their departure from the northern coast of Yucatan 

 (fig. 4-42). Most birds depart in the hours before mid- 

 night and, owing to the 772 km (480 mi) distance 

 of the Gulf crossing, are still over water at dawn and 

 must continue flying until they reach land. Figure 

 4-43 shows the hour to hour change in densities of 

 arriving trans-gulf migrants on the northern Gulf 

 coast under conditions of moderate southerly winds, 

 the most frequent condition during the peak period 

 of spring migration. Peak arrival time may be shifted 

 to an earlier hour when southerly winds are stronger 

 and may be delayed when winds are northerly 

 (Gauthreaux 1971). 



14 



12 



o 



O10 



o 



- 8 



01 6 



.? 4 



8 9 10 11 12 1 2 3 4 



Hour o( the night |c s t I 



Figure 4-42. Fhght densities at different departure 

 times for land birds that migrate from 

 the northern coast of Yucatan to 

 southern Louisiana. 



100 - 

 w 90 



■t 80 

 < 70 



(A 



■D 60 



" 50 



o 



«i 40 



01 



" 30 

 c 

 S 20 



} 10 



in 



ill I III II I 



11 13 15 17 19 



Time of Arrival (est) 



21 



Figure 4-43. Percentage of land birds arriving in 

 southern Louisiana from the coast of 

 Yucatan during different hours of the 

 day. 



In spite of the rigors of a Gulf crossing, most 

 trans-gulf migrants do not usually alight on the first 

 available land, but overfiy the coastal marshes and put 

 down in an extensive forested zone about 56 km (35 

 mi) north of the coastline in the Chenier Plain (Lowery 

 1945). Radar studies by Gauthreaux (m Mclntire et al. 

 1975) revealed that in fair weather with southerly 

 winds only about 10% of the migrants landed south 

 of Lake Charles. However, under conditions of strong 

 northerly winds and thunderstorms that often accom- 

 pany a spring frontal passage, as many as 60 to 80% 

 of individuals in a trans-gulf flight might alight in 

 isolated coastal woodlands such as are found on 

 cheniers. At such times these woodlands temporarily 

 support tremendous densities of land birds, and the 

 presence of these areas may be extremely important 

 to the survival of many individuals. 



Orientation patterns of all migrants were studied 

 by Able (1972) who utilized weather radar facilities 

 at Lake Charles. He found that trans-gulf migrations 

 per se were relatively infrequent during the fall migra- 

 tion season (August, September, October). Instead, 

 flights were oriented to the southwest, along the 

 Louisiana and Texas coasts. These coastal flights can 

 result in concentrations of land birds in small wood- 

 lands on coastal ridges as great as, or greater than, 

 those resulting during the spring trans-gulf migrations. 



4.14 UPLAND FOREST HABITAT 



The upland forest habitat in the Chenier Plain is 

 usually designated as a pine and hardwood forest. 

 This name is synonomous with the term "loblolly 

 pine- shortleaf pine type" described by Walker (1962). 

 The pine and hardwood community occurs on acidic 

 soil that is primarily clay or loam. This forest also 

 colonizes disturbed soils and disposal areas (Parker 

 et al. 1975). It can be cultivated on suitable sites and 

 managed for specific tree species. The faunal compo- 

 sition of this forest is altered by changes in the age 

 classes and species composition of plants. Upland 

 forest habitat occurs extensively in Texas and Louisi- 

 ana, but it occupies only about 1.7% of the Chenier 

 Plain ecosystem (fig. 4-44). 



4.14.1 A FUNCTIONAL OVERVIEW OF THE UP- 

 LAND FOREST HABITAT 



The forest community (fig. 4-45) is characterized 

 by a high biomass of standing vegetation and surface 

 litter material (2 on fig. 445), which provides refuge 

 (8) for many animals. The contribution of organic 

 matter (6) to bordering wetlands (3) is probably low 

 and is influenced by rainfall (9), which exports litter 

 and nutrients from the forest community. Gross 

 primary production and the synthesis of vegetational 

 structure (2) is a function of solar energy (1) and 

 nutrient inputs (4) from the atmosphere and the 

 earth's crust. Plants (2 and 6) are consumed by 

 herbivores or incorporated into soil litter with its 

 associated decomposers. These first level consumers 

 (herbivores) are in turn utilized by higher level con- 

 sumers (carnivores), many of which move between 

 upland and wetland areas. Adverse conditions (5) 



218 



