Science-- there's something for everyone

Thursday, June 28, 2012

Lead bullets stunt California condor recovery

The California condor (Gymnogyps californianus) is one of the rarest birds in the world. It came very close to extinction in 1982 when the last 22 individuals were caught and placed in captive breeding programs. That program was largely successful, and as of 2010, there were almost 400 condors, half of them in the wild. That’s the good news.

The bad news is that the birds still require regular interventions to keep them from dying out, including food supplementation and vaccination. Free-flying condors are caught twice a year for health checks and often need prolonged stays in treatment facilities. And even with all that assistance, only 24 chicks have fledged in the wild. The rest of the free-flying birds were all hatched in captivity and released as adults.

One of the leading culprits for the original decline of California condors was lead poisoning. Unfortunately, Myra Finkelstein of the University of California, Santa Cruz and her colleagues have found that lead poisoning is still a huge problem for the raptors. Lead is a heavy metal that has adverse effects on a number of tissues and organs, including the nervous system. The Centers for Disease Control sets an acceptable threshold for human adults of 25 micrograms of lead per deciliter of blood (µg/dl) and for children of 5 µg/dl. Children with blood lead levels above 45 µg/dl require chelation therapy. Wildlife managers have similar guidelines for condors.

The researchers measured the blood lead levels of 150 birds between 1997 and 2010.  Because each bird was caught multiple times, there were over 1100 independent blood samples. Each year, from 50 to 88% of captured birds exceeded the safe limit, and about 20% had levels above 45 µg/dl, requiring chelation therapy for acute lead poisoning. Almost half the birds required chelation at least once during the thirteen years of testing, and many were poisoned multiple times. The highest recorded blood lead level was 610 µg/dl.

In the U.S., lead has been banned from gasoline since 1986 and from household paints since 1987. So where is all this lead coming from? To find out, the researchers used stable isotopic analysis. Lead, like many elements, comes in more than one isotope or weight. Lead paint has a different ratio of these isotopes than lead ammunition or background environmental lead. Captive condors that have never been in the wild have lead with the background ratio, but the lead found in most poisoned free-flying birds has the same ratio as that of lead ammunition. It’s not that condors are being shot (though unfortunately, and illegally, some are), it’s that they eat carcasses that have been shot with lead bullets. Remember, condors are obligate scavengers, meaning they only eat dead animals. The birds inadvertently swallow bullet or buckshot fragments while consuming their prey.

Without an outright ban on lead ammunition, what’s the outlook for the condor? The authors estimate that with the current monitoring and interventions (but without future releases of chicks raised in captivity) there should be a stable population of a hundred and fifty or more birds… in about 1800 years. In other words, things are not going well for the California condor.

No comments:

Post a Comment