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.