Al Jaramillo said to Jack Bowling -

>Great to have someone who knows something about this stuff step in! OK,
>so the jet stream can go from Alaska to California about 4 times a year.
>My interpretation would be that this is really a rare occurrence rather
>than common. Does this tend to happen most commonly in the late
>summer/fall? If so then they may be common enough for birds to rely on
>them?

I hope I don't come off too pedantic here. The seasonality and duration of
highly meridional flows ("meridional" means pole to pole rather than "zonal"
which means paralleling the equator) varies from year to year but there is
some long term consistency. It is actually a continuous process, whereby
strong zonal flows eventually buckle and lead to meridional flows (zonal
flows restrict the transfer of energy poleward from the equator; eventually
the buildup of potential energy becomes too high and the flow becomes
unstable and transforms into a meridional flow whereby the energy difference
eventually becomes small enough that a zonal flow ensues, etc. etc.). The
jet streams are strongest during the winter when the temperature difference
between the equator and the poles is at a maximum. Likewise, they are at
their weakest during the summer months. The shoulder months of spring and
fall are often the most interesting for jet streams as their meandering path
often undergoes large changes. This also happens to be the times of bird
migration. Nice coincidence. Mind you, the winds can hinder as well as
help migrating birds. Case in point: this past spring, an anomalous high
pressure area anchored itself over Yukon from the last week of April to the
third week of May producing unrelenting stiff northeast winds at all levels
of the troposphere. The consequence was that the later waves of migrants
were delayed, some to the point where many B.C. birders were wondering if
something drastic had happened.

(JB had said) -
>>Therefore it is not much of a stretch to believe that birds would use
>>favourable winds when possible to assist in their passage. Many hours of
>>watching Sandhill Cranes pass over Fort Nelson at heights which you need
>>a scope to see them let alone b inoculars lends some credence to this.
>>So I suppose we disagree here.

>I don't think we really disagree. I am convinced that birds use favourable
>winds while migrating. This has been made clear to me from countless of
>hours hawkwatching. However, I am not convinced (not that I don't believe
>its not possible) that birds use the jet streams. Now its much more
>conceivable that the birds are using the strong winds at a lower level than
>the jet streams, but I don't know much about the average heights birds
>travel at during migration.

I agree. Much more likely that the birds will fly as high as necessary to
find favourable winds which in almost all cases would be much lower than the
jet stream. Seeing as how a typical eastern Pacific jet core speed is
faster than 100 knots, I think that it would be counter-productive for a
bird to immmerse itself in such a high-velocity medium anyway. They most
likely find their "comfort zone". It would seem logical that smaller birds
would be restricted to lighter flows.

>I know that Cranes, geese, hawks etc. will go quite high while migrating,
>but how about a peep or a warbler? From listening to nocturnal flight
>calls during migration I know that a good number of migrants, especially
>thrushes and sparrows fly quite low, however I do not know what proportion
>are flying low versus high. Anybody know about what the radar data has to
>offer on this point?

I have kept this tidbit around just in case someone asked this question.
The article this is cut from was a summary of a meteorological conference
written by a Montreal meteorologist. The conference was concerned with the
state of the art in atmospheric sounding techniques but, as you can see for
yourself, had some enlightening ancillary information for birders. The clip
follows (I will try to explain the pervasive "met" jargon).

"Finally, Wilczak presented a very informative (and entertaining)
presentation entitled: "Contamination of Wind Profiler Data by Migrating
Birds". That's right folks, by birds. Winds measured with 915 MHz radar
and with the WPDN (Wind Profiler Data Network) profilers often indicated
non-random errors as large as 15 m/s when compared to simultaneously
measured rawinsonde winds. Characteristically, the errors occur below 4 km;
they are a nocturnal phenomenon; beginning shortly after sunset and ending
before sunrise; the errors almost always have a southerly component in
spring and a northerly one in the fall; and there are sharp increases in the
measured signal power and spectral width. Furthermore, contamination can
occur simultaneously over broad areas, with the errors not being detected by
the quality control software. At times, the whole WPDN indicated the errors
which then looked reasonable to a data assimilation system because the
errors were consistent between the profilers. Such an example was shown and
all of the WPDN data were accepted by the NMC (National Meteorological
Centre in Suitland, Maryland) data assimilation system. Detailed studies of
these errors indicated that they are due to the profilers' detection of
migrating songbirds. Fortunately enough, new QC (quality control)
algorithms based on characteristics of the returned power spectra are being
developed and showing promising results. McLaughlin also discussed this
contamination problem at the White Sands Missile Range. One of his comments
was, and I quote: 'Notably though, some of the most interesting atmospheric
events observed have been biological. Since the beginning of operations at
the APRF in December 1991, many examples of insect, bird, and bat activity
have been seen including thin and thick layers, migrations, and after-sunset
phenomena.' A paper on this topic will appear in BAMS."

Unfortunately, I do not know which journal is BAMS. Wild presumptions to be
drawn from the above include:

1) Detected birds fly below 4 km (Which birds? Likely mostly warblers and
thrushes as you mentioned). 2) The migrations are nocturnal phenomena. 3)
The migrations happen across a wide front (the WPDN network is national in
scope); apparently they can even be nationwide! This has likely been
suggested before by banding station data. 4) Flight speed of migrants can
reach 15 m/s or about 30 miles an hour (I have seen other references stating
the 12-15 m/s speed).

I find it both amusing and amazing that small birds moving en masse have the
capability to screw up the high-tech gadgets of the weather cognescenti -
"contaminating" the data just as surely as all that radar energy is
contaminating the airspace the birds must travel through.

,Jack