Graduate Thesis Or Dissertation
 

Post-hatching survival and productivity of American avocets at drainwater evaporation ponds in the Tulare Basin, California

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  • Evaporation ponds that dispose of agricultural drainwater provide attractive habitat for waterbirds but may result in contaminant exposure that impairs reproduction. I estimated post-hatching survival and evaluated productivity estimates for American avocets (Recurvirostra americana) nesting at evaporation ponds in the Tulare Basin, California from 1991 to 1993. Avocets and Black-necked stilts (Himantopus mexicanus) comprised the majority of 5,969 nests found. Nest success and embryo mortality varied among ponds and between species. Embryo deformity and mortality rates associated with selenium were higher in stilts. The majority of reproductive losses were attributed to predation, especially at ponds without islands. Post-hatch survival of radiomarked avocets after 5 weeks ranged from 0.375-0.729 and differed among ponds (log-rank X²=7.12, 2 df, P=0.028). Predation accounted for 55.4% (36/65) of known mortalities. Depredated chicks ranged from 1-30 days of age with a median age of 6 days. Arsenic concentrations in avocet livers were below detection limits while boron concentrations were below detection limits at Westlake-South (WLS) and Tulare Lake Drainage District-South (TLDD-S) in 1993. Mean boron concentration in livers was 3.7 ppm at TLDD-S in 1992 and 22.9 ppm at Lost Hills Water District (LHWD). Liver selenium concentrations increased with age and differed among ponds (P=0.0001). Mercury concentrations differed among ponds (P=0.0001), but were similar to background. Growth, survival and selenium exposure were consistent to support post-hatching effects of selenium. Mortality in avocet chicks was highest at LHWD, intermediate at TLDD-S and lowest at WLS, consistent with selenium exposures. I observed lower survival, histologic lesions, lower hatchling weights, slower growth, and elevated selenium and boron in tissue at LHWD. I used capture-recapture analysis also to estimate post-hatching survival and calculated avocet productivity. Survival estimates for the first 4 weeks post-hatching, based on model averaging, ranged from 0.572 to 0.751. Productivity at some ponds was insufficient to offset adult mortality. Estimates of average annual productivity were 0.49 and 0.45 in 1992 and 1993, respectively. Juvenile survival rates necessary for recruitment to offset 10% and 20% adult mortality ranged from 0.204 to 0.408 and 0.222 to 0.444, respectively.
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