Ecological Analysis and Population Estimates of the Birds of the Atchafalaya Basin in Louisiana.

Monthly population estimates and data on ecological relation­ ships of birds were obtained in the three main forest overstory types— bottomland hardwood (BH), cottonwood-willow-sycamore (CWS), and cypress-tupelo (CT)— in the Atchafalaya River Basin in south central Louisiana from September 1973 to August 1976. Woodland birds were censused on study plots, raptors by aerial surveys and road counts, and owls by automobile and boat survey routes at night. Individual bird species and communities in which they reside were analyzed in the light of recent concepts of niche and competition theory with concentration on three components of each species niche and of the community structure: l) a ’'horizontal'' or between-habitat component dealing with the three overstory types; 2) a "vertical’’ or within-habitat component dealing with three vertical levels within the forest; 3) & temporal component dealing with changes that occur within and between overstory types seasonally and over the 12 months of the year. Habitat (%) and vertical (By) niche breadths for species, as well as habitat (Cg), vertical (Cy), and temporal (Cqj) overlap for pairs of species, were computed to study the coexistence mechanisms between congeners and members of closely related genera. Bird species diversity (BSD), bird species richness, bird species abundance, plant species diversity (PSD) and foliage density all increased from the youngest serai stage, CT, to the oldest serai stage, BH. BSD and PSD were highly correlated (r = 0.9255) as were bird species richness and foliage density (r = 0.8807). The relation­ ship between BSD and PSD was contrary to reports of investigators elsewhere, who found no correlation between the two. The number of species preferring certain forest levels was found to vary proportionally with the amounts of foliage in those layers. Foliage height diversity (FHD) was not correlated with BSD probably because the three vertical levels within each forest type (i.e. those used to calculate FHD) contained similar proportions of foliage. This finding was also contrary to many previous reports. In the Basin, the structure of the avian communities seems to be dependent upon foliage density and complexity of vegetation in the overstory types. The number of species increased and BSD decreased in all forest types from summer to winter. The number of individuals increased dramatically in BH and CWS from summer to winter primarily because of the influx of wintering Yellow-rumped Warblers and Whitethroated Sparrows. The number of individuals did not vary much seasonally in CT. BH contained 86 species of birds; CWS, 8lf; and CT, 7k. BH and CWS shared 77.1 percent of their species and had a community overlap value (C) of 0.79*+; BH and CT shared 6k.9 percent and had a C of 0.483; and CWS and CT shared 75.5 percent and had a C of O.69I. Of the 102 species recorded in the three forest types 71 were noted 10 times or more. Among these 71 species, 26 preferred BH, l̂f preferred CT, 9 preferred CWS, and 22 showed no preference. Vertical distribution of birds differed between habitats and seasonally. In BH, more birds used Level 1 (0.0-0.61 m) than in CWS or CT probably as a result of the development of the ground vegetation and the number of wintering White-throated Sparrows in this layer. Annual flooding in CT inhibits the use of the lower forest levels by understory species of birds. More birds used Level 1 in winter than at any other season, primarily a result of the huge wintering populations of White-throated Sparrows. Of the 86 genera recorded in my study, only 17 contained 2 or more species. Competition between congeners and between members of closely related genera was avoided or reduced in all but two cases (Picoides and Parus) by one or more of the following strategies listed in descending order of importance: 1) a potential competitor being rare or occurring only during migration; 2 ) differences in habitat usage; 3) reduced temporal overlap; U) reduced vertical level overlap. Changes in niche breadths as the competitive environment shifted seasonally were noted in only five species (Downy and Hairy Woodpeckers, Carolina Wren, Blue-gray Gnatcatcher, and Cardinal). The wren and the Cardinal apparently responded to shifting food resources, the woodpeckers and the gnatcatcher to the presence (or absence) of competitors. Most bird species in the Atchafalaya River Basin were habitat generalists and vertical or within-habitat specialists. These results suggest that competition for within-habitat resources is stronger than competition for habitats.