Bird Mercury Concentrations Change Rapidly as Chicks Age: Toxicological Risk is Highest at Hatching and Fledging

Abstract

Toxicological risk of methylmercury exposure to juvenile birds is complex due to the highly transient nature of mercury concentrations as chicks age. We examined total mercury and methylmercury concentrations in blood, liver, kidney, muscle, and feathers of 111 Forster's tern (Sterna forsteri), 69 black-necked stilt (Himantopus mexicanus), and 43 American avocet (Recurvirostra americana) chicks as they aged from hatching through postfledging at wetlands that had either low or high mercury contamination in San Francisco Bay, California. For each waterbird species, internal tissue, and wetland, total mercury and methylmercury concentrations changed rapidly as chicks aged and exhibited a quadratic, U-shaped pattern from hatching through postfledging. Mercury concentrations were highest immediately after hatching, due to maternally deposited mercury in eggs, then rapidly declined as chicks aged and diluted their mercury body burden through growth in size and mercury depuration into growing feathers. Mercury concentrations then increased during fledging when mass gain and feather growth slowed, while chicks continued to acquire dietary mercury. In contrast to mercury in internal tissues, mercury concentrations in chick feathers were highly variable and declined linearly with age. For 58 recaptured Forster's tern chicks, the proportional change in blood mercury concentration was negatively related to the proportional change in body mass, but not to the amount of feathers or wing length. Thus, mercury concentrations declined more in chicks that gained more mass between sampling events. The U-shaped pattern of mercury concentrations from hatching to fledging indicates that juvenile birds may be at highest risk to methylmercury toxicity shortly after hatching when maternally deposited mercury concentrations are still high and again after fledging when opportunities for mass dilution and mercury excretion into feathers are limited.

Lead, cadmium and mercury in the blood of the blue-footed booby (Sula nebouxii) from the coast of Sinaloa, Gulf of California, Mexico

“…The low Hg concentration in chicks suggests that the growing and molting process can dilute and eliminate Hg burdens in chicks (Ackerman et al, 2011;Condon and Cristol, 2009;Wayland et al, 2002). Blue-footed Booby chicks went from 50 g in recently hatched chicks to as far as 2100 g when they were sampled, and they had already completed the juvenile plumage molt, this implies the growth of down and replacement by juvenile plumage.…”

“…Then, intersexual differences in heavy metals have been documented in several species, but without a homogeneous pattern, and possibly with several mechanisms involved. In chicks, heavy metals can experience a dilution in blood as a consequence of gaining weight (Ackerman et al, 2011;Wayland et al, 2002), eliminating metals through feathers (Condon and Cristol, 2009), or being fed on smaller prey than adults that contain lower heavy metal concentrations (Evers et al, 2005).…”

Marine foraging ecology influences mercury bioaccumulation in deep-diving northern elephant seals

“…Lastly, the increased mass and body condition (i.e. growth dilution effect due to increase in body mass) of seals at the end of the long foraging trip, in preparation for the extended lactation and fasting period associated with breeding, probably reduces mercury concentrations in internal tissues because changes in body condition can influence mercury concentrations in vertebrates [51,52]. Thus, the higher mercury concentrations we observed in seal blood and muscle after the short foraging trip (about 150 days shorter than the long foraging trip) were probably caused by decreased body mass (mass approx.…”

Avian mercury exposure and toxicological risk across western North America: A synthesis

“…Third, data were included only for eggs, adult tissues, or post-fledged juveniles. Samples collected from pre-fledged juveniles were excluded, because chicks undergo rapid changes in mercury concentrations in internal tissues as they grow and age (Ackerman et al, 2011) making any comparisons difficult. Fourth, data for both total mercury and methylmercury were included.…”

“…Down feathers also can be a useful tissue, because THg concentrations in down feathers represent in ovo exposure and can be translated into equivalent THg concentrations in whole eggs (Ackerman and Eagles-Smith, 2009). Besides chick down feathers, sampling juvenile birds for contaminant monitoring purposes is not advised, because THg concentrations in internal tissues (including blood) change rapidly as chicks age due to mass dilution and mercury transfer into growing feathers (Ackerman et al, 2011; Kenow et al, 2007) and, therefore, are difficult to interpret.…”