BACKGROUND: Nicarbazin is an anti-coccidial product sometimes used as a contraceptive to reduce the size of feral pigeon populations. However, its effectiveness in reducing pigeon population size in cities has generated some controversy. Here, we evaluate its effectiveness in Barcelona city. RESULTS: We set 23 feeding stations in which we provided nicarbazin and 10 feeding stations with a placebo (untreated corn). Censuses were carried out before and after one year of treatment, in radii of 200 m around each feeder. We additionally censused 28 circles of 200 m of radius distributed randomly 200 m away from the feeders and 28 circles distributed >500 m away from the feeders, which acted as controls. Population size across the whole city was also evaluated pre- and post-treatment. We found that feral pigeon density did not change after one year of treatment, either in the circles around feeding stations with nicarbazin or in the areas around control stations distributed at 200 and >500 m from the feeders. Population size in placebo circles rose after a year by 10%. The pigeon census for the whole city of Barcelona showed a 10% increase. CONCLUSION: Overall, our results indicate that the nicarbazin treatment had no effect on the feral pigeon population size, and we advise against its use as a pigeon control method, at least in large cities. Keywords: Feral pigeon, Columba livia; nicarbazin; contraceptives; population size control