On a recent, bright and early morning at Lake Murray, a dozen birding enthusiasts gazed through binoculars, seeking a glimpse of the feathered species singing in the brush- and tree-covered landscape.

“I have a California thrasher in my scope,” called out the group’s leader, Gabriel Mapel, offering others a peek in his lens.

“A California specialty — awesome bird,” Mapel exclaimed.

Some of the delicate winged vocalists the group spotted can be found in San Diego County year-round, but many are visitors, just like Mapel and his fellow bird lovers who traveled from across the country to see them.

San Diego is a stopover point along the the Pacific Flyway migration path that stretches from South America to Alaska. It’s a crisscross this time of year, with winter flocks returning north and spring birds just arriving. More than 520 species have been documented in the county — the most in the nation, but that number is expected to decline in coming years as climate change takes hold.

“I would say that every species in San Diego County could be threatened by climate change, said Phil Unitt, curator of birds and mammals at the San Diego Natural History Museum.

“Each species is going to be its own really complex story,” Unitt explained, as he walked along a path in the San Diego National Wildlife Refuge, using his special chirping call to attract nearby birds.

Unitt said annual migration patterns are being disrupted by rising temperatures, causing some species to seek new habitats that are wetter and cooler.

“Species like the house wren and the Cassin's kingbird that aren’t migrating south in the numbers that they did previously,” Unitt said.

The National Audubon Society estimates about half of North America's bird species will be forced to find a new habitat or face extinction within approximately half a century.

Unitt said many bird species take their migration cues from extended daylight hours.

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“That stimulates the hormones that, ‘Okay, now it’s time to migrate north, or northeast,’” he said.

But warming temperatures are causing spring to arrive increasingly earlier, with plants and trees budding sooner. The fresh new greenery and flowers attract volumes of insects — the primary food source for birds.

Unitt said birds that don’t adapt to the changing season by migrating earlier will miss the peak food abundance when they arrive at their breeding grounds.

“If the insects just aren’t available in the numbers that they were, that’s going to be a strong signal to skip nesting,” Unitt said.

He said breeding season is also expected to be disrupted during droughts, another growing impact of climate change.

“Birds reproduce much less, they don’t even attempt to nest during a severe drought," Unitt said.

He worries about drought outlasting a bird’s lifespan and having a profound effect on the population, such as the five-year dry spell San Diego recently experienced.

“When we’re at one extreme that extends through the entire life cycle of five, six, seven, eight years of some small bird, and I become really concerned.”

Unitt said bird populations are also threatened from increased heat waves, invasive insects decimating forest habitats and wildfires.

San Diego’s major blazes have drastically upended sensitive ecosystems, he said, including the Cuyamaca forest following the 280,000-acre Cedar fire in 2003.

“The mountain chickadee, pygmy nuthatch — some birds that were the most abundant in the forest are now rare,” he said.

The small songbirds used to thrive in the coniferous canopy of Jeffrey pine trees, probing into clusters of pine needles for insects.

Several other species in San Diego County are also facing grim prospects due to the changing climate, he said, including the willow flycatcher, tricolored blackbird and the loggerhead shrike — a little gray and white bird with a black mask that was fairly common 25 years ago.

“Very few left on the coastal slope now,” he said. “There’s a few in the desert.”

Unitt emphasized that birds have adapted to evolving habitats for millions of years, but current climate changes are happening at a pace and scale never seen before.

Birds are looked upon as messengers when it comes to climate change. They’re highly reactive to changes in the environment, and the most studied species on the planet. Predicting which species will adapt to a changing habitat is difficult, said Unitt. Some will learn to live and feed in urban environments.

What is clear is many bird species are increasingly losing their habitats, and the effects of climate change are just beginning, said Dan Cayan, a research meteorologist with Scripps Institution of Oceanography.

“Things in the future are going to get successively more altered as the decades go forward," Cayan said. “We’re looking at a future that really is decidedly warmer than what we’ve seen historically."

Cayan studies natural variability and climate change, including heat waves, floods, droughts and sea level rise.

Cayan said average daytime temperatures from March through May in San Diego County have increased nearly 2 degrees over the past five decades, and nighttime temperatures have soared approximately 3 degrees. He said the warming trend is expected to continue.

“In the middle part of the 21st Century we’re thinking that temperatures will be probably 3 degrees-plus warmer than the averages today,” he said.

“This is a phenomena that will obviously impact ecosystems and along with that, bird populations,” Cayan said.

He said the future will also be drier, with a decline in fall and spring precipitation.

“Looking at climate model simulations we are seeing more years that are overall dry, and occasionally we’ll get a really wet year,” he said.

Cayan said the earth is just beginning to show signs of greenhouse gas emissions already in the atmosphere.

“We have this commitment to climate change because of what we’ve already done, and the earth is still catching up to that,” Cayan said. “It’s still adjusting.”

For now, spring is arriving about three days earlier than it did in previous decades, but flowers are expected to bloom three weeks sooner by the end of the century, according to data from USGS.

Birds will likely be among the first to notice the change.

On a recent, bright and early morning at Lake Murray, a dozen birding enthusiasts gazed through binoculars, seeking a glimpse of the feathered species singing in the brush- and tree-covered landscape.

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