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In the United States,

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a not-so-silent spring marks
the arrival of several billion

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birds migrating northward
from Central and South America.

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But curiously enough, the exact

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timing of this journey varies each year.

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So why is that?

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The key to this mystery may not lie in
looking at the traditional flyways,

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but instead in radar, climate models,
and a little bit of math.

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At the Global
Modeling and Assimilation Office,

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we routinely produce a comprehensive

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climate data set by combining
observations and models,

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and these are freely
available to the public.

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This data can be used to explain
the characteristics

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of ecosystems,
like bird migration patterns.

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In fact, by using a network of 143

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NOAA radar stations
across the continental United States,

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the team analyzed 23 years of data
to see if they could group stations

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that had similar year-to-year variability
in migration observations.

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And this is different
from the concept of flyways.

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This is specifically looking at which
radar stations across the United States,

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in what regions have a similar

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variability in bird migration timing.

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What they found is that the U.S.

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can be divided into two regions -
east and west, each with its own

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specific pattern of variability
of migratory bird arrival times.

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So now that we have divided
the continental United States

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into two regions,
we can look at each region specifically

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and understand the
climatic drivers of this region.

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The team found that the
behavior of Rossby Waves,

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huge waves of high-latitude westerly
winds, was a major influence

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for bird migrations in the East.
As Rossby Waves

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are triggered, particularly
those in the tropical Pacific,

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they influence climate patterns
and bring warm temperatures to the eastern

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United States, which in turn correlates
with earlier arrivals of migratory birds

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in that region.

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Cold temperatures
have the opposite effect.

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Variations in the West, however, seem
to be linked to more regional

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climate conditions, such as sea surface
temperatures of adjacent waters.

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In this study, we tried to highlight
the fact that we can use our data

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for applications that would bridge
climate science with other disciplines.

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The team hopes that future research
will build upon this study

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to help us better understand how changes
in the climate impacts specific

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migratory bird species.