The approaching cold front has prompted me to contemplate and categorize various ways in which particular weather phenomena can produce birding opportunities. So here goes an attempt at delineating them, with an emphasis on south Louisiana. All of these, however, are pretty well known to experienced birders across the continent.
1. Cold front passages
Cold front passages produce concentrations of migrants. In spring this is crucial in coastal areas along the Gulf, where trans-Gulf migrants are induced by these fronts to stop in our area instead of overflying the coastal belt and heading inland. A cold front produces a headwind (and often rain) that makes the Gulf crossing a struggle, inducing them to stop on the immediate coast or somewhere in the coastal belt. These "fallouts" are best viewed in the Nature Conservancy properties on Grand Isle, but can be impressive even in New Orleans (where the best spot to experience them is usually the Couturie Forest in City Park). The grounded migrants will often linger for some days until wind directions change to give them a tailwind to continue north. If the front arrives at the coast in time to intercept the arrival of migrants from across the Gulf (usually around midday), the fallout can happen that same afternoon. If not, the next day is a better shot.
In fall, cold fronts often usher in waves of migrants utilizing the northerly tail winds that occur in the days that follow them. The pulse of migrants is usually greatest on the morning following the first night dominated by northerly winds after the front passes.
Because cold fronts are more frequent in the cooler months, cold front birding opportunities are most common early in spring and late in fall. However, most years we will get a front as late in spring as early May, and as early in fall as August. At these times they may not usher in much change in temperature, but will still produce the desired wind direction change that can produce good birding.
2. Intense and widespread rainstorms
These can produce fallouts just as cold fronts do in spring, with the birds wanting to pause because they have been struggling through the rain (vs. bucking a headwind after a cold front). Rain-grounded birds will usually depart more rapidly than after a cold front, often at first nightfall if the rain has abated. This is because wind direction is usually southerly and thus favorable for continued migration at times when a lot of non-frontal rain is in our area.
Major regional rain events in spring are generally considered the best conditions for grounding shorebirds. The rain not only grounds them, but often creates rain pools in large open grassy areas, into which birds will gather. The "Exxon Fields" at Grand Isle are well known for this. Before the construction gobbled up so much of the University of New Orleans campus lawns, they also were famous for this. Sometimes rain pools can have birds along Lakeshore Drive. There are a few other lawns around, but they are owned by private interests who have not yet been courted to gain permission for general birder use so I will not name them. However, during a rain event earlier this month, lawns on the West Bank of Jefferson Parish produced gatherings of Greater and Lesser Yellowlegs, and Pectoral, Least, and Solitary Sandpipers, and a Dowitcher sp. Shorebirds do not generally stay grounded long- it is best to get out and find them while the rain is still falling.
3. Tropical weather
Ah, the love-hate relationship of birders with tropical weather systems! We love 'em for the birding they create, but we fear their destructive potential! Tropical weather regularly displaces coastal and oceanic birds inland, sometimes hundreds of miles (I recall a Cory's Shearwater in Oklahoma). For this reason many birders key in on them and get out and about searching for storm waifs as soon as they are able to do so safely.
The most predictable outcome of a tropical weather event in New Orleans is that Magnificent Frigatebirds and Black Terns will appear around the city. These occur on the coast in numbers, so they may have merely been displaced inland by tens of miles- though we don't really know from whence they came. Overall in the Eastern USA, Sooty Tern may be the most common truly pelagic species to be displaced inland after tropical systems make landfall, although it seems like just about anything is possible. Jaegers, shearwaters, petrels, storm-petrels, phalaropes, tropicbirds- have all been reported in the last few years somewhere in the eastern USA after tropical systems passed inland.
The best place to search for storm waifs is large bodies of water- such as Lake Pontchartrain. Birders saw a Great Shearwater from the Causeway after one recent storm. There is some indication that birds may also be found following the Mississippi River back out to sea up to several days after a storm, but this needs further study.
At this point it is not really clear how much of a correlation there is between numbers of waifs carried inland and strength of the storm. Birds appear to show up mostly on the track or to its east. While it seems reasonable to think some have become stuck in the eye (kept within it by their avoidance of the eyewall), and others merely pushed onshore by the winds, we don't really know.
Sometimes other weird phenomena accompany tropical weather. For instance, Hurricane Juan in 1985 lingered in late October on the northern Gulf, and resulted in unusual numbers of tardy land bird migrants occurring along the Gulf Coast- species that should have been in the tropics by then. Even flamingos have occurred along Gulf and Atlantic shorelines after tropical systems.
The widespread rain of a tropical system can in itself produce fallouts that are similar to those described in the rain events under # 2 above. Waterbirds and shorebirds are often in good numbers in flooded grassy fields during tropical weather. One fancy record I recall: Hurricane Opal in 1995 induced a Sabine's Gull to pause at Southshore Harbor on the New Orleans lakefront.
4. Extended drought
Less of a weather event
than a prolonged pattern
, extended dry periods produce low water levels in ponds and impoundments. This can produce extensive shorebird habitat, often otherwise hard to come by in the immediate New Orleans area. It can draw birds like a magnet. The place with the best track record in this regard in our area is the impoundment on Recovery Rd in Bayou Sauvage National Wildlife Refuge- especially the pond accessed by walking beyond the landfill mound, or via the levee from Chef Highway farther east. At such times this spot can have thousands and thousands of birds, ranging in size from herons to peeps.