title: It's what's inside that counts: egg contaminant concentrations are influenced by estimates of egg density, egg volume, and fresh egg mass.
abstract: In egg contaminant studies, it is necessary to calculate egg contaminant concentrations on a fresh wet weight basis and this requires accurate estimates of egg density and egg volume. We show that the inclusion or exclusion of the eggshell can influence egg contaminant concentrations, and we provide estimates of egg density (both with and without the eggshell) and egg-shape coefficients (used to estimate egg volume from egg morphometrics) for American avocet (Recurvirostra americana), black-necked stilt (Himantopus mexicanus), and Forster's tern (Sterna forsteri). Egg densities (g/cm(3)) estimated for whole eggs (1.056 ± 0.003) were higher than egg densities estimated for egg contents (1.024 ± 0.001), and were 1.059 ± 0.001 and 1.025 ± 0.001 for avocets, 1.056 ± 0.001 and 1.023 ± 0.001 for stilts, and 1.053 ± 0.002 and 1.025 ± 0.002 for terns. The egg-shape coefficients for egg volume (K v ) and egg mass (K w ) also differed depending on whether the eggshell was included (K v  = 0.491 ± 0.001; K w  = 0.518 ± 0.001) or excluded (K v  = 0.493 ± 0.001; K w  = 0.505 ± 0.001), and varied among species. Although egg contaminant concentrations are rarely meant to include the eggshell, we show that the typical inclusion of the eggshell in egg density and egg volume estimates results in egg contaminant concentrations being underestimated by 6-13 %. Our results demonstrate that the inclusion of the eggshell significantly influences estimates of egg density, egg volume, and fresh egg mass, which leads to egg contaminant concentrations that are biased low. We suggest that egg contaminant concentrations be calculated on a fresh wet weight basis using only internal egg-content densities, volumes, and masses appropriate for the species. For the three waterbirds in our study, these corrected coefficients are 1.024 ± 0.001 for egg density, 0.493 ± 0.001 for K v , and 0.505 ± 0.001 for K w .
authors.affiliation: Western Ecological Research Center, Dixon Field Station, U.S. Geological Survey, 800 Business Park Drive, Suite D, Dixon, CA, 95620, USA. mherzog@usgs.gov.
authors.forename: Mark P
authors.initials: MP
authors.lastname: Herzog
authors.affiliation: Western Ecological Research Center, Dixon Field Station, U.S. Geological Survey, 800 Business Park Drive, Suite D, Dixon, CA, 95620, USA.
authors.forename: Joshua T
authors.initials: JT
authors.lastname: Ackerman
authors.affiliation: Forest and Rangeland Ecosystem Science Center, U.S. Geological Survey, 3200 SW Jefferson Way, Corvallis, OR, 97331, USA.
authors.forename: Collin A
authors.initials: CA
authors.lastname: Eagles-Smith
authors.affiliation: Western Ecological Research Center, Dixon Field Station, U.S. Geological Survey, 800 Business Park Drive, Suite D, Dixon, CA, 95620, USA.
authors.forename: C Alex
authors.initials: CA
authors.lastname: Hartman
mesh_terms: D000818:Animals; D046091:Charadriiformes; D004528:Egg Shell / Q000737:chemistry*; D004784:Environmental Monitoring; D004785:Environmental Pollutants / Q000032:analysis*; D010063:Ovum / Q000737:chemistry*
chemical_list: D004785:Environmental Pollutants
keywords: Egg contaminants; Egg density; Egg volume; Egg-shape coefficients; Eggshell; Fresh wet weight