A single egg from a clutch of four or five eggs was collected from dipper nests throughout the watershed in 1999, 2000 and 2001. Viable and non-viable eggs were selected at random from each of 33 different clutches from resident dippers on the main river (n = 17) and from migrants on the tributaries (n = 16). Considerable variation in contaminant levels occurs within and between clutches allowing for equal treatment of eggs regardless of their position in the clutch or in repeat clutches (C.A. Morrissey, unpublished data). Whole eggs were refrigerated for up to 4 weeks. Egg contents were then transferred into an acetone : hexane-rinsed glass jar to be frozen at −25 °C until analysis.
Chemical analyses of eggs were carried out at the National Wildlife Research Centre (NWRC), Hull, Quebec, Canada. OC analyses included determination of chlorobenzenes (tetrachlorobenzene, pentachlorobenzene, hexachlorobenzes), hexachlorocyclohexanes (α-, β-and γ-HCH), chlordane-related compounds (oxychlordane, trans-chlordane, cis-chlordane, trans-nonachlor, cis-nonachlor and heptachlor epoxide), DDT and metabolites (p,p′-DDE and p,p′-DDD), mirex and photomirex, and dieldrin. Total PCB were calculated by summing the peaks of 62 individual congeners identified. Samples were analysed quantitatively by capillary gas chromatography coupled with a mass selective detector, operated in selected ion monitoring mode according to CWS method no. MET-CHEM-OC-04B (Won, Mulvihill & Wakeford 2000). Briefly, samples underwent neutral extraction with 1 : 1 dichloromethane (DCM) : hexane after dehydration with anhydrous sodium sulphate, removal of lipids and biogenic materials by gel permeation chromatography, and further cleanup by Florisil (Floridin Co., Quincy, FL, USA) column chromatography. All samples were spiked with internally labelled 13C standards prior to extraction. Each sample extract was injected twice, once for determination of OC and once for PCB. As part of the quality control, blanks and CWS reference material (1989 Lake Ontario Herring Gull QA) were run concurrently. The nominal detection limit for all compounds was 0·0001 µg g wet weight−1. Internal standard recoveries were typically between 80% and 110% and residues were not recovery corrected. Egg concentrations are reported on a wet weight basis (Peakall & Gilman 1979) with arithmetic corrections of desiccated samples that deviated by more than 5% from the mean moisture content (78·6%) of freshly collected, undeveloped eggs.
Total Hg and Se were analysed according to CWS method no. MET-CHEM-AA-02E (Neugebauer, Sans Cartier & Wakeford 2000). The sample homogenates were freeze-dried to determine moisture content and analysed for Hg without prior acid digestion on the AMA-254, Advanced Mercury Analyser, which employs direct combustion of the sample in an oxygen-rich atmosphere. Samples for which there was sufficient material remaining were digested in nitric acid according to standard techniques for Se analysis. Se was analysed by graphite furnace atomic absorption spectrometry (GFAAS) using a Perkin Elmer 3030b equipped with a Deuterium background corrector and HGA-300 Graphite furnace. Accuracy of analysis was determined using certified reference materials Dolt-2 and Dorm-2 (National Research Council of Canada, Ottawa, ON, Canada) and blank samples. Recoveries of reference materials were within the certified range (95–121%). Additionally, random egg samples were analysed in duplicate to check precision. All values for Hg and Se are reported on a dry weight basis and detection limits under these conditions were 0·18 µg g dry weight−1 for Hg and 0·10 µg g dry weight−1 for Se.