow-billed cuckoos. In the
case of the cuckoos, for instance, spring greenery started growing
1.2 days earlier per year, although the birds arrived on average 0.2
days early. Put another way, the timing mismatch increased by an average
of one day annually.
The
report combined satellite data with bird sightings all over North
America, splitting the continent into 120-by-120 mile sections. “The
novel thing about this paper is the scale at which they are showing the
effect,” said
Wesley M. Hochachka, an ecologist at the Cornell Lab of Ornithology in New York who was not involved with this report.
Using satellite imagery, the study authors tracked the start
of green-up, the sudden burst of photosynthetic activity that begins in
early spring in North America. As seen from the sky, green-up is an
explosion of leaves. This brings out droves of hungry caterpillars and
other plant-eating insects. These bugs are a crucial food supply for
songbirds, which travel northward to eat and breed after spending the
winter in South or Central America.
This invertebrate buffet
lasts for a limited time. In oak forests, for instance, insects find
the young leaves quite tasty. But as the foliage ages, the oak trees
deposit bitter tannin compounds in their leaves, making the plant matter
difficult to digest or downright inedible. If birds' timing is off,
they may arrive to find their habitats impoverished of food.
Songbirds
leave Central or South America timed according to changes in daylight.
Departure dates vary yearly, but not wildly. Meanwhile in the north,
Mayor said, “time for green-up is shifting with climate change and
becoming more unpredictable.” In the eastern United States, spring
green-up started earlier and earlier.
In Townsend's warbler
habitat and some other western regions, green-up was delayed later each
year during the study period. The reasons for the lag are not yet fully
understood, the scientists said, although Hochachka theorized that a
lack of rainfall could play a role.
The study authors also
tracked when birds arrived in the north using data from Cornell
University's eBird project, a compendium of
400 million sightings submitted
by birdwatchers since the early 2000s. (The citizen-science
eBird program fuels North American bird research, Hochachka said, in a
way no other continent can match.)
There
was some good news from these sightings, Mayor said. “At least 80
percent of the species don't seem to be dramatically affected yet,” he
said. Some songbird species may make up for lost springtime by flying
north faster.
Mayor emphasized that the researchers selected the
48 songbird species to study because they were commonly spotted. It's
harder to get reliable data on rare species that are threatened with
extinction.
The fact that scientists found nine pronounced
mismatches in a relatively short timespan was notable, Hochachka said.
Most studies of this type focus on smaller regions but use time scales
longer than a single decade. “They have identified the really blatant
cases where species' arrivals are diverging,” he said.
It was too
early for Mayor to speculate what characteristics separated the
most-mismatched nine from the other 48 species, he said. But the
ecologist said he expected these bird populations to decrease because
of their poor timing. He was also worried that a lack of songbirds would
go beyond silent habitats.
“If birds aren’t arriving when
insects emerge in the spring, we could see things like insect outbreaks
or defoliation,” he said. “There are many potential impacts that we
don’t have a good handle on yet.”
And unlike the case of “Silent
Spring,” any given EPA ban cannot curb this trend. Mayor recommended
that bird fans continue to contribute data to eBird, given its
scientific value. “Getting outside and observing these birds is important,” he said.
Likewise, Hochachka said, it would be difficult to directly
ameliorate the impact of this mismatch. But it is still possible to
make birds' lives easier in other ways, he said, such as planting native
or bird-friendly
plants in our back yards.
“There
are things I think we can do in the northeast states to compensate
somewhere else in the life cycle,” Hochachka said. As birds return to
South and Central America along the East Coast, for instance, city
lights may cause them to lose their way. Or worse,
crash into windows. Reducing light pollution or installing non-reflective glass could help make the long trip a little easier.
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