Mercury contamination of terrestrial songbirds
Songbirds may be contaminated with mercury when they consume adult aquatic insects, like dragonflies, that spend part of their life cycle in the water.
Life cycles of adult aquatic insects (dragonfly example shown below) involve several steps:
Larval forms live in the water, feeding on mercury-contaminated food items then crawl out of the water when mature to molt and become an adult. The emergence of adult aquatic insects from the water transports mercury from the aquatic to terrestrial systems.
Emergent aquatic insects can be the dominant component of the diets of some songbirds. For example, parent red-winged blackbirds, like the female shown below, often feed dragonflies to their nestlings.
Because larval dragonflies are predators, they accumulate very high concentrations of mercury before they emerge from the water. The concentrations of mercury in adult dragonflies are high enough to pose a risk to the health of nestling red-winged blackbirds.
Terrestrial birds may also become contaminated with mercury when the feed on spiders that live along shorelines and feed on aquatic insects. For example, the long-jawed orb weaver (shown below) spins its web in vegetation along shorelines, enabling it to capture insects emerging from the water.
Because the spiders are predators on aquatic insects, they can have very high mercury contamination levels. Adult songbirds not only consume spiders but also feed spiders to their nestlings. Concentrations of mercury in spiders can be high enough to pose a health risk to birds. The risk posed by mercury-contaminated spiders to bird health depends on the size of the bird – nestlings being more at risk than the adults.
Sources of information:
- Gann, G., C. Powell, M.M. Chumchal, R. Drenner. 2015. Mercury-contaminated terrestrial spiders pose a potential risk to songbirds at Caddo Lake, Texas/Louisiana, USA. Environmental Toxicology and Chemistry, 34:303-306. (link to paper)
For more information about our mercury research, go to our Aquatic Ecology Lab website.