A comparison of nesting data recorded in the early 1900s with similar data today for more than 200 species of California birds shows that over all they are breeding(繁殖)5 to 12 days earlier than they did 75 to 100 years ago.
Earlier studies found that many but not all birds in California's mountains are moving north or to higher altitude to find cooler temperatures in the face of global warming.
The shift to earlier breeding that we detected allows birds to nest at similar temperatures as they did a century ago, and helps explain why half the bird species in the mountainous areas of California did not need to shift upward in altitude in response to climate warming over the past century, said Steven Beissinger, a UC Berkeley professor of environmental sciences.
Early spring arrivals have long been noted by the public and reported by scientists, but the assumption has been that the birds are tracking resources, primarily food: with warming temperatures, plants produce leaves and seeds earlier, and insects appear earlier.
The new study spotlights another major reason: by nesting a week earlier, birds produce eggs and young at a temperature about 1 degree Celsius lower than if they nested at the normal time in the same place. This exactlycounterbalancesthe approximately 1 degree Celsius increase in global temperatures over the past century.
"By nesting a week or 10 days earlier, birds are avoiding some of the negative effects of climate warming,"Beissinger said." the good news is that there may be more flexibility for species to respond to climate change than we thought, and not all species may need to move farther north or to higher altitudes, "he added. "But we don't know yet whether staying in place and shifting schedules earlier is a permanent solution, or only provides temporary relief from the 2 degree Celsius rise in temperatures forecast to occur."
Birds may find, for example, that the window of good temperatures for breeding becomes shorter, which may limit the opportunity to re-nest if they fail the first time. Larger species that have a longer nesting period might not have an enough time to complete their nests before it starts to become too warm, he said.
