Dr. Tingley and his colleagues discovered that most species now nest about a week earlier than they did 70 to 100 years ago.
.. Ecologists generally believe that birds adapt to rising temperatures by moving to higher elevations or heading north. They shift their nesting time for a different reason: to sync with food availability, like an early appearance of plump caterpillars or swarms of insects.
.. But in 2012, researchers found that about half of the bird species in certain regions of the Sierra essentially stayed put over the past century, not significantly extending their ranges to cooler elevations even though the climate was warming.
The new study offers a plausible explanation. If the birds lay their eggs earlier, they can stay in their centuries-old range, with no need to migrate to higher altitudes.