Baker: Migrating birds facing more hurdles

“Every day during migration, they’re on this trade-off between starving to death and being able to continue forward… When they’re not flying, they’re mostly voraciously eating.” – UCLA Ornithologist, Morgan Tingley

Some 700 species of birds nest in North America, and of these more than 400 are migratory. For these species, migrating at the right time is a life and death situation. It takes a lot of energy to power their bodies over distances that may extend thousands of miles from overwintering grounds in Central and South America to their North American breeding sites.

So, the migrants will need to find plenty of food along the way, and even more once they reach their breeding area and have chicks requiring nearly constant feeding. But migration is risky.

Besides the very real chance of dying from sheer exhaustion, many migrants will be killed by encounters with severe weather, predators, and crashes into human-made structures.

Why the birds migrate north

A reasonable question, then, is why migrate at all? Why leave the rich foraging grounds of their tropical and neotropical overwintering areas to hazard the trip north? Ecologists and evolutionary biologists have explored a number of possible explanations for long-distance migration, and the story is complex.

But for me, one of the most surprising factors turns out to be that a spruce forest in Quebec or an oak-hickory woodlands in southeastern Ohio provides a much richer and more reliable source of high-caliber foods in late April and May than a tropical forest in Puerto Rico or Columbia.

With the arrival of warm weather after a long, cold winter, northern hemisphere plants kick into gear in a big way. It’s called the annual “spring green-up,” and it translates into a bonanza of succulent growth for untold numbers of insects, the ideal fare for most of our songbirds.

The birds come for the insects available

But this super-abundance of energy-rich insects (even most birds that eat seeds as adults feed insects to their chicks) dies down rapidly as the season advances, and a migrant that misses the peak of spring green-up will pay a price by producing fewer and less healthy offspring.

And increasingly, many species − especially those that migrate from tropical overwintering sites − are missing that peak. Over the past several decades, climate change has led to warmer weather arriving a little earlier, on average, each year.

Unsurprisingly, peak insect hatches are following this shift in spring greening. A 2022 study of the 25 years from 1989 to 2014 in upstate New York found various insect species to be emerging three to 12 days earlier and booming for a shorter span of time.

Insects are peaking earlier each year

A further study by Oklahoma State University’s Scott Loss showed this pattern to be happening across the nation. His team found that spring green-up (and associated peak insect abundance) was occurring earlier within the migration paths of 150 bird species across North America … and he noted, “Most of these species were more in sync with past long-term averages of green-up than current green-up.”

It makes a difference whether you’re a short-distance migrant like an American robin or Eastern phoebe that overwinters in southern U.S. and Mexico, or a long-distance migrant overwintering closer to the equator like many of our warbler species.

Birds use a variety of environmental cues to tell them when to initiate migration. Among the most important are circadian (day-to-day) changes in temperature and the of hours daylight. But the closer a bird’s overwintering grounds are to the equator, the less seasonal change it will see in either of these cues.

So long-distance migrants tend to rely on internal, genetically hard-wired processes that keep track of the passage of time over the course of a year. It’s these species that are increasingly falling out of sync with changes in peak periods of green-up far to the north.

But even birds that have been able to adjust to earlier green-up are finding that doing so also has its costs. Some species start migrating about the same time as they traditionally have, but are speeding up their rate of travel, with less time for resting and refueling along the way.

Then too, one of the hallmarks of climate change is the unpredictability of extreme weather events. When a period of late-winter warm weather is interrupted by a sudden drop back into freezing temperatures, the delicate leaf and flower-producing buds of plants suffer frost damage, and insects die.

These “false springs” not only reduce available resources for the spring migrants, but impact the availability of high-quality food throughout the rest of the year.

And yet, each spring and fall, hundreds of millions of birds undertake what ornithologist, Scott Weidensaul has called, “the largest, greatest natural spectacle on the planet.” I recommend Googling “Audubon Bird Migration Explorer” for a link to an interactive site that shows the migration routes of 450 North American bird species and discusses challenges they face along the way.

Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.

This article originally appeared on Fremont News-Messenger: Baker: Climate change making bird migrations more challenging