How Weather Patterns
Influence Bird Movements
Bryan Guarente
University of Illinois at Urbana-Champaign
Atmospheric Sciences Department
COS Presentation
Monday March 21, 2005
Outline
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Bird Navigation
How To Read Weather Maps
Two Caveats
Case Studies
Helpful Resources
Bird Navigation
• How do birds migrate?
– Using landmarks (Point Peleé)
– “Staying within the lines”
• Highways (I-95)
• Ridges (Blue Ridge: Hawk Mountain)
• Coastlines (Atlantic Coast, Lake Michigan)
– Magnetism of the earth
– Sun, Stars, Moon
– “Believing their instincts”
– Avoiding weather or go with weather
Bird Navigation (cont)
• Most important thing:
– Which family of birds?
– What habitat is needed?
– What time of year?
• “Circlers” (Soaring)
– Falconiformes
– Gruiformes
• Direct Flights (short and
long distance)
– Anseriformes
– Charadriiformes
– Passeriformes
Bird Navigation (cont)
• “Circlers”
– Thermals: pockets of warm rising air
• Thermals enhance a good migration day for
circlers
• Strong upper level winds also make for a good
“circler” migration day
– Thermals are not required but circlers will
limit distances on days without thermals
Bird Navigation (cont)
• Short Distance vs. Long Distance Migrants
– Long migration is not effected greatly by weather
patterns
• More dependant on season or amount of sunlight
• Examples: Arctic Tern, Sandpipers, and Barnacle Goose
– Short migration is effected greatly by weather
patterns
• Less dependant on season or amount of sunlight
• Examples: Most Passerines and Waterfowl
Problems encountered
• What would you do if you were flying
and you encountered
– Significant Cloud Coverage?
– Frozen Precipitation?
– Strong Winds opposing your motion?
–Land!
Birds experience these conditions and
are forced to land all the time.
How’s the weather up there?
• Upper atmosphere makes all the
difference
– BIRDS DO NOT MIGRATE AT SURFACE
• Surface winds seldom important
– Between 1km and 5km AGL winds are the
biggest weather factor
• 850mb (1km) to 500mb (5km) most important
levels for winds
• However, surface weather systems
effect upper atmosphere
– Surface cannot be ignored
How to read weather maps
• Birds prefer to follow
winds (cross country
flight example)
• Most maps show
winds
– wind barbs

– All directions are the
same as a compass
– Plotted as the
direction the wind is
FROM
How to read weather maps
• Rotation of highs and lows visible on
surface obs
How to read weather maps
• Convergence and divergence will show areas
of preferential movement to and from a
location
• Multiple kinds of convergence and divergence
(directional and speed)
How to read weather maps
• Areas of convergence and divergence
visible
How to read weather maps
• Streamlines
– Shows direction a bird would travel… without
outside forcing
– Can see where migrants will come from or end up
– Easiest way to see convergence at any level
– Fronts become obvious (lines of convergence)
• Fronts make some birds stop migration (strong pressure
decline, assume bad weather)
– Uncommonly plotted… especially for forecasts
How to read weather maps
How to read weather maps
• 850 mb (1km up)
– BEST place to look for strong winds that
will promote movements
– Strong winds from preferential direction =
GOOD MOVEMENTS
• Season dependant
– Stronger winds at this level are called “lowlevel jet”
– The end of a low-level jet has speed
convergence (great place to look for
migrants)
How to read weather maps
How to read weather maps
• 700mb (2-2.5km up)
– Good place to look for strong winds for
migrants
– Not as good as 850mb, but still important
– Low-level jet also present at this level
– Stacked strong winds (700mb strong winds
above 850mb strong winds) = GOOD
MOVEMENT
How to read weather maps
How to read weather maps
• 500mb (5-6km) and 250mb (10-12km)
– Winds at these levels only effect high
altitude fliers
– Hard to track these birds
– Track vertical motions
• Upward vertical motion may lead to movement
– Barring cloud cover (which accompanies rising
motion)
– Unproven, but feasible
How to read weather maps
How to read weather maps
How to read weather maps
• Jetstreaks and vertical motions
– Jetstreaks: areas of fastest moving winds in
jetstream
– Dynamical effects cause divergence
How to read weather maps
• Jetstream convergence and divergence
How to read weather maps
How to read weather maps
How to read weather maps
• Infrared (IR) Satellite Imagery
– Low pressure systems look like commas
– High clouds versus low clouds
– Figure out low level winds from low clouds
(if present)
– Sometimes see areas of preferred heating
of the ground (areas good for circlers)
– Nighttime cloud cover
How to read weather maps
• Visible Satellite Imagery
– Low pressure systems look like commas
– Cloud levels not clear in some cases
– Visible satellite not available at night
How to read weather maps
How to read weather maps
• Water Vapor (WV) Satellite Imagery
– Low pressure systems sometimes look like
commas
– Rotations can be seen very clearly
• Rotations are usually associated with vertical
motion and bad weather (can ground birds)
– Jetstream pattern is often clearly visible
– Can infer streamlines
– Images available at night
– Cannot see below 700mb
How to read weather maps
• Radars
– Radar Reflectivity
• Two different modes used (for small particles and large
particles)
– Clear air mode
– Precipitation mode
• Clear air mode:
– Easy to pick up fronts and smaller particles (clutter issues)
– Precipitation cannot be present
• Precipitation Mode
– Hard to pick up fronts and smaller particles
– Precipitation will be present
– May be hard to distinguish birds from precipitation
How to read weather maps
Two Caveats?!
• Good weather patterns do not always equal
good migration
• All weather forecasts are model produced
– Models differ
– Models are made using our current understanding
of the atmosphere
– Weather forecasts beyond three days are
pipedreams
• Because of this: models are NEVER correct!
– BUT, we need to look for guidance somewhere
Experience is the key
• As it is with birds:
– The more you look, the more you understand
• This may not come easy for some of you
• Every day forecasts change
– Look often and be flexible
Are you ready for your first try?
What happened on this day?
• Black-legged Kittiwake (Rissa tridactyla)
showed up near Chicago (reported by
Eric Walters)
• Refer to emails on IBET (8077, 8078,
8082, 8087, 8091) for discussion of
patterns
• For sightings refer to IBET (8094 BLKI,
8096 BLKI, 8100 FEHA, 8101 SACR)
What happened on this day?
• Sandhill Cranes (Grus canadensis)
“clouded” the Chicago area skies (1,269
reported by Matt Karabestos; 400
reported by Leslie Cummings)
• First real push of Sandhill Cranes (G.
canadensis) all season
• IBET posts (9514 – 9518, 9521, 9526,
9528)
Summary
• Upper-level winds are the most
important factor (weather-wise)
– The longer time scale the better
• Areas of convergence = runways
• Surface fronts play a role but can be
avoided by larger birds if they are weak
fronts
• Clouds, precipitation, and strong winds
opposing motion will make a bird land
Useful Resources
• http://www.atmos.uiuc.edu/~bguaren2/
birdweather/index.html
• http://www.rap.ucar.edu/weather
• http://weather.cod.edu/
• http://vortex.plymouth.edu/
• http://www.crh.noaa.gov/
Other helpful sources
• “Severe and Hazardous Weather” 2nd
edition
– Rauber, Charlevoix, and Walsh
– Excellent introductory weather book
– Written with non-science students in mind
– Figures from today’s presentation from
book
Thank you!
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COS
IBET
COS member: Tom Kelly
Bruce D. Lee and Catherine Finley
Albert Guarente (my dad)