This system is forecast to move into the Gulf during Saturday, and the models look great for fallouts– especially in the TX/LA coastal corridor, which should be seeing rain by early tomorrow morning.  Over the course of the day, winds aloft will gradually shift toward the westerly, steering migrants toward MS/AL for possible late fallouts there (with storms expected to peak mid-afternoon).  Trans-Gulf travelers will have nice tailwinds for most of their trip tonight, so the question is whether they’ll arrive soon enough to beat the front.

They sure have been moving over the last couple of days.  Arrivals yesterday were extremely heavy, especially on the Upper Texas Coast, as you can see from the animation below.  At their early-afternoon peak, when reflectivities reached 30 dBZ, the number of birds passing within range of the Houston beam must have easily numbered in the millions.  Riding a stiff tailwind, birds coming ashore in Louisiana can be seen burning past the coast and arriving in north MS by the end of the day.

Last night, with 925 mb winds reaching a gale-force 40 knots over parts of the South, the pace of nocturnal migration was breakneck.  Birds taking off from coastal forests whizzed completely through Mississippi in a matter of a few hours.  Here’s last night’s animation:

The beginning of May is bittersweet, since it represents the turning of the corner for spring migration, which will be slowing to a trickle in a few short weeks.  But for now, it’s still going full-bore.  Even in the noontime heat yesterday, when I finally got around to stepping outside, the woods were dripping with birds (mostly Tennessee, Chestnut-sided, and Yellow Warblers and American Redstarts; a Swainson’s Thrush, a Warbling Vireo, and a flock of Indigo Buntings were also hanging around). 

Fortunately, Chazz Hesselein didn’t get wind of this forecast, and he showed up on Dauphin Island yesterday afternoon to witness a superb fallout.  Hundreds or even thousands of Scarlet Tanagers foraged tamely in shrubs and on the ground; thrushes of several species allowed point-blank looks; Orchard Orioles and Indigo Buntings were present in large numbers; and as many as four Black-billed Cuckoos (a species of which most of us feel fortunate to see one) were reported across the island.

Given that anyone at all made the flight Monday night, it stands to reason that they’d be too exhausted by Tuesday to worry about snooping birders.  But why would they depart at all?  A headwind is a big deal, especially for smaller birds (a headwind of just 10 mph can cut down a hummingbird’s flight range – the distance its fat reserves will last – by more than half).  There don’t seem to have been many warblers present on Dauphin yesterday, so there may have been a size bias among Yucatan birds willing to depart.  I’m just surprised that there was a flight at all.  Migrants are full of surprises.

Meanwhile, the high over the mid-South has been moving along somewhat more quickly than predicted, and we started to see the back side of it last night – allowing considerable migration on light tailwinds as far east as Jackson.  Here’s the national animation:

What did all this mean for birding?  Winds aloft never developed the strong easterly bias that had been predicted over the Gulf, so most coastal areas received arrivals, and persistently nasty weather meant fallouts were possible throughout the weekend – although the storms were patchy enough that many birds may have threaded the gaps and continued inland on the south-ish winds ahead of the stalled front.  On Saturday, though TX and LA received the most birds, migrants were visible over the Gulf from every coastal station, from Brownsville all the way around to Key West:

Incoming flights on Sunday were much thinner, although a rather substantial arrival event could perhaps be seen along the MS/AL coast in mid-afternoon.  The drop-off in traffic from Saturday to Sunday intrigues me, because wind conditions in Mexico appear to have been similar for departees; increasing cloud cover may have been the difference, cluing migrants in to distant trouble.  Sunday’s animation:

Finally, arrivals on Monday seem to have been practically nonexistent (although it’s important to remember the possibility of migrants coming in below the radar beam, as they well might under adverse conditions).  Fortunately, though, coastal areas had accumulated enough birds through the weekend to make for good birding.  15+ warbler species were reported from both Grand Isle and Fort Morgan on Monday.  Yellow, Chestnut-sided, Magnolia, and Tennessee warblers seem to have been prominent constituents of this latest wave, as well as the transient spotted thrushes (Swainson’s, Gray-cheeked, and Veery); good numbers of tanagers, orioles, and grosbeaks continue to be reported as well.

Migration is currently on pause, but it will be the briefest of pauses:  by tomorrow night, the southerlies will have re-exerted themselves, and the Yucatan Express will be rolling again.  I highly recommend getting out before then if you can; grounded migrants in good diversity could be present just about anywhere in the Southeast. 

Meanwhile, there’s some rough weather in the forecast for this weekend, and that, of course, means that birding could be interesting.  Two separate cold fronts are actually scheduled to pass through:  a weak one on Saturday that will have little, if any, impact on arriving migrants, and a stronger one on Sunday that will stretch into the Gulf and will likely have considerable impact.  According to the latest model runs, the storms attending this front will be passing over Houston sometime Sunday morning, and over New Orleans by afternoon, so some degree of fallout in these areas seems likely.  The bad news for those of us in the mid-South is that winds aloft over the Gulf will have little if any westerly component, so most of the action will probably be in Texas and southwestern Louisiana.  I hope someone will prove me wrong.

Looking a little further out, Monday is worth keeping an eye on.  According to forecasts, conditions on Sunday evening will still be favorable for Yucatan departure; winds aloft over the northern Gulf will have a strong westerly component, nudging more migrants in our direction; and onshore winds may have shifted enough to the north to force birds to take a breather in coastal hotspots.  I will try to post an updated forecast sometime later this weekend.

That the Wandering Glider (Pantala flavescens) is something more than your garden-variety dragonfly is suggested by its many nicknames – Globetrotter, Globe Skimmer, Storm Chaser, Typhoon Dragonfly – as well as its selection for the Worldwide Dragonfly Association’s official logo (see below). In fact, this extraordinary species is both the most abundant and the most cosmopolitan dragonfly in the world, found in both hemispheres to about 40 degrees latitude, and on such remote landmasses as Hawaii and Easter Island. It’s been recorded from ships in the middle of the ocean, and at 20,000 feet in the Himalaya.

P. flavescens owes its ubiquity to its peripatetic lifestyle. One of a group of dragonflies known as rainpool gliders, it lays its eggs in temporary pools created by heavy rains, and is finely adapted – much like our last MOD, the Cedar Waxwing – for capitalizing on such an unpredictable resource. An aerodynamic teardrop-shaped body (with ample room for flight-fueling fat storage) and unusually broad wings help make it a champion flyer; in fact, it’s somewhat notorious among dragonfly collectors for being impossible to catch.(2) A highly accelerated life cycle allows it to get in and out of those ephemeral pools before they dry up: glider larvae feed with unparalleled aggression and appetite, and can go from egg to adult in a blistering six weeks. The emergence of adult gliders from a rainpool can be a real spectacle, with huge swarms forming (3), catching updrafts towards the stronger tailwinds aloft, and moving off en masse in search of new breeding sites – which could be on an entirely different continent.  In southern India, rice paddies serve as prime glider breeding habitat, and swarm emergence is often timed to coincide with Onam, a rice-harvest festival that in many areas is the biggest event of the year.  People in these parts know P. flavescens as onathumbikal, or the dragonfly of Onam.

Migrating gliders are apparently unfazed by rain, and can be observed flying in rains heavy enough to drive just about every other insect undercover.  Some evidence suggests that this species may actually hitch a ride with storm systems – even, perhaps, with hurricanes – in order to be perfectly poised to take advantage of fresh breeding habitat.  In 1967, a team of Japanese entomologists recorded insect observations from a weather ship located far out at sea.  When the ship took a direct hit from a typhoon, the intrepid observers climbed abovedecks in the eye of the storm, and to their amazement saw “an enormous number” of Wandering Gliders passing by.

Although P. flavescens is often referred to as the most highly evolved dragonfly in the world (whatever that means), a few of its behaviors still need some work.  Specifically, its ability to spot a rainpool is so acute that any smooth, shiny object can serve as a glider magnet; females can often be found absorbed in the quixotic task of laying eggs on the roof of a parked car.

(1) Though soon to be outdone by the Soviets.

(2) Gliders on Easter Island are less accomplished flyers; many of these adaptations have fallen into evolutionary disfavor in one of the most remote places on Earth, where long-distance flight means almost certain death.

(3) One gilder swarm allegedly covered 13 square miles.

Photo credit:  John C. Abbott, odonatacentral.com

And here’s an extreme instance of the anaprop itself, from the Mobile station at 1 AM this morning:

The phantom ‘spokes’ of high reflectivity emanating from the station are caused by the radar beam being refracted downward, so what you’re actually seeing is the reflectivity of the ground.  A common cause of this phenomenon is an atmospheric event known as a temperature inversion, which occurs when a layer of warmer air forms above a layer of cooler air, reversing the normal altitudinal trend and interfering with the radar beam’s path.  This often happens on calm, clear nights within areas of high-pressure; after sunset, the ground begins to lose heat quickly, cooling the near-surface air, while subsiding air aloft (associated with the high) warms as it sinks, eventually forming a relatively stable blanket atop the cooler air below.  These conditions prevailed over most of the Southeast last night, and soundings confirm that an inversion developed.  Here is the temperature profile over Mobile at 1 AM.  The red line follows the change in temperature with altitude; note the near-surface jag representing a warming trend (a pretty substantial one, of about 8 degrees F) from the ground up to about 1200 feet.

Jackson’s temperature inversion was slightly less dramatic, with a maximum differential of about 6 degrees.  Now let’s check out the bird activity over Jackson, using filtered velocities to eliminate insects and other slow-moving targets.  As often happens, peak movement here occurred in the middle of the night, once a wave of coastal departees made it to our area; here’s a snapshot of their passage at 12:30 AM:

What stands out to me is the tight clustering of targets near the radar station (mostly 20-30 miles away), reflecting birds traveling at unusually low altitudes (<1000 ft).  With no cloud layers or headwinds aloft to keep them down, my best guess is that they were favoring the highly stable, slightly cooler air below the temperature inversion – or, more precisely, striking the best balance between staying low (to keep cool and avoid turbulence) and flying high (to catch stronger tailwinds).  This is a hypothesis that could be evaluated by keeping close tabs on bird movements and temperature profiles . . .

On a side note, I was interested to see high reflectivities last night over parts of Georgia and Florida, despite easterly winds prevailing aloft.  Here in the mid-South, this could be a good time to look for those elusive Florida migrants – birds like Cape May Warbler, Black-throated Blue Warbler, even (dare I speak its name?) Connecticut Warbler . . .

The Knights of Oxford (northern MS) are outstanding birders who regularly contribute sightings to the state mailing list.  This weekend they had an enviable cast of visitors to their yard, including Tennessee, Nashville, and Cape May Warblers.  This is an intriguing trio of species, because all three follow highly divergent migratory strategies.  TEWA is a classic trans-Gulf migrant, making the 600-mile trip from the Yucatan each spring and moving across the Southeast in a broad front; it’s perhaps the most abundant warbler in North America, and (partly thanks to its irrepressible spring song) is easy to find in our area at this time of year.  NAWA, meanwhile, is one of a few warbler species that are classified as circum-Gulf migrants, preferring an overland route that leads them along the Mexican and Texan coastlines and up the Mississippi and Ohio valleys (NAWA’s relatively northerly wintering range may lend this strategy more appeal).  Finally, CMWA is a member of the group sometimes known as “Florida migrants” – because CMWAs largely winter in the West Indies, it makes sense for them to use the islands as stepping stones on their way north, eventually hopping from Cuba to Florida and continuing along an eastern corridor.  Here’s a rough diagram of the routes followed by these latter two species:

As one might guess from the maps, both NAWA and CMWA can be harder to find in Mississippi than many other warblers.  Most NAWAs pass to our west, while most CMWAs pass to our east, although we’re close enough to the main flyways of each species to pick up some fairly regular representatives of both.  However, it might seem counterintuitive to have both species turn up here on the same day, since the east winds that might nudge a CMWA our way would keep NAWAs well away from Mississippi, and vice versa.  But as the weather maps from Friday show, you can sometimes have your cake and eat it too:

As it turns out, everything was aligned just right for a weekend convergence of warblers upon the mid-South.  Clockwise flow out of a high off Florida meant that winds aloft over the Greater Antilles had a strong easterly component, which would be likely to nudge a few “Florida migrants” slightly off track towards the west.  Meanwhile, an advancing cold front over Texas tightened a pressure gradient that produced strong southwesterly winds along the coastal corridor used by circum-Gulf travelers.  While there’s no way of confirming that these two warblers arrived by exactly these means, it’s certainly plausible – and a birder confronted with this weather scenario should be alert to the possibility.

This isn’t earth-shattering analysis, but it’s a nice textbook illustration of the power of weather systems to shuffle birds around.  Hopefully, the weeks to come will bring many more opportunities for this kind of sleuthing.

Here in the Deep South, after getting banged up by cold fronts on about a weekly basis through early April, we’re now baking under a nearly motionless high, with little relief in the forecast.  Although it won’t pack the wallop that the last few storm systems have packed, there is another cold front slated to come through our area over the coming weekend, with associated rains perhaps moving offshore into the Gulf by Monday.  There is some chance of this producing another fallout, depending of course on its timing and strength, and I’ll be giving updates on that later in the week.  Otherwise, though, the forecast is just full of great flying weather – warm southerlies aloft – and the birds will be moving pretty quickly through our area.  Diversity at this time of the year should be outstanding, and every morning should have the potential for a great find or two, given luck and persistence.

Here’s an example, from Saturday night, of the major flights that have characterized recent evenings.  The extremely intense reflectivity blanketing the NW Louisiana area in the early morning hours is a red herring, mostly resulting from anomalous propagation.  This animation nicely illustrates the “dead zone” (here in the Midwest) that immediately follows the passage of a low pressure cell; the low itself can be seen moving into the mid-Atlantic states, with bands of precipitation spiraling around it in a counter-clockwise direction:

Below is an animation of the same night as seen from Jackson.  Reflectivity is on the left; velocity, filtered to eliminate slow-moving targets (see previous posts for explanation of this technique), is on the right.  Again, the velocity images reveal hidden complexities in what first appears to be a straightforward “exodus” event.  Although there are substantial numbers of fast-moving targets (i.e. birds) visible all through the night, they make up a much smaller proportion of targets during the early nighttime hours.  There seems to be a basic pattern here in central Mississippi of insects being dominant for a period after sunset, until their contributions are overwhelmed by those of birds arriving from coastal areas.  (At least, that’s how I feel most comfortable interpreting it.)  Note that the birds seem to “drop out” for a few hours in the middle of the night.  This is something of an illusion; when wind speeds inch above one of the NEXRAD velocity thresholds (26 kts in this case), it causes a whole group of pixels to be eliminated.  (In other words:  due to the conservative filtering process, targets moving at anything between 36 and 50 kts will show up when the wind speed is 25 kts, but not when it’s 27.)  This is a drawback of my filtering method which I’ll try to find a way around in the future.

It’s interesting to me that birds can still be seen in the air in the hours just pre-dawn, after winds have visibly shifted to the west.  I wonder who these birds are and what, if anything, they’re accomplishing by catching a ride eastward.

With the same conditions prevailing tonight, expect more of the same–and since the moon will be nearly full, it should be a great night for moonwatching!  Break out your scope and see if you can flesh out some of these radar blips . . .

The approaching cold front, as currently forecast, will be timed poorly to generate a fallout.  It’s scheduled to pass through inland Mississippi tomorrow afternoon, in time to stifle tomorrow night’s migration but too early to precipitate any birds aloft.  By dusk tomorrow, it’ll be far enough into the Gulf that conditions will likely be poor for migrant departure in Mexico.  Then again, migrants have already proven themselves capable of surprising behavior this season . . .

I continue to fret about the difficulty of disentangling insects’ contributions to radar reflectivities from those of birds–and I’ve now developed a new tool that I hope will facilitate judgment.  Briefly, it’s a script that processes radar velocity imagery to filter out targets traveling at less than a specified number of knots above wind speed (based on hourly wind speeds at 925 mb from the RUC model - I’d be glad to share details with anyone who’s interested).  I’ve currently set the threshold at 10 knots over wind speed, a level which should eliminate most insects and preserve most birds, although I may twiddle this in the future.  So the animation below shows targets over Jackson last night that were unequivocally traveling at least 10 knots over wind speed.  Because NEXRAD velocities are measured very coarsely, this is a very conservative filtering method.  Also, remember that these velocities are relative, which accounts for why fast-moving targets (i.e. birds) only appear in “lobes” aligned with their direction of flight, although they’re of course pretty evenly distributed in space.  Consider this experimental, and look for more about it in posts to come.

Tonight (and tomorrow night), look for things to pick up considerably, with large numbers of grounded migrants seizing the opportunity to head north.  With auspicious wind forecasts over the Gulf for the next two nights, there should be a fresh load of new arrivals to fill the void.  (Some incoming birds have already been visible this morning from the Brownsville and Corpus Christi stations in south Texas, despite unfavorable winds last night along most of the Mexican coast; it will be interesting to see if this flight develops further.)  The next front is scheduled to move through on Friday, potentially laying the groundwork for some more good weekend birding on the coast.

Now that we’re officially into the second half of April, look for the tone and tempo of migration to change.  With warm southerlies gaining the upper hand, the flow of birds will be steady and moderate, without being punctuated by the regular fallout conditions that characterize early spring; but the flights will make up in diversity for what they’re missing in drama.