STASIS IN LATE QUATERNARY BIRDS FROM THE LA BREA TAR PITS DURING THE LAST GLACIAL-INTERGLACIAL CYCLE

The emphasis on gradual evolution in response to climatic change, as typified by the studies on Galapagos finches, suggests that birds should respond with changes in body size and shape to significant changes in climate. We examined the most commonly fossilized birds at Rancho La Brea to see if there were measurable changes during the last glacial-interglacial cycle. Measurements of the tarsometatarsi of the common birds, such as the bald eagle (Haliaeetus leucocephalus), the extinct California turkey (Meleagris californica) and the caracara (Caracara plancus prelutosus) showed complete stasis for the last 35,000 years, with no statistically significant size or robustness changes even during the peak glacial interval at 20,000 years ago, when the climate at Rancho la Brea was dominated by closed-cone coniferous forests and snowy winters. Even though birds such as the bald eagle and caracara show a strong Bergmann’s rule effect, with larger body sizes in colder climates, the bald eagles and caracaras at Rancho La Brea did not change in size during the coldest period of the glacial maximum. These results, along with previously documented examples of La Brea condors and golden eagles, suggest that birds are not as sensitive to long-term changes in climate as is traditionally thought.