Bird Migration
by Darrell Vollert

Bird migration, particularly trans-gulf migration across the Gulf of Mexico, is one of the great

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.


When winds are strong out of the south and southeast, migrants will bypass the coastline

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.


Once neotropical migrant landbirds have replenished their fat reserves sufficiently they strike

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.


Spring migration begins in early March and continues into early June. The bulk of the neotropical

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.


Fall migration is more prolonged. There is not that immediacy to reach a destination. Fall migration

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