Sunday, April 20, 2014

Playing the weather to see more spring migrants

Along the Gulf Coast (and in the New Orleans area), spring migration is notoriously boom and bust- there can be huge numbers of migrants around one day, and virtually none the next.  Although the birding and ornithological communities don't have this entirely figured out yet, it is generally understood that most of this variation can be explained by weather patterns, and how migrants respond to them.

Some basic background information:

1)  warblers, buntings, thrushes, and other smallish land bird migrants tend to wait for favorable weather conditions before they migrate.  The most critical deal-breaker is a headwind, but active precipitation can also make them delay.

2) the bulk of these species migrate nocturnally.

3) they usually fly a couple thousand feet overhead.

In spring, these migrants are arriving from across the Gulf of Mexico.  They are generally understood to leave the tropics (especially the Yucatan) near sunset, fly all night, and arrive over the Louisiana coast in the middle of the next day- most commonly between 11 am- 3 pm but it may be earlier if they are hastened by a strong tailwind, or later if delayed by a strong headwind. Thus, while over land they would normally land shortly before sunrise, they are forced to continue flying during daylight by being over the water.  Their arrival can usually be seen on unfiltered weather radar (eg, as a light blue haze of echoes that appears to our south and then envelopes us. 

Amazingly, the norm under favorable weather (lack of precip or headwind) is for these migrants to head straight over us undetected, and land in a dispersed fashion in the parishes to our north, beyond Lake Pontchartrain, maybe tens of miles beyond.  LSU ornithologist George Lowery, who figured most of this stuff out decades ago, dubbed coastal and near-coastal areas of state the "coastal hiatus" for this reason.

Any inclement weather (a switch to headwinds with the passing of a cold front, or rain) confronting birds migrating across the Gulf will cause increased numbers of birds to stop in New Orleans (and on the coast), which we call a fallout.  However, timing is critical- if the headwinds arrive in late afternoon or later, the migratory flight may have passed us and fallout is less likely.  Likewise, if a rain event happens before or after they are passing overhead, it may produce little or no grounding.   

Rain-induced fallout birds will generally leave us at nightfall if the rain has moved on, while many frontal-fallout birds will hang around for days until tailwinds resume.  Thus, for a rain event, it is advisable to ditch work and get out biriding in the afternoon immediately after the rain falls.  For a frontal fallout, you can wait and call in sick the next morning.
There are no fronts scheduled for the next few days, so we may have to wait a bit for our next major pulse of birds, unless we luck out and get a midday rain event.  In general, frontal passages become farther and farther apart as spring progresses, providing fewer opportunities for fallouts.   There are enough migrants passing to make decent fallouts possible until May 12 or so, after which the volume of migratory flow drops sharply.

Good birding,


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