[texbirds] Monitoring spring bird migration using NEXRAD weather radar
- From: "John Arvin" <jarvin@xxxxxxxx>
- To: "New Texbirds" <texbirds@xxxxxxxxxxxxx>, "LABIRD-L" <LABIRD-L@xxxxxxxxxxxxxxxx>
- Date: Sun, 17 Mar 2013 15:52:46 -0500
For the last several days we have been having the kind of spring weather
that figuratively sucks up migrating birds and propels them northward
toward their breeding grounds in sufficient numbers to use NEXRAD weather
radar to good effect to monitor this movement. I will not go into the
technicalities of how NEXRAD works because there are many internet sources,
mosty university sites, that do a far better job than I could in their
tutorials. But a couple of simple things need to be said: 1) migrating
birds almost always move with favorable winds. And the winds I am talking
about are not surface winds but those at higher levels of the atmosphere
where most birds move when they are headed long distances. It is only when
they begin a migration flight on favorable conditions only to have those
conditions deteriorate en route when they cannot easily cease migrating to
wait for better conditions that they move against winds. The reason is
obvious. It takes lots more energy to move against winds when moving with
winds helps conserve energy to complete the journey. Any bicyclist can tell
you that. Then 2) there are times of the day when monitoring NEXRAD is
more profitable than others, and this depends whether you are birding the
coast or birding inland. In the latter case, since most over-land migration
is at night, then clearly night is the time to study this. However, some
birds, primarily large bodied, long-winged species like cranes, pelicans,
and raptors using rising air currents off heated ground to provide them the
lift they need to move long distances in an energy-efficient manner.
Therefore these migrate by day when they have the sun to provide the
heating effect. Some ducks, herons, gulls, terns, and shorebirds also
migrate during the day if conditions make this a viable strategy, although
these groups also migrate at night under other conditions. Songbirds
migrate from south of the Gulf of Mexico to the northern Gulf coast in one
necessarily non-stop flight (Yes, I know there are several thousand
offshore petroleum platforms in (mostly) the near-shore Gulf, and I spent 3
months on one studying trans-Gulf migration with an LSU 3 year project and
I can discuss that at length, but for now I'll let that slide for a while).
These also arrive during daylight hours between mid morning and mid
afternoon depending on how much help from tailwinds there is. Thus daylight
hours are the time to look for trans-Gulf migrants approaching the coast.
Other songbird species, primarily those that winter north of the Isthmus of
Tehuantepec in southern Mexico, although there are many exceptions to that
statement, choose a circus-Gulf path in which they migration over land,
paralleling the coast. These migrate primarily by night (again there are
exceptions). 3) Timing of migration seasonally: there is a regular
progression of suites of songbird species throughout the spring migration
season, beginning for all practical purposes in January with Purple Martins
and continuing through March when southern breeders arrive (Ruby-throated
Hummingbird, Chimney Swift, a few early warblers and Yellow-throated Vireo,
etc. and peaking in mid April through early May with too many species to
list. May is diverse early but beginning about the 10th the flycatchers are
the bulk of songbird migrants, especially the Empidonax and pewee groups.
The seasonal timing of individual species is remarkably consistent year to
year and if an Empidonax is reported in March then it's time to take a
second look. As I have over the past 5 or 6 springs I will post bird
movements that I detect on NEXRAD when there is something significant
visible (not every day as in the past). One thing needs to be said. NEXRAD
is not a panacea that can predict where a migratory flight will land. Often
on strong south winds migrants may take advantage of the boost to move well
inland (50 - 100 or more miles) before landing. On the other hand if they
encounter bad weather en route they may land at the first opportunity,
often exhausted, a situation that has come to be called a "fallout". These
events are much anticipated by birders. The classic fallouts of the 50s,
60s and 70s may be a thing of the past but a weakened version continues.
So read these bulletins if you want to, and if not, the delete key is
within easy reach. I may post some particularly illustrative radar images
to the FB Texbirds site from time to time. John C. Arvin
Research Associate
Gulf Coast Bird Observatory
103 West Hwy 332
Lake Jackson, TX 77566
jarvin@xxxxxxxx
www.gcbo.org
Austin, Texas
John C. Arvin
Research Associate
Gulf Coast Bird Observatory
103 West Hwy 332
Lake Jackson, TX 77566
jarvin@xxxxxxxx
www.gcbo.org
Austin, Texas
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