Mercury-exposed songbirds act more cautious

Songbirds in the wild must maintain a delicate balancing act. If they’re not vigilant enough, they risk being attacked by predators. But if they are so fearful that they don’t spend enough time looking for food, they may eventually starve.

Now scientists have found that mercury pollution may upset this balance. A new study suggests that finches exposed to methylmercury forage more cautiously, leading them to lose weight.

Mercury pollution comes from a variety of sources, including coal-fired power plants and small-scale gold mining. In water, mercury is often converted into another form called methylmercury. Methylmercury can pass from an animal’s blood into its brain and affect behavior.

The team raised 40 zebra finches in the lab, half of which were exposed to methylmercury starting as embryos. When the birds were three to six months old, the researchers placed pairs of finches in arenas with food, water, and fake evergreen trees.

During five-day trials, the team gradually changed the conditions in the arenas. On the second day, the researchers removed some of the fake trees, which had provided cover. On the fourth day, they hung a stuffed red-tailed hawk in one corner and played recordings of calls by two other predator species for an hour.

Birds exposed to methylmercury responded differently to the presumed threats, the team found. On the morning the hawk was introduced, the unexposed finches began foraging after less than half an hour, but the mercury-exposed finches waited about 45 minutes. Ninety percent of the unexposed birds foraged at some point while the hawk was visible, but only 63 percent of mercury-exposed birds did. And the weights of the mercury-exposed finches dropped by an average of 0.85% between the fourth and fifth days, while the unexposed finches’ weights stayed the same.

Granted, the finches’ methylmercury dose was similar to what they would experience at a “highly contaminated industrial site,” the authors write, or the equivalent of “a human eating a lifetime diet consisting entirely of swordfish”. Still, the findings raise concerns that this pollutant could put songbirds at greater risk of starvation — particularly those struggling to survive lean winters. Roberta Kwok | 9 April 2015

Source: Kobiela, M.E., D.A. Cristol, and J.P. Swaddle. 2015. Risk-taking behaviours in zebra finches affected by mercury exposure. Animal Behavior doi: 10.1016/j.anbehav.2015.02.024.

Image © Wang LiQiang | Shutterstock