Increasing gaps between the emergence of spring and the internal clock birds rely on to migrate to northern breeding grounds is preventing several species from arriving on time to successfully mate and produce a healthy generation of new fledglings, according to a study published Monday in Scientific Report.
To maximize fitness, birds must time their arrival and breeding activities to coincide with optimal habitat conditions and food availability. This means there is evolutionary incentive to correctly anticipate breeding-site conditions while birds are still at their winter grounds, often thousands of miles away.
As warming temperatures cause plants to emerge earlier or later than usual – throwing delicate biological cycles out of sync – birds are increasingly unable to anticipate when conditions will be optimal for their arrival, according to the study.
"It's like 'Silent Spring,' but with a more elusive culprit," Stephen Mayor, a postdoctoral researcher with the Florida Museum of Natural History at the University of Florida and first author of the study, said in a statement.
"We're seeing spring-like conditions well before birds arrive. The growing mismatch means fewer birds are likely to survive, reproduce and return the following year. These are birds people are used to seeing and hearing in their backyards. They're part of the American landscape, part of our psyche. To imagine a future where they're much less common would be a real loss."
Mayor and his multi-organizational team combined satellite data and data collected from citizen scientists to determine the rate of change between spring plant growth, a process called green-up, and the arrival of 48 songbirds across North America from 2001 to 2012.
They found this gap increased an average of half a day each year across all species, rounding out to roughly five days per decade.
While most species were able to adjust their arrival dates to accommodate changes in green-up, nine of the studied species had gaps at double or triple this rate, indicating they are unable to keep up with shifting conditions.
These species are the great crested flycatchers, indigo buntings, scarlet tanagers, rose-breasted grosbeaks, eastern wood-pewees, yellow-billed cuckoos, northern parulas, blue-winged warblers and Townsend's warblers.
Conducted primarily at the University of Newfoundland, the study is the first of its kind to analyze at the continental level the ability of birds to shift the timing of migration, breeding and egg-laying in response to climate change.
Though the study found most birds are adapting to these shifts, the key question is whether their adaptations will result in long-term success or failure.
"If anything could adapt to climate change, you'd think that birds that migrate thousands of miles could," Mayor said. "It's much easier for them to move in response to climate conditions than salamanders, for example, or trees. But because every species relates to another, one of our fears is that climate change can disrupt these relationships between organisms such that their critical life events are not timed optimally, putting them at risk."
Birds migrate north from the Southern Hemisphere based on physiological responses to levels of sunlight exposure, which is consistent from year to year.
But the state of their migration grounds is highly dependent on climate, meaning they could show up at the right cosmic time of year but at the wrong time for the habitat to suit their seasonal needs.
Too early, and they could freeze to death or not have enough food available for chicks; too late, and birds could have fewer nesting sites, fewer available mates, and a lack of resources for newly hatched fledglings.
The researchers found that green-up is occurring earlier than usual in eastern North America, causing birds to arrive later. In the West green-up is happening later, causing birds to arrive ahead of prime conditions.
Though birds have successfully adapted to changes in climate since the emergence of their early ancestors in the Late Cretaceous Period, the rate and extent to which climate is currently changing are far greater than in the past, making scientists wonder if birds will be able to keep up.
To that end, Mayor’s research team will examine why some species adjust better than others.
"The natural world is very complex," Mayor said. "When you kick it with a big change by altering the climate, different parts of that natural world respond in different ways. We're just beginning to understand the consequences of this grand unnatural experiment."
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