H. H. Hildebrand | USGS Science

USGS1978

Shell thinning and pesticide residues in Texas aquatic bird eggs, 1970

Significant decreases in eggshell thickness were found in 15 of 22 species of aquatic birds in Texas in 1970. Shell thickness reductions of 9 to 15 percent were found in white pelicans (Pelecanus erythrorhynchos), brown pelicans (P .occidentalis), and great blue herons (Ardea herodias). DDT family compounds were found in all eggs, and mean residues ranged from 0.4 ppm in white ibis (Eudocimus albus) to 23.2 ppm in great egrets (Casmerodius albus). GDDT residues were negatively correlated with shell thickness in five species; PCBs were negatively correlated in two. Residues in marine birds were generally lower and more uniform than levels in birds feeding in fresh and brackish water. DDT and dieldrin residues were higher in eggs from colonies near agricultural areas where these insecticides were heavily used; higher PCB residues were consistently associated with urban and industrial areas. Populations of five species have declined and deserve continued study: brown pelican, reddish egret (Dichromanassa rufescens), white-faced ibis (Plegadis chihi), laughing gull (Larus atricilla), and Forster's tern (Sterna forsteri). Population trends of four other species were undetermined and should be followed closely in future years.

Pesticides Monitoring Journal↗

USGS1977

The decline of brown pelicans on the Louisiana and Texas Gulf Coast

Before 1920, native populations of brown pelicans (Pelecanus occidentalis carolinensis) were estimated at 50,000 to 85,000 birds in Louisiana and 5,000 in Texas. By 1958, brown pelicans had vanished from two wintering areas in each state, and wintering and breeding birds disappeared from additional areas between 1959 and 1961. By 1963, the disappearance of the brown pelican from Louisiana was complete, and in Texas only about 100 birds were observed annually. Mortality before 1939, particularly in Texas, was attributed mostly to hunters and fishermen. Losses in both states were attributed to hurricanes, disease, and extended periods of freezing weather. Pesticides may also have contributed significantly. Widespread endrin-caused fish mortality was documented between 1958 and 1964 in Louisiana. Pelicans undoubtedly were exposed to endrine through eating the fish. Significant levels of DDE were found in Texas fishes in the late 1960's. DDE residues in pelican eggs were high enough to be responsible for nesting failure. Shell thinning averaged 10% in Texas pelican eggs collected in 1970, 1972, and 1974, but in earlier years greater thinning was associated with the collapse and loss of eggs. Poor reproductive success during recent years was believed due to (1) failure of many adult pelicans to nest and (2) a high rate of nestling mortality in the few pairs that did nest and hatch young. The remnant population of brown pelicans on the Texas Coast will disappear unless reproductive success improves, because recruitment is presently insufficient to offset mortality.

Louisiana, Texas↗