Every spring, migratory birds arrive in the continental United States from south and central America to breed. But precisely when they arrive each spring varies from year to year. In a NASA-led study published in the Bulletin of the American Meteorological Society, scientists have linked this variability to large-scale climate patterns originating thousands of miles away.
Migratory birds benefit ecosystems by helping to control pests, pollinating plants and serving as food for other wildlife. The more land managers know about the current migration patterns of these birds – and the migration patterns that are likely to develop in the future due to climate change – the better they can direct their efforts to protect the birds and to restore and conserve their habitats. This study brings them one step closer to that goal.
Every spring, migratory birds arrive in the continental United States from south and central America to breed. But precisely when they arrive each spring varies from year to year. To better understand what is driving variability, scientists decided to turn to climate models, radar stations and a little bit of math. By studying the link between climate patterns and bird migration, scientists can better prepare to protect these birds and the habitats on which they rely. Credits: NASA's Goddard Space Flight Center
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The scientists analyzed 23 years of bird migration data collected via NOAA’s Next Generation
Radar system – a network of 143 radar stations across
the continental U.S. – to determine the variability in the birds’ arrival times
each spring. This is where they made their first discovery: The U.S. could be
divided into two regions, east and west, each with a distinct pattern of variability
in bird arrival times.
The east region includes all areas east of 102 degrees west longitude – a
line that, in the U.S., bisects North Dakota and extends down through Texas.
The west region includes all areas west of that line.
Bird migration in the U.S. is monitored in the context of four “flyways” or
principle migration routes, two in the eastern U.S. and two in the western U.S.
The new research digs into the different influences on each half of the country
that affect the timing of environmental cues, such as temperature and weather
patterns, that prompt birds to travel along their flyway.
“Our approach doesn’t replace the ‘flyways’ but rather
introduces a different geographic framework that reflects the interannual
variability of bird migration at the continental scale,” said Amin Dezfuli,
scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and
lead author of the study. “This framework helps us to better understand how
climate patterns influence broadscale bird movements and migration
variability.”
In order to identify what was driving the variability in bird migration in
each of these newly-defined regions, the science team analyzed meteorological
and climate model data for both. They found that variability in the west region
was strongly linked to regional air and sea surface temperature in the adjacent
(Pacific) Ocean. For instance, above-average temperatures in the region in 2005
resulted in the birds arriving earlier than average.
They found that variability in the east
region, however, was more strongly linked to large-scale atmospheric
disturbances called Rossby waves. Rossby waves form due to the Earth’s rotation and geography. They help
transfer warm air from the tropics poleward and cold polar air to the lower
latitudes. Flowing east to west, they can be thousands of miles long and impact
weather and climate patterns.
“Using the climate data, we were able to attribute the bird migration
patterns, especially in the eastern region of the U.S., to Rossby waves,” said
Dezfuli. “Rossby waves can be triggered in the tropical Pacific, thousands of
kilometers away, and propagate all the way over to the U.S., providing the
climatic conditions we associate with these bird migration patterns.”
Because Rossby waves can be as long horizontally as the country itself, a
peak in the west can bring warm temperatures to that region while a
low-pressure trough in the east could bring cooler temperatures and storms to
the eastern region at the same time. This ultimately impacts bird migration in
both regions.
“Knowing the timing of migration is fundamental to our overall
understanding of migration systems,” said Kyle Horton, co-author and biologist
at Colorado State University. “This study adds a new and important dimension to
this understanding, highlighting how interconnected migration systems are with
atmospheric circulation – near and far.”
Dezfuli is now looking toward how these results may be applied in the
future.
“Now that we have established regional-specific associations between
climate variability and bird migration patterns, we could next investigate the
potential changes in migratory patterns under future climate scenarios,”
Dezfuli said.
The more scientists and other stakeholders know about climate change’s
effects on bird migration, the better they can prepare to protect these birds
and the habitats on which they rely.
Banner Image: More than 350 species of birds are counted on the Pacific
Flyway every year. Credits: courtesy of Davis Ranches/John Brennan
By Esprit Smith NASA's Earth Science News Team
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