Life is never simple. When we try to solve a problem, our solution may entail unexpected side effects. Social scientists call such effects unintended consequences.

Just listen to a litany of negative side effects of a new drug in a TV commercial. The drug may well help a patient but at the risk of incurring undesirable side effects. Sometimes an unintended consequence can be helpful. Aspirin relieves pain but it also has anti-coagulant properties that can reduce the likelihood of heart attacks and lower the severity of strokes.

Natural communities are ripe for unintended consequences because of the many interactions among organisms. I’ll start with my favorite example that has an absurd but ultimately happy ending, and then move on to some ornithological examples.

In the early 1950s, the incidence of malaria increased dramatically among the Dayak people in the interior of Borneo. Malaria is spread by mosquitoes, so targeting mosquitoes seemed like a reasonable solution. So, the World Health Organization (WHO) sprayed DDT broadly. The DDT killed many mosquitoes and malarial cases decreased dramatically.

But there were unintended consequences. First, the thatched roofs of the Dayak people started to fall in. There was a great increase in caterpillars that fed voraciously on the thatch. The reason for the caterpillar explosion is a bit murky, but it seems that DDT killed both parasitic wasps that lay their eggs in caterpillars (ugh!) and lizards that made themselves at home in Dayak houses, feeding on the caterpillars.

But we’re not done. DDT gets more and more concentrated as it moves up a food chain, so these levels are typically higher in top predators. In Borneo, domestic cats began to die and the result was a population spike in rats. The rats spread the plague and typhus. So perversely, the application of DDT resulted in the substitution of one horrible disease, malaria, with the plague and typhus.

So, what was to be done? DDT application was halted. The WHO decided that more cats needed to be brought in to control the rats. But getting cats into these dense forests with few roads and no airports posed an obstacle. In 1956, the United Kingdom’s Royal Air Force parachuted 14,000 cats into a village. What a tangled web!

Some recent research from England describes unintended consequences of feeding birds. This effect involves members of the chickadee family so the research may well be applicable to our local bird fauna.

We know that providing food to birds increases their survivorship but birds do not become dependent on our handouts. One unintended consequence of bird feeding is that aggregations of birds can facilitate the rapid spread of infectious diseases.

Kate Lee and her colleagues examined the effects of garden bird feeders in England. These feeders are in urban and suburban backyards. Over the past 40 years, the number of species using feeders has increased significantly at the garden level. Furthermore, the many feeder-using birds show growing population sizes while the populations of species that do not use feeders has held steady.

Garden-feeding birds in the UK include four species of tits – the equivalent British common name for chickadees. The great tit and blue tit are dominant birds at feeders, chasing off willow tits and marsh tits.

Like our chickadees, tits nest in cavities. Willow tits are unusual among the feeder-frequenting birds in that they are in decline. The authors attribute this decline to the increase in blue tits, which aggressively compete with willow tits for nest cavities. About 40% of willow tit nesting attempts fail because they are usurped by blue tits.  So, an unintended consequence of garden-feeders is the decline of willow tits.

At our feeders, black-capped chickadees are usually the dominant birds in mixed flocks of chickadees, tufted titmice, red-breasted nuthatches and white-breasted nuthatches. All are cavity nesters. It would be interesting to see if we are tilting the balance in favor of chickadees.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at [email protected]

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