Bird evolution may not keep up with climate change

Short-term changes in birds’ body shapes may not indicate an ability to evolve with the fast pace of modern climate change.

Researchers recently compared fossils of relatively small birds from the last 37,000 years with the bones of modern species. They found that the average sizes and shapes of some of the birds’ bones don’t change with time through ice ages and the warmer intermediary periods.

“What it’s telling us is that we have to be careful about thinking that small scale changes mean changes in speciation,” said Donald Prothero, a paleontologist at the Natural History Museum of Los Angeles County and one of the co-authors of a study published recently in Integrative Zoology. He worries species of small birds may not be able to evolve fast enough to keep up with the pace of climate change. Current changes are taking place about 10 times faster than during the glacial period, he said.

In Los Angeles’s La Brea Tar Pits, natural asphalt seeps from the ground. The tar pits have trapped animals and preserve their bones for tens of thousands of years, providing an ongoing record of fossils and life in the area dating back at least before the last ice age.

Katherine Long, one of the co-authors of the recent study, measures bird bones at La Brea Tar Pits in Los Angeles. Credit: Donald Prothero

A number of studies in recent years have shown that the fast pace of ongoing climate change has resulted in morphological changes in bird species like Galapagos finches, or that hurricanes increased due to climate change have resulted in adaptations to the size of lizard toes and leg morphology in survivors, even several generations afterward.

But paleontologists have often countered this with older, long-term results that indicate morphological changes as a species don’t happen that quickly.

“Unfortunately, there’s a battle between biologists and paleontologists,” Prothero said.

Previous studies on all the common large birds like golden eagles (Aquila chrysaetos) and bald eagles (Haliaeetus leucocephalus), condors (Gymnogyps californianus) and others at La Brea had shown little change over this period. But Prothero and his team wanted to use the long fossil record at the site to see whether they could see similar changes in morphology and body size in small birds during long-term climate change events in the past such as past ice ages.

The oldest fossils of the three species they looked at — western meadowlarks (Sturnella neglecta), the yellow‐billed magpie (Pica nuttalli) and the common raven (Corvus corax) — dated back to about 37,000 years ago. The newest fossil dated back about 9,000 years. But the team also looked at relatively recent specimens in museum collections.

The team didn’t determine significant changes in the specimens and fossils over time, despite huge climactic changes during the glacial-interglacial period that occurred during this time.

Fossil leg bones from meadowlarks, ravens and magpies taken from the La Brea Tar Pits. Credit: Donald Prothero

“The long-term time scale shows that most animals and fossils are not responding to climate change in the short term,” Prothero said. “They just wobble around a certain norm, then come back to where they started.”

Colin Donihue, a postdoctoral research fellow in biology at Washington University, was not involved in Prothero’s study but was involved in a recent study showing lizards on islands recently hit with major hurricanes have shown adaptations for several generations afterwards. Donihue said that numerous studies have shown phenotypic changes in populations in response to extreme climate events.

“In some instances, those changes are just fluctuations around some average — for instance over the last several decades, Rosemary and Peter Grant have shown that evolutionary changes in Galapagos finch traits seem to ultimately cancel each other out,” Donihue said, adding that connecting localized changes with macro-evolutionary changes is a challenge for researchers.

“This study takes an even longer perspective over a few millennia and finds substantial variation in morphology at several time points during that period, but no indication of consistent directional change, despite concurrent changes in the environment,” Donihue said. “More studies of populations and species on this millennial-scale will be exciting contributions to bridging the gap in our knowledge between rapid short-term evolutionary dynamics and long-term macro-evolutionary changes in species and clades.”

Header Image: Western meadowlarks haven’t changed much over 37,000 years, despite experiencing an ice age during that period. Credit: Becky Matsubara