Mercury reduces avian reproductive success and imposes selection: an experimental study with adult- or lifetime-exposure in zebra finch

Abstract

Mercury is a global pollutant that biomagnifies in food webs, placing wildlife at risk of reduced reproductive fitness and survival. Songbirds are the most diverse branch of the avian evolutionary tree; many are suffering persistent and serious population declines and we know that songbirds are frequently exposed to mercury pollution. Our objective was to determine the effects of environmentally relevant doses of mercury on reproductive success of songbirds exposed throughout their lives or only as adults. The two modes of exposure simulated philopatric species versus dispersive species, and are particularly relevant because of the heightened mercury-sensitivity of developing nervous systems. We performed a dosing study with dietary methylmercury in a model songbird species, the zebra finch (Taeniopygia guttata), at doses from 0.3 - 2.4 parts per million. Birds were exposed to mercury either as adults only or throughout their lives. All doses of mercury reduced reproductive success, with the lowest dose reducing the number of independent offspring produced in one year by 16% and the highest dose, representing approximately half the lethal dose for this species, causing a 50% reduction. While mercury did not affect clutch size or survivorship, it had the most consistent effect on the proportion of chicks that fledged from the nest, regardless of mode of exposure. Among birds exposed as adults, mercury caused a steep increase in the latency to re-nest after loss of a clutch. Birds exposed for their entire lifetimes, which were necessarily the offspring of dosed parents, had up to 50% lower reproductive success than adult-exposed birds at low doses of methylmercury, but increased reproductive success at high doses, suggesting selection for mercury tolerance at the highest level of exposure. Our results indicate that mercury levels in prey items at contaminated sites pose a significant threat to populations of songbirds through reduced reproductive success.

Figure 1. Average blood mercury values for each dietary dose of adult-exposed and lifetime-exposed zebra finches.

Adult-exposed averages are represented by filled circles and solid lines; lifetime-exposed averages are represented by hollow circles and dashed lines. Values are means and bars are one S.E.

Figure 2. Effects of dietary mercury on zebra finch reproduction.

All points are model averages from the generalized linear mixed models. Bars are one S.E. A) The average total number of independent offspring produced per pair in one year of reproduction. B) The average clutch size. C) The proportion of eggs laid that hatched. D) The proportion of hatched chicks that survived to leave the nest. E) The number of days between removal of the first clutch of eggs and laying of the second clutch. F) The probability that both members of the pair survived for one year.

Figure 3. Differences in the effects of mercury on reproduction between adult-exposed and lifetime-exposed zebra finches.

Adult-exposed averages are represented by filled circles and solid lines; lifetime-exposed averages are represented by hollow circles and dashed lines. All points are model averages from the generalized linear mixed models. Bars are one S.E. A) The average total number of independent offspring produced per pair in one year of reproduction. B) The average clutch size. C) The proportion of eggs laid that hatched. D) The proportion of hatched chicks that survived to leave the nest. E) The number of days between removal of the first clutch of eggs and laying of the second clutch. F) The probability that both members of the pair survived for one year.