By now, it's well understood that a warming planet is slowly advancing the arrival of spring. Flowers, like the famous cherry blossoms, are experiencing record-early blooms. Bees and other pollinators are missing early blooms. This trend is alarming many ecologists and climate scientists as heat records are shattered and ocean temperatures soar.
Now, a new study published in Proceedings of the National Academy of Sciences takes a look at its impact on another beloved spring event — hatching songbird chicks — and what the potential consequences are for the future of birds.
The study, led by scientists at the University of California, Los Angeles, and Michigan State University, found that birds are producing fewer chicks when they start breeding too early or too late in the spring season. This type of research is known as phenology, or the study of periodic events in biological life cycles. As climate change results in earlier springlike weather, birds have been unable to adapt their reproductive readiness.
"Understanding the links between phenology and demographic processes is critical to predicting the future response of species to ongoing climatic change," the study authors write. "For North American birds, many of which have undergone large-scale phenological shifts over the last several decades, this is a topic of particular concern."
As the start of spring begins earlier and earlier, the researchers anticipate that this trend will only worsen, generating a large-scale impact on many bird populations that could even lead to extinction.
"By the end of the 21st century, spring is likely to arrive about 25 days earlier, with birds breeding only about 6.75 days earlier," said the study's first author, Casey Youngflesh, who led the research as a postdoctoral researcher at UCLA and is now a postdoctoral fellow at Michigan State University, in a statement. "Our results suggest that breeding productivity may decrease about 12% for the average songbird species."
These findings are surprising because they contrast "what has been observed at the individual level, where the earliest breeding individuals in a given year for a given population tend to have higher breeding productivity," the study authors report. That makes some sense — longer warmer seasons can mean more time for more clutches, a group of eggs fertilized at the same time. But this benefit weakened significantly at the population level. Nonetheless, some species did come out on top, including northern cardinal (Cardinalis cardinalis), Bewick's wren (Thryomanes bewickii), and wrentit (Chamaea fasciata.)
Biologists have been trying to better understand the potential consequences of early spring on birds for a while. In an interview with Salon, Youngflesh emphasized that understanding the timing for songbird ecosystems is "very important," but it's not an easy task to do. For this study, researchers used data from a bird banding project called Monitoring Avian Productivity and Survivorship (MAPS) where people use mist nets to capture songbirds and place a lightweight, numbered aluminum leg band on their leg. The birds are later released, unharmed, but the band allows scientists to collect data that can be used to estimate key demographic parameters, including breeding patterns.
To compare this with spring's arrival, the researchers used satellite imagery to literally measure when greenery is appearing. Plants are imperfect way of measuring this relationship but they do mean caterpillars, which are one of the primary food sources birds share with their young. As climate change worsens, pollinators are also affected, meaning less insect food for birds.
For every four days that vegetation appears earlier in the spring, birds are only breeding earlier by about one day
Based on their continent-wide analysis of 41 bird species, the researchers concluded that birds can't keep up with the early arrival of spring and as a result, they're raising fewer chicks when spring arrives early. Morgan Tingley, a UCLA associate professor of ecology and evolutionary biology and the study's senior author, said the results of their work — which took six years to complete — importantly demonstrates that there is an optimal time for birds to reproduce.
"There actually is this period when birds are aligned in their timing of reproduction and with springtime, as determined by plants, that does lead to sort of the maximum reproductive success," Tingley told Salon. "When you are asynchronous it leads to a mismatch, and so that if birds are too early, or if they are too late, then their reproductive success goes down."
Tingley said that the general, long-term trend is that for every four days that vegetation appears earlier in the spring, birds are only breeding earlier by about one day.
"Over time, this can rack up to falling further and further behind," Tingley said. "And so the final result we had was that because of this trend — and because we know that spring is going to keep on getting earlier on average over the course of this century, given ongoing trends of climate change — we estimate that by the end of the century this could lead to a decline in bird productivity somewhere around 5 to 12 percent."
The higher end could be "catastrophic," Tingley said.
Indeed, in 2019, a study published in Science estimated that North America has seen a net loss of 2.9 billion birds since 1970. At the time, the National Audobon Society declared the findings indicative of "a full-blown crisis that requires political leadership as well as mass individual action." Taking this into account, coupled with the findings of the impact of early spring on breeding, scientists say there needs to be an immediate shift to doubling down on conservation efforts.
"They are kind of adapting, but they're not doing it quickly enough, and that's a concern."
"It's hard to predict for any given species what might happen," Tingley said. "But the level of declines that we're seeing are declines that could, if species has no other way to deal with it, could certainly lead to extinction."
Tingley said as individuals we can't stop climate change, but that there are small actions individuals can take to protect birds — like keeping outdoor cats inside.
But aren't birds adaptable? They survived the dinosaur apocalypse, after all. Youngflesh said yes, they are adapting — it's just not happening fast enough in the face of accelerating climate change. He added it's especially tricky for migratory birds because they don't have great information on what's happening where they'll arrive come warmer weather.
"Birds only breed about a third of a day earlier, so they basically need to be responding about three to four times faster, to actually be keeping pace," Youngflesh said. "So yes, they are kind of adapting, but they're not doing it quickly enough, and that's a concern."
CORRECTION: Due to a transcription error, a previous version of this article read "keeping bottle caps inside" could help protect birds. Tingley said "outdoor cats."
