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A Birder’s Eye View of the Seasons in the Lower Rio Grande ValleyWINTER
Resident bird species and winter residents are at least predictable in their presence or absence, but light breezes of change still whisper. Even as the those intrepid pioneers of the coming spring migration—the Purple Martins—begin to arrive at scattered locations, there are still some individuals of many species moving south to escape the most severe weather of the season at more northern latitudes. Bird movements that could be termed “migrations” go on each day of the year, though the first week or so of January and the last week of June represent dead low tide in the annual ebb and flow of bird populations. Most flocking waterfowl don’t ever really settle in to spend the winter in one particular spot, but constantly shift up and down the coast as food supplies become deleted in one area and rise in another. Most insect and seed eating landbirds either establish individual winter feeding territories join mixed species flocks that roam throughout a shared territory, or winter in flocks of their own kind, not forming territories, but moving about much like waterfowl, following the availability of their preferred foods. The mixed species flocks of wintering insectivorous birds—Blue-gray Gnatcatchers, Ruby-crowned Kinglets, Blue-headed Vireos, a pair of resident Black-crested Titmice, and various warbler species, are common in Rio Grande riparian woodlands and wooded residential areas in Valley towns. In the United States only southern Florida can compare with the lower Rio Grande Valley in the numbers and variety of wintering warblers. Regular wintering species include Black-throated Green, Yellow, Yellow-throated, Pine, Black-throated Gray, Black-and-white, Orange-crowned, and Nashville warblers, American Redstart, Ovenbird, Northern Waterthrush, and Common Yellowthroat. Mixed flocks of larger woodland birds—Green Jays, Altamira (and upriver along the Rio Grande, Audubon’s) orioles, also roam riparian woodland and mature thorn woodland, feeding on insects and small fruits (Anacua, Sugar Hackberry, Granjeno, Coma, and Coyotillo, as well as nectar and exotic fruits from ornamental plants in wooded residential areas). In the drier uplands flocks of Western Meadowlarks, Pyrrhuloxias, Lark Buntings, mixed sparrows (Lark, Savannah, Clay-colored, White-crowned, resident Black-throated, and Vesper) shift about looking for rich concentrations of seeds. A special aspect of the region in winter is the unpredictable presence of vagrant neotropical species from Mexico. Over the years some remarkable species have been discovered in various locations across the Valley, but Bentsen State Park has always had more than its share of rarities. This is partly due to the intense daily coverage it has received from birders. A complete list just for Bentsen would run too long, but some of the more notable finds have been Roadside Hawk, Short-tailed Hawk, Collared Forest-Falcon, Ruddy Quail-Dove, Mottled and Stygian Owls, Elegant Trogon, Dusky-capped Flycatcher, Masked Tityra, White-throated Robin, and Yellow-faced Grassquit. Some species like Clay-colored Robin that began as vagrants, have now become regular breeding species. Others, like Blue Bunting, show up nearly annually and may in time also become residents. In some years there seems to be an obvious correlation between environmental stress (severe cold periods or freezes, droughts, etc.) and the appearance of vagrants, but in other instances the influences are unclear. The circumstances of each occurrence are a little different, but a typical scenario is that a bird will be discovered in the late fall or early winter and will be present until middle March. Some years there are few or even no vagrants. In others there may be several. In the back of each birder’s mind at this season is “what next?” This is one of the special birding thrills unique to the Rio Grande Valley. SPRING
The lower Rio Grande Valley is the southernmost mainland area in the United States, and spring weather begins by late January or early February. Those plants which are deciduous begin to put out new foliage and many begin to flower by early February. Certain birds, like waterfowl and Yellow-rumped (Myrtle) Warblers, begin moving northward. Cave and Northern Rough-winged Swallows that both have small wintering populations in the area; arrive in numbers by late in the month. Other early arrivals are American Golden-Plovers, still in winter (basic) plumage. By March 1 Barn Swallows are inspecting their traditional nesting sites and within a few days the earliest of the warblers - Black-and-white, Yellow-throated, and Northern Parula - begin to show up in coastal thickets. These are soon followed by White-eyed Vireo and other swallows. There is a lot going on in spring. In addition to migration and the continuing presence of many wintering species, many resident species begin breeding activity. A few, like Great Horned Owl, Crested Caracara, and American Oystercatcher, may have eggs as early as January, but most landbirds do not begin laying until mid to late February. Black-crested Titmouse, Verdin, woodpeckers, and some wrens, are examples of early nesting landbirds. Most other species, whether resident or those that migrate from the region for the winter, begin nesting activities in April. Often, most species will have completed a nesting cycle before summer officially gets underway. South Texas is the main pathway for those species that migrate from the Neotropics around the Gulf of Mexico as opposed to straight north across it. This includes most of the long distance migrating raptors, which cannot get the lift they require for soaring flight (which uses little energy compared to powered flight) over water, and thus must migrate over land exclusively. Turkey Vultures are the first migrating raptors to show up in our area during early to mid March. Raptor migration gets underway in earnest during the last week in March, when Broad-winged Hawks begin to appear in large flocks. Broad-winged and Swainson’s Hawks, Turkey Vultures, and Mississippi Kites are long distance migrants, most originating in South America. These species move northward up the narrow Gulf coastal plain of eastern Mexico, sandwiched into a narrow corridor between the mountains and the sea, into southern Texas before dispersing broadly to the north of the Texas “Coastal Bend”. The resulting stream of flocks of hundreds of hawks and vultures, often accompanied by flocks of Anhinga which migrate in much the same manner, can be spectacular. Although birds from almost all families engage in migratory movements, Spring migration in the region has become synonymous with Trans-Gulf migration, the incredible flights of millions of colorful warblers, orioles, tanagers, buntings, and many other landbirds, that may cover a minimum of 500 miles and take 18 to 24 hours to accomplish, all over the open ocean with nowhere to put down in case of trouble. That this feat is undertaken by birds as tiny as the 4 gram (one eighth of an ounce) Ruby-throated Hummingbird makes it all the more astonishing. Birders, weary of the drabness of winter across the country, and ready for some colorful bird activity, eagerly await the famous “fall-outs” of spring, when tens of thousands of birds may be grounded on or near the Gulf coast by poor flying weather. Most evenings flights of land birds that have wintered in the tropics and moved to the northern coasts of Middle America, depart north and northwestward over the open Gulf waters. If weather conditions are favorable for northbound over-water flights, these birds may arrive over the coast of Texas as early as dawn of the next morning, but most will require more time to make the 500 – 800 mile crossing (depending on starting point) and will arrive from midday through the afternoon. Under these conditions most birds continue their flights inland until evening before landing, and the Texas coast may have very few migrants on the ground. However, if migrating birds run into bad weather, especially if it is a widespread event like the southward passage of a cold front with head winds and perhaps rain and turbulent conditions, their landfall may be greatly delayed, and when they do finally arrive their energy reserves have been spent so that they land at the first possible opportunity. Under these conditions the coastal areas may swarm with hundreds of brightly colored birds. It is sobering, however, to think of the losses that occur under such conditions. Thousands of birds may perish at sea, and others at the hands of predators, automobiles, and collision with manmade objects such as tall antennas, buildings, and power lines, as they arrive exhausted and disoriented. Yet, despite the hazards, Trans-Gulf migration persists as a successful strategy because those birds which do survive arrive earliest on the breeding grounds, and are able to choose the best territories, ensuring that their offspring will have the best chances for survival. SUMMER
Southbound “fall” migration begins in late June, even while some individuals are still northbound. Southbound Purple Martins and Long-billed Curlews appear by the last week in June. Orchard Orioles and Black-and-white Warblers are moving by the first of July. About the same time adult shorebirds appear, still in breeding (alternate) plumage. By the latter part of July shorebird migration is in full swing, still mostly composed of breeding plumaged adults. A little later juvenile shorebirds begin to arrive and adults that will winter locally have largely molted into nonbreeding (basic) plumage. Small flycatchers join Louisiana Waterthrushes and Yellow Warblers on their way south. By August many species are on the move. At night one can hear the mellow whistles of Upland Sandpipers and the buzzy calls of Dickcissals as they pass over in the darkness. Accurate statements about climate trends in far South Texas are difficult to make. This is a land of extremes where averages mean little. One of the more predictable patterns however, is the advent of tropical weather systems over the Atlantic, Caribbean, and Gulf of Mexico in late summer. These systems may have a tremendous or almost no effect on local weather patterns in any given season. Drought-breaking rains after a long dry summer are one of the more common (and hoped for) effects. Few years pass without at least some peripheral effects from tropical depressions, storms, or hurricanes. Direct hits by major hurricanes are rare, but when they do occur, the effects upon birdlife, both in the short and long terms, are profound. The birds most directly affected by tropical weather systems are seabirds. It is common after the passage of tropical storms or hurricanes to find exhausted or dead seabirds far inland. But migrating land birds are also affected. When long distance migrants encounter a tropical cyclone those that do not land in a sheltered situation may be killed by the violent weather conditions. But the larger impacts of these storms on landbirds are often indirect and long term. These come in the form of habitat alteration. Critical habitat near the coast is often destroyed outright, and inland habitats may be altered by wind damage to vegetation or by flooding. In a chronically drought prone region like the lower Rio Grande Valley heavy tropical rainfall may have very positive impacts on habitats, and by extension to the birds that live in them. FALL
Landbirds including warblers, swallows, orioles, buntings, cuckoos, and flycatchers pass through in peak numbers and diversity in September. Fall migration in this region differs a bit from spring migration in that more of the species are on a southwest trajectory that takes them around the Gulf. A very special example of this phenomenon is that of the Ruby-throated Hummingbird. The migration peak of this species is during the first week in September and hundreds may be concentrated at favorable feeding locations around this time. Those species that are Trans-Gulf Migrants in both tend to cross the Gulf farther east in fall than they do in spring and hence are uncommon to rare on the lower Texas coast at this season. Some over water migration does depart from our coastline in fall but it tends to occur later in the season with the beginning of the passage of cold fronts strong enough to penetrate to the southern Gulf of Mexico. By early October fall migration has largely shifted from those species which are passing through toward the tropical wintering areas to arriving winter residents. The first real cold fronts often bring early arrivals like Ruby-crowned Kinglet, Orange-crowned Warbler, Eastern Phoebe, House Wren, and Lincoln’s Sparrow. Greater White-fronted Geese, American White Pelicans, and Sandhill Cranes, followed by rapidly increasing duck populations mark waterbird activity during this period. The arriving of winter residents continues well into November, and even into December, but after early November most passage migrants, that is, birds headed for points south, is primarily over. The hardier species like Western Meadowlarks, many sparrows, more northerly breeding water birds like Common Loon continue arriving in November. By the end of the month most winter residents are settled into their quarters and migration has trailed off to a small trickle. Thus, the annual cycle is completed. The resulting avifauna in deep South Texas is the most diverse north of the Mexican border. The four county area that makes up the lower Rio Grande Valley has recorded just at 500 species of birds. This is more species of birds than have been recorded in all but two or three entire states. Small wonder that birders from across the continent make pilgrimages to the region. John C. Arvin *Subject's current title as of July 2006 is: |
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The New Year finds the Lower Rio Grande Valley at the southern tip of Texas in mid winter, potentially the coldest few weeks of the year. “Potentially” because winter here is not a season per se, but a series of Fall-through-Spring-like transitions compressed into about ten day cycles that recur from late November through mid February with the passage of periodic cold fronts and the usually rapid warm ups that follow them. Freezing temperatures during this period are possible, but are very rare, and most of the region is free from killing frosts.
Spring, as defined in birding terms, is the period of the year when birds are moving northward from warmer areas where they have passed the winter, to their breeding grounds. This process begins in mid winter and continues through May and even into June, though the peak movement of most species is between late March and early May. As we noted in the winter summary, species like Purple Martins may begin to arrive shortly after the first of the year, but most species that winter in the Neotropics (that is, the tropical parts of the Americas) do not leave their winter homes until conditions on their breeding grounds will be suitable for their survival once they arrive. This is between mid April and mid May for most species that breed in the northern parts of North America.
Every Valley resident agrees on one thing: summers are long and they are hot. As we mentioned in the spring account, most birds actually complete one nesting cycle before June gets underway. Summer from a birding perspective is not just about weather, as summer-like conditions may begin in mid April and continue through mid October. Two major life conditions for birds occur in summer – breeding and “fall” migration. While many birds may have completed one breeding cycle by early June, not all do or do not do so successfully. Some species may breed more than once in a season and others may renest after a breeding failure from one cause or another. Other species may wait to initiate breeding if conditions are unfavorable early in the season. This is especially true in drought years (all too common in our area). If very dry conditions exist in spring and early summer some birds may breed after the more predictable rains of the late summer tropical weather season.
As we have seen above, fall as a birding season actually begins before mid summer. By the calendar, when fall officially begins in September, the southbound migration has reached its peak. Migratory birds from all groups are moving in numbers. Among raptors the first species to show large movements is Mississippi Kite, which begins migration in August and whose passage is largely finished in our area by the end of September. About the time the kites are winding down Broad-winged Hawks are just beginning to appear and numbers can be staggering. Daily counts of 5 figures are common during this period. As Broad-winged Hawks begin to decline, Swainson’s Hawks and Turkey Vultures dominate the migration through October. American Kestrels begin to show up in early September and remain to be common winter residents.