Rising global temperatures are making it harder for birds to tell when it’s spring and breeding time, researchers have found. Research led by researchers at UCLA and Michigan State University found that birds produce fewer young if they start breeding too early or too late in the season. Birds have not been able to keep up with climate change, which experts say has resulted in early spring conditions.
The authors go on to write that the mismatch between the start of spring and birds’ readiness to breed is likely to worsen as the world warms, which could have far-reaching effects on many bird species. The breeding season for birds begins with the appearance of the first green plants and flowers, which appear earlier and earlier with warming.
“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 study first author Casey Youngflesh, who led the research as a postdoctoral researcher at UCLA and is now a postdoctoral fellow at State Michigan. “Our results suggest that breeding productivity can be reduced by about 12 percent for the average songbird species.” The authors emphasize that conservation strategies should address the responses of bird species to climate-induced changes.
“For nearly 30 years, scientists have hypothesized that animals could become out of sync with plants because spring started earlier,” said Morgan Tingley, associate professor of ecology and evolutionary biology at UCLA and lead author of the study. “Although there are some very good case studies of this phenomenon, it remains a big mystery whether advancing springs will be a general problem for most species.”
Timing is all about birds when it comes to raising their young. If they breed too early or too late, harsh weather could damage their eggs or newborns. But timing relative to food sources also matters: If birds forage before or after it’s naturally available, they may not have the resources to keep their young alive.
“We definitely found evidence of both absolute and relative bird timing effects on bird reproduction,” Tingley said.
Using data from a large collaborative bird banding program run by the Institute for Bird Populations, researchers calculated the timing of breeding and the number of young produced for 41 migratory and resident bird species at 179 sites near forested areas across North America between 2001 and 2018.
Then the authors used satellite imaging to determine when vegetation appeared around each site. They found that each species had an optimal time to reproduce, and that the number of young produced decreased when spring came very early, or when reproduction occurred early or late compared to plant hatching.
While most birds were adversely affected by the changes in early spring, several species—among them the northern cardinal, Bewick’s wren, and white-throated warbler—bucked the trend and showed increased breeding productivity when spring began earlier. These species are mostly non-migratory species that can respond more quickly to the emergence of spring plants that signal the beginning of the breeding season.
By breeding earlier and without the temporal constraints imposed by migration, the study noted that non-migratory species may also be able to reproduce more than once per season.
But these species were the exception to the rule. Even most non-migratory species could not keep up with the earlier spring arrivals. Overall, for every four days earlier that leaves appeared on the trees, the species reproduced only one day earlier.
For migratory species, this discrepancy means that the time between arrival at their breeding grounds and actual breeding is likely to be shortened because spring conditions begin earlier. Birds need time to establish territories and physiologically prepare to lay eggs and raise young, so the change could cause even greater reproductive disruption.
“North America has lost nearly a third of its bird populations since the 1970s,” Tingley said. “While our study shows that the worst impacts of the timing mismatch are unlikely to occur for several decades, we need to focus now on concrete strategies to boost bird populations before climate change takes its toll.”
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