DIRECT AND INDIRECT EFFECTS OF INSECTICIDE APPLICATIONS ON GROWTH ANDSURVIVAL OF NESTLING PASSERINES

We conducted an experiment in 1989 and 1990 to determine the effects o f malathion ultra-low volume (ULV) aerial spray on reproduction of pas serine birds in shrub-steppe habitat of southern Idaho. The experiment consisted of reducing the insect food base with malathion, a broad-sp ectrum insecticide, while monitoring nestling growth variables and dai ly nest survivorship. Relative arthropod abundance and biomass were re duced on the treatment plot in 1989 and 1990 (F. P. Howe et al., unpub lished manuscript). Wing and tarsus lengths of nestling Sage Thrashers (Oreoscoptes montanus) were shorter on the treatment than on the cont rol plot after malathion application in 1989 but not in 1990. Nestling Brewer's Sparrow (Spizella breweri) wing length and mass were lower o n the treatment plot in 1990 but not in 1989. Primary length and foot length were unaffected for either species in either year. Multivariate ANOVA indicated that nestling Brewer's Sparrow ''size'' (a combinatio n of five growth variables) was smaller on the treatment than on the c ontrol site after the 1990 treatment, but similar between sites after the 1989 malathion application. Sage Thrasher nestling ''size'' was sm aller on the posttreatment malathion plot in 1989, but similar between plots in 1990. The percentage of eggs hatched and percentage of nestl ings hedged were unaffected for either species. No differences were de tected in Mayfield estimates of nest survivorship for either species i n either year, The mean number of Sage Thrashers fledged per nest atte mpt was Lower on the treatment than on the control plot in 1989, but n ot in 1990. The mean number of Brew er's Sparrows fledged per nest att empt was not affected on the treatment plot. No adult mortality for ei ther species was observed, and nestling mortalities could not be attri buted to malathion. Thus, few differences were detected in nestling gr owth variables and nest survivorship between the control and treatment plots, and these differences were not consistent across years. Under the conditions of this study, malathion had no observable direct effec ts, and only marginal indirect effects, through food-base reduction, o n Brewer's Sparrow and Sage Thrasher nestling growth and survival.