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Bird Migration
by Darrell Vollert |
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miracles in nature. Millions of neotropical migrant birds migrate through Texas each fall and spring. These are bird species which winter in Latin America and nest in temperate zone of North America. Included in this group of birds are shorebirds, raptors, cuckoos, nightjars, hummingbirds, flycatchers, vireos, swallows, thrushes, wood warblers, grosbeaks, buntings, and orioles. They are among the most colorful birds which nest in the United States. A number of these migrant birds circum-vent the Gulf of Mexico during their southbound and northbound flight, while many others make the much more treacherous journey across the gulf. In order to make this nonstop flight north across the gulf, neotropical migrants gorge on protein-rich fruit and insects almost 24/7 to build their fat reserves. At dusk on days in the spring when weather conditions are favorable(clear skies) neotropical migrants strike out singularly or in small, loose flocks over the Gulf of Mexico heading north to the gulf states. Many depart from the tip of the Yucatan Peninsula, while others take flight north from Central America and even northern South America. Migrants prefer to make this flight across at night when the temperature is cooler and the humidity is higher allowing moisture to flow through their lungs. They navigate across the gulf using the North Star and the earth's magnetic field. If conditions remain favorable throughout the flight across the Gulf of Mexico small migrants such as warblers will arrive at the coastline around 2pm the following day. Larger migrants like thrushes will arrive around 11am to noon. Shorebirds and raptors prefer to migrate during the daylight hours over land rather than over open water.
and land as far as one hundred miles inland. A strong late season cold front associated with rain over the gulf is a neotropical migrant's worst nightmare. It is one thing to fly into the face of a strong head wind. It's another thing to fly with rain pelting their bodies. A bird's instinct is to come out of the sky when it is raining, but what is a bird to do when they are flying over open water? They will dip down and fly just above the surface of the water. Some will be overtaken by waves.Others will run out of energy and perish in the gulf. After a strong spring cold front millions of tiny birds will wash ashore. Many of these dead birds have completely metastasized their flight muscles. It is a form of self-cannibilization. The keel of the breast bone is protruding through the skin. Only the most physically fit survive the flight across during these weather conditions. It is nature's way of weeding out the genetically and physically weak. A weather event such as this produces what is known as a fall out. Exhausted birds literally fall out of the sky when they reach land during these weather condtions. Coastal oak-hackberry mottes and cheniers and mulberry trees are a neotropical migrant's saving grace. They provide shelter and foraging habitat for tired migrants. A single bush along the coast line can look like a living Christmas tree during a fall out. Colorful wood warblers, tanagers, and orioles adorn trees and shrubs like Christmas ornaments during fallouts. Not only do these tired birds have to survive the elements, but also lurking raptors. Critical habitat for migrant shorebirds and land birds can be found at national wildlife refuges, state parks, Houston Audubon sanctuaries, and Texas Ornithological Society sanctuaries along the coastline.
out from the coastline for points farther north. Flying over land, migrants will also use familiar landmarks for navigation along with the previously mentioned North Star and magnetic field tools. During spring migration neotropical migrants are on a tight time budget to reach their nesting grounds and set up a territory to defend. They will reach their nesting grounds in a matter of days once they reach the coastline.
migrants migrate through Texas the last two weeks of April and the first week of May. One might ask why would they risk their lives to make such a treacherous flight across the Gulf of Mexico? Why not stay year-round in the tropics? The answer lies in the billions of insects which emerge in the spring in the temperate zone from their winter slumber. Migrants take advantage of a literal buffet of insects to sustain themselves through the nesting season. Extended daylight hours in the United States and Canada during the spring and summer months allow neotropical migrants to feed hungry young for longer periods of time during the day. Shorebirds nesting on the Arctic tundra can feed young in almost twenty-four hours of daylight. Plus, there are relatively fewer native bird species in the temperate zone to compete with for food during the nesting season.
begins in early July with the arrival of shorebirds along the coastline. At times a representation of almost every shorebird species can be found at Houston Audubon Society's Bolivar Flats in late summer. Fall migrants continue to trickle through until mid November. Migrants will join their tropical cousins until it is time to begin the risky migration northward again to sustain the species existence. |