[texbirds] Monitoring spring bird migration using NEXRAD weather radar
- From: "John Arvin" <jarvin@xxxxxxxx>
- To: "New Texbirds" <texbirds@xxxxxxxxxxxxx>
- Date: Sun, 17 Mar 2013 12:06:23 -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. 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 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).
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
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