It may take just a scent, while passing by a restaurant, to jog a memory of a meal gone bad. Many people, after eating something that makes them ill, refuse to consume that food again. Some birds apparently feel the same way about eating insects that have been exposed to insecticides.
A study, conducted by Lowell K. Nicolaus and Hansoo Lee of Northern Illinois University in DeKalb, Illinois, and published in the August edition of Ecological Applications, found that Red-winged Blackbirds (Agelaius phoeniceus) will consistently avoid mealworms (Tenebriosp.) which have been injected with sub-lethal doses of the insecticide parathion, after only a single exposure. Parathion is an organophosphate, one of the most widely used classes of insecticide in both industrialized and developing nations.
This avoidance of toxic prey, known as conditioned taste aversion (CTA), began after the blackbirds were exposed to the contaminated worms for only one day. The birds were exposed to extremely low levels of parathion, levels too low to induce obvious illness. This suggests the possibility of a widespread, undetected exposure to these insecticides and a loss of important food sources for birds and their nestlings.
All the work in this study was done among free-ranging birds, which allowed the researchers to avoid many of the artificialities of captive studies. This was possible because birds in the study were in separate breeding territories.
The benefits of avoiding toxic foods include reducing the risk of subsequent poisoning of adults, and also of adults feeding toxic prey to their young. These results suggest the possibility, however, of CTA playing a role in reported declines of insect-eating birds populations. CTA could limit both feeding efficiency and breeding success in birds that must exploit many resources to flourish.
CTA could also have widespread agricultural implications. A decline in bird predation on insects might have economic consequences if crops were damaged by rising insect populations. The largest effects of this prey avoidance might be felt in developing nations, where insecticide application is heavy, and its use is much less regulated, than in industrialized nations. The majority of bird diversity is also found in these areas.
"The long-term effects of CTA on bird populations could have widespread implications," says Nicolaus. "This study produced the first clear evidence that a single exposure to an organophosphate insecticide can produce a lasting change in behavior among free-ranging birds. Even if the toxin is no longer present, we have observed that these birds will continue to avoid insects they experienced as toxic."
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