Maybe you’ve heard the children's song about the old woman who swallowed a fly. Who knows why she swallowed a fly, but she tried to get rid of it by swallowing a spider to catch it. To catch the spider, she swallowed a cat, and then a dog to catch that. After that, she swallowed a goat to catch the dog, a cow to catch the goat, and finally, a horse to catch the cow. In the end, she dies without ever having gotten rid of the fly. 

The old woman’s idea seems kind of ridiculous, but it’s not far off from something people do in real life. “Biological control” is a method for dealing with pests by using other organisms to control them (without having to swallow them, of course). Have a problem with insects? Bring in predators or parasites that prey on them, or a pathogen to make them sick. Plagued by weeds? Introduce an herbivore to eat them. 

It’s a powerful tool, but just like the old woman and her stomach full of animals, biological control can often go very wrong. In a recent study, ecologist Pavel Pipek traces the story of a bird that was brought to New Zealand as a biological control agent, but went from hero to villain after it failed at its job and became a pest itself. At the same time, he shows that wildlife researchers can pull information about the species they study from some unlikely places. 

She swallowed a bird…

In the mid-1800s, New Zealand’s farmers were struck with a problem. Actually, it was thousands and thousands of little problems, in the form of insects like army worms and black field crickets that ravaged their crops. Pests like these would normally be kept under control by bug-eating birds, but New Zealand’s European settlers had cleared a lot of forest for farmland and development, and many of the islands’ native birds disappeared without any place to call home. 

As native plants and animals went into decline, organizations called “acclimatization societies” sprang up all over the country with the goal of importing species from Europe and other parts of the world and introducing them in New Zealand. Among these were a number of insect-eating birds meant to solve the farmers’ pest issues. The yellowhammer (Emberiza citrinella) seemed like a good candidate for the job. These small yellow and brown birds, the acclimatization societies said, would feast upon insects and their beautiful songs would also remind settlers of their European homes. 

Starting in the 1860s, ships bearing yellowhammers and other birds ordered by the acclimatization societies and individual farmers and land owners left London for ports around New Zealand. Over the next decade, 25 of these shipments were made, with almost a quarter of the yellowhammers coming from one Brighton man named Richard Bills, who did brisk business catching the birds near his home.

As the birds arrived, they were released and quickly made themselves at home. Their new environment, Pipek says, was similar enough to their European range that they would have had no problems establishing themselves, and the fields and pastures they were meant to protect were ideal habitat. They were further helped along by relocations that spread the birds further inland (one acclimatization society employed a full time bird catcher to move the birds from the coasts to other locations where they were needed), organized efforts to kill native birds of prey like the New Zealand falcon and morepork (a type of owl) and legislation like the Protection Act, which made the killing of introduced birds illegal and punishable by hefty fines.

Devil in Disguise

By 1871—a year when 300 yellowhammers were released in the Auckland area—the bird’s introduction was declared a success. The Auckland Acclimatization Society said that the species was established and spreading, and even wrote to their agent in London not to send any more birds. 

Across New Zealand, though, farmers were learning that the yellowhammer was not all it was cracked up to be. While the birds do eat insects during their breeding season, they primarily feed on seeds and grain. And feed they did, digging into the very crops they were supposed to protect from bugs. Farmers complained to the acclimatization societies and the government, and even New Zealand naturalist Richard Taylor pointed out that the birds were often considered pests in their home countries. These complaints were ignored, though. The acclimatization societies defended the yellowhammers and other introduced birds, and called the farmers short-sighted for not seeing the obvious benefit of their new neighbors. 

It would take almost another decade for the acclimatization societies to realize their mistake, and in 1880, the yellowhammer finally appeared in their records as a granivorous species. By then, the birds were nearly all over the country, and new shipments had to be turned away and sent on to Australia. 

Given how badly things got botched, Pipek says it’s tempting to write the acclimatization societies off as a bunch of amateurs, but this wasn’t the case. Their members included many respected scientists and high-level government officials (including New Zealand’s governor-general and prime minister at the time), and Pipek says the whole ordeal is more indicative of the gap in knowledge and experience between scientists and farmers. 

With the mistake realized and the damage done, yellowhammers were relieved of their government protection and farmers declared open season on them. Hunters were hired and rewards were offered for dead birds, young boys competed in egg collecting contests and poisoned grain was spread over fields during the winter. In 1902, the birds became enemies of the state and were listed as “injurious birds” in the Birds Nuisance Act, which empowered the government to organize and direct efforts to destroy them. A few years later, the acclimatization societies tried to solve the yellowhammer problem the same way they did the insect problem, with similar results. They imported another species, the little owl, to hunt yellowhammers, but there’s little evidence the owls put a dent in the yellowhammer population. They did, however, have a taste for the endangered Cromwell chafer beetle, and limit the bug’s numbers even today. 

Yellowhammers were killed in large numbers for the next few decades, but the effort was too little, too late, and the birds were already too common and widespread to be wiped out or controlled. The genie couldn’t be put back in the bottle, nor the bird back on the ship. Today, the yellowhammers are more or less tolerated in New Zealand, and while they’re still listed as a crop pest, they’re no longer considered a serious threat to agriculture. 

Besides revealing a fascinating story of a little bird that threw a whole nation into disarray and reminding us that messing with nature can blow up in our faces, Pipek’s study shows that there’s a lot to learn about natural history by looking at social history. Just studying the yellowhammer’s biology doesn’t tell the whole story of how it spread so successfully across New Zealand—most of the details were tucked away in ship’s logs, letters, newspaper clippings, and the account books of long-gone acclimatization societies, and only unearthed when Pipek and his team went looking off the beaten path of information. Finding all those details would have been a lifetime’s work until recently, but the digitization of newspaper archives, books, and historical documents is making it easier than ever before. The invasive species that cause us problems today are often the result of past human activities, Pipek says, so historical information could play a big role in how we study and deal with them. Hopefully, none of it involves swallowing a horse.