DDE Still High in White-Faced Ibis Eggs from Carson Lake, Nevada

Abstract

-White-faced Ibis (Plegadis chihi) eggs collected in 1996 at Carson Lake, Nevada, showed no decrease in p,p'-DDE (DDE) concentrations from levels in 1985 and 1986 which is contrary to DDE patterns shown for most avian species. An estimated 40-45% of the population was adversely affected by DDE in 1985, 1986, and 1996 with a probable net loss of about 20% of the expected productivity. One segment of the nesting population at Carson Lake in 1996 averaged 18.3% eggshell thinning, although the mean for the whole population is not known. Obvious population declines of White-faced Ibis have not been reported, although quantitative population data are incomplete; however, the excellent and predictable food sources on the breeding grounds in Nevada (due to flood irrigation by farmers) appear to contribute to a high reproductive potential. While adverse consequences of DDE were not documented at the population level, it is important to locate the source(s) of the DDE/p,p'-DDT (DDT). Results of a previous study showed that prey from the breeding grounds were not contaminated with DDE/DDT. The White-faced Ibis DDE/DDT loads are suspected to originate from localized areas on the wintering grounds and/or staging areas. Use of satellite transmitters placed on nesting birds is proposed to locate the source of the DDE/DDT. Received 27 May 1997, accepted 23 August 1997, final revision received 8 September 1997.