Population Trends of Colonial Waterbirds in Galveston Bay




McFarlane, Robert W.

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Texas Natural Resource Conservation Commission


The Texas Colonial Waterbird Census data for the 26 year period 1973 to 1998 were analyzed in search of population trends for the 22 species of colonial waterbirds nesting within the Galveston Bay ecosystem. The minimum, average, and maximum number of breeding pairs for each species is presented. The trends are graphed in the accompanying figures for each species. A trend line has been incorporated into each graph to aid interpretation but the reader is cautioned that colonial waterbird data seldom meet the inherent assumptions associated with linear regression trend analysis (independence, linearity, normal distribution, and homoscedasticity of variance). Trend lines near the horizontal indicate a stable, albeit fluctuating, population. Year-to-year variation is rather extreme for most species and breeding was absent in some years for some species. The total population for each species is double the pair number plus an unknown, but substantial, number of nonbreeding individuals. Ten species (45 percent) appear to have stable populations. Eight species (36 percent) appear to have increased in abundance to some extent. Only 4 species (18 percent) exhibit evidence of population decline.


pgs. 170-171