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The people of Toronto are fed up with pigeons. The birds crowd around subway stations, overload balconies with poop, and build their flammable nests in hazardous places. The city has tried trapping and relocating them, scaring them away with falcons, and even debated imposing . Nothing has worked.

The latest tactic? Giving the birds food laced with birth control.

Toronto is far from the only city dealing with pigeon problems. Adaptable and prolific, Rock Pigeons are extremely common—and often despised. City health and sanitation departments caution that pigeon droppings can damage buildings and historic landmarks, and their nests can cause fires on train tracks. This has prompted culling, trapping, or poisoning programs, but pigeon numbers keep bouncing back, pushing some civic leaders to seek more creative solutions.

Now Toronto is placing its bet on a form of avian birth control called nicarbazin. Nearly a year ago, the animal control department deployed four feeders around the city that automatically dispense nicarbazin-infused wheat pellets at a set time each day. The pilot program aims to reduce the metropolitan pigeon population by annually—and results are set for release later this summer.

Nicarbazin was originally developed in the 1950s to treat a poultry disease called coccidiosis. But the drug came with a curious side effect: It made hens lay infertile eggs. The compound pokes holes in the membrane around the yolk, and so the embryo can’t develop, says Christi Yoder, a former wildlife biologist who helped conduct early research on the contraceptive capabilities of nicarbazin in the early 2000s. The drug was first tested in the laboratory with chickens and domestic Mallards, and then later in the field on nuisance , both scenarios where nicarbazin performed quite well, Yoder says.

For pigeons, however, the results have been mixed. The drug has had some success, especially with smaller pigeon populations confined to areas like industrial lots or small towns. But the right conditions need to align for nicarbazin to do its job. Pigeons can be conditioned to return to the same places at the same time each day, but ensuring that the birds eat the necessary dose isn’t always guaranteed, especially when other food is available. And if a bird stops consuming the drug for a few days, its fertility will return to normal. “You gotta be taking this stuff consistently for it to work,” says Erick Wolf, CEO of Innolytics, LLC, which sells nicarbazin under the brand name , including for Toronto’s pilot.

That fickleness makes nicarbazin especially challenging to use effectively in cities. A in Barcelona found that, while the drug reduced some pigeon colonies by about 55 percent over 3 years, other colonies appeared unaffected. This is likely because of humans. In areas where people feed pigeons, the birds may not always eat the birth-control bait, says Carlos González-Crespo, the lead author of the paper who is currently a researcher at University of California Davis. A on nicarbazin in Barcelona also found that the drug had little effect on the overall population of pigeons across the city, likely because of these confounding human factors.

Giving pigeons birth control also seems to merely stabilize populations rather than decrease them, says Nadia Xenakis, a biologist at BC SPCA who led in 2019 on a pilot program in Vancouver. That’s because contraception impacts fertility but not survival. A pigeon’s typical lifespan is two to seven years, so bringing down the numbers solely using nicarbazin can be a waiting game, she says. And for the duration, feeders must be monitored to make sure that pigeons are eating enough for a proper dose and cleaned regularly so they don’t attract rats and other pests. (Nicarbazin poses little risk to those animals, Wolf says, because of its specific effect on bird eggs and its daily dosage requirement.)

All of which raises the question: With its many uncertainties, is birth control the most effective way to reduce urban pigeon populations? The answer, experts agree, is no; stopping people from feeding the birds would likely have a bigger impact. “Many studies for urban pigeons are like, if you’re just reducing the food available to them, their populations will reduce by 50 percent before you even try contraceptives,” says Page Klug, a U.S. Department of Agriculture biologist who led on avian contraceptives.

The problem is that human behavior is difficult to manage. González-Crespo recalls how challenging it was to stop city residents from feeding pigeons for his study, since they thought that they were doing a good deed. In reality, feeding the birds can harm them, he says, by leading to overcrowding and unsanitary conditions.

Indeed, it was opposition to a feeding ban that ultimately led Toronto to try the approach the press has dubbed “planned pigeonhood.” Although the program’s impact won’t be clear until the summer, the city recently added a fifth birth control dispenser in a new location. Officials are also encouraging residents to seal off food sources and potential nesting nooks. Pigeon problems can’t be solved with contraceptives alone, González-Crespo says. “You cannot expect wonders from doing just one thing.”