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“Moon-Watching”
Studying Birds that Migrate at Night
General Information
Many species of birds choose to migrate at night.
Unfortunately for most bird watchers, millions of birds pass undetected
through the night sky in the spring and fall as they make their journey
between their breeding and wintering grounds.
In 1952, an organized, continent-wide network of 1,391
birders and astronomers performed counts of nocturnal migrant birds crossing
the full moon (Lowery and Newman, 1966). From the 1st to the 5th of October
that year, observers across North America manned 265 observation points in
three Canadian provinces and every state except Alaska, Hawaii, Maine,
Nevada and Utah. A total of 35,407 birds silhouetted against the moon were
counted. Data were then compared with weather patterns across North America
to discover relationships and patterns that affect the movement of birds.
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What species of birds migrate at night? For reasons
known only to themselves, many species populate the list of nighttime
migrants, including owls, thrushes, thrashers, catbirds, wood warblers,
vireos, kinglets, nuthatches, creepers, wrens, gnatcatchers, cuckoos,
buntings, rails, woodcocks, tanagers, orioles, blackbirds, bobolinks, and
most species of sparrows. Some species that migrate either by day or by
night include loons, grebes, ducks, geese, swans, shorebirds, swifts, and
swallows hummingbirds, auks and murres.
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Figure 1 – CCD Image of the Moon. No bird silhouettes are shown on this
or following images. We hope to obtain them this fall.
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So how can one observe nighttime birds? One tried and
true method is to use a spotting scope or telescope to spot migrating birds
as the pass across the full moon. A magnification of 40x or more gives the
best results (we use 80x here). The silhouettes of migrant birds up to 2
miles distant or more can be seen as they wing their way through the sky.
You will be amazed at the crispness of the silhouettes as birds pass by
singly or in loose aggregations. As you gain more skill, you will learn to
recognize families of birds by their silhouette as they pass by.
Higher-flying birds take longer to transit the moon than
birds at lower altitudes, and once in a while, you will be treated to a
high-flying aircraft or an orbiting satellite as it transits the moon.
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Figure 2 – CCD Image of the Moon
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How high do these birds fly? Using radar,
ceilometers, and other instruments, researchers have determined that birds
typically migrate at altitudes ranging from 1,500 to 5,000 feet, but on some
nights, altitudes may range from 6,000 and 9,000 feet. The highest altitudes
observed range from 15,000 to over 20,000 feet. This may sound surprising,
but some species regularly cross mountain ranges above 20,000 feet. An
incident in 1973, over the Ivory Coast of Africa, involved a collision
between a commercial aircraft and a Griffon Vulture at 37,000 feet! (Terres
1995).
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Be sure to keep an eye on the edges of the moon as you
watch. Some birds will transit the very edge of the moon, and you will miss
them if you are not observant. Typically, birds begin their migration right
after sunset, and continue until about 2 AM. Migration peaks between 11 PM
and 1 AM.
Try doing some timed counts to quantify your observations. |
Figure 3 – CCD Image of the Moon.
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Figure 4 – CCD Image of the Moon
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How do these birds navigate at night? In the 1950’s,
German ornithologists Franz and Eleanore Sauer demonstrated that night
flying birds have the ability to navigate by the stars. In the 1970’s,
Stephen Emlen took this a step further and identified the precise star
patterns that Indigo Buntings use when navigating at night. It was assumed
that the North Star was important as it is fixed in the night sky, but the
surprising result was that the birds instead used star patterns within about
35 degrees of the North Star. |
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Constellations such as the Big Dipper, the Little Dipper,
Draco, Cepheus and Cassiopeia were important star patterns used by these
birds!
Interestingly, there is little evidence that the moon plays a role in
nighttime navigation by birds. In fact, the evidence seems to suggest that a
bright moon actually interferes with stellar navigation. This brings us to
the topic of man made light pollution that blots out the stars across much
of North America, especially around highly populate areas. What actually is
the impact of light pollution on bird migration and other flora and fauna,
and on man himself? We will deal with this topic in a future posting.
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Using NEXRAD Radar
Today, NEXRAD radar has pretty much supplanted visual observations of
nighttime migrating birds. The numbers of birds on the move during a mass
migration is truly staggering. Radar provides amazing information on the
timing and direction of bird movements, especially as they relate to
existing weather patterns. Radar, however, is not without its limitations.
Visual observations of birds against the lighted moon can still provide
useful ground truth regarding families and species of birds being observed
and other detailed data.
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Figure 5 – NEXRAD weather image at Indianapolis
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A number of good sites on the Internet can be accessed to
view real time radar images across North America. Some sites provide
specific information on how to read these radar images to interpret what is
being seen, such as precipitation, winds, ground clutter, the presence of
birds, bats, insects, and other reflective targets.
NEXRAD radar images for your geographic location can be
accessed at the web site for the National Center for Atmospheric Research.
http://www.rap.ucar.edu//weather/radar/
Click on the US map nearest your location to view the radar
image closest to your location.
So when is the best time to look for night migrating
birds? In the spring and fall, massive movements of migrant birds will
coincide with the arrival and passage of weather fronts. Birds will take
advantage of air masses that are moving in the direction they want to go.
Northern moving fronts bring spring migrants, and southward moving fronts
bring the fall migrants. So watch the weather forecasts and study the moon
at night to compare the radar images with what you are seeing.
Keep a journal and record your observations. Compare the
radar images and weather patterns with what you are seeing through your
telescope or spotting scope. When did the migration start? When did it seem
to end? How many birds per hour were passing by? Again, try to get some
counts to quantify your observations! There is still much to be learned
about these amazing creatures that share our planet. Your data may prove
valuable to scientists seeking answers to the many perplexing questions that
surround these nighttime migrants. And most of all have fun!
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Additional Reading
Biebach, H. W., Freidrich, G., Heine, L., Jenni, S., Jenni-Eiermann, and
D. Schmid, 1991. The daily pattern of autumn bird migration in the northern
Sahara. Ibis 133: 414-422.
Bolshakov, K. V., 1985. Moon-watch method for quantitative studying of
nocturnal bird passage (collection and analysis of data). Pp. 14-36, in V.
E. Dolnik, ed. Spring nocturnal bird passage over arid and mountain areas of
middle Asia, and Kazakhstan. Proc. Zool. Inst. Leningrad, USSR Academy of
Sciences.
Bruderer, B. 1971. Radar studies on nocturnal bird migration in the Negev.
Ostrich 65:204-212.
Dolnik, V. R. 1990. Bird migration across arid and mountainous regions of
Middle Asia and Kazakhstan. Pp. 368-386, in E. Gwinner, ed. Bird migration.
Springer-Verlag, Berlin, Germany
Emlen, S. T. 1975. The stellar-orientation system of a migratory bird.
Scientific American 233(2):102-111.
Gauthreaux, S. A. Jr., 1971. A radar and direct visual study of passerine
spring migration in Southern Louisiana. The Auk 88:343-365.
Gauthreaux, S. A. Jr. 1972. Behavioral responses of migrating birds to
daylight and darkness: A radar and direct visual study. The Wilson Bulletin
84(2): 136-148.
Liechti, F, and B. Bruderer. 1995. Quantification of nocturnal bird
migration by moon watching: Comparison with radar and infrared observations.
Journal of Field Ornithology 66(4): 457-652.
Lowery, G. H. 1951. A quantitative study of the nocturnal migration of
birds. University of Kansas Publ., Mus. Nat. Hist. 3:361-472.
Lowery, G. H. and R. J. Newman. 1966. A continent-wide view of bird
migration on four nights in October. Auk 83:547-586.
Nisbet. I. C. T. 1959. Calculation of flight directions of birds observed
crossing the face of the moon. Wilson Bulletin 71:237-243.
Nisbet, I. C. T. 1969. A migration wave observed by moon watching and at
banding stations. Bird Banding 40:243-252.
Pyle, P., N. Nur, R. Philip Henderson, & D. F. DeSante. 1993. The effects of
weather and lunar cycle on nocturnal migration of landbirds at Southeastern
Farallon Island, California. The Condor 95:343-361.
Sauer, E. G. F. and E.M. Sauer. 1958. Celestial navigation by birds.
Scientific American 1999: 42-47.
Terres, J. K., 1995. The Audubon Encyclopedia of North American Birds. Wings
Books, New York, NY. 1109 Pp.
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Any use of these images must have permission of CWBO. |
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