Sustained contraceptive‐level plasma concentrations of 4,4′‐dinitrocarbanilide in free‐ranging urban feral pigeons fed a custom bait

Abstract The increasing presence of feral pigeons ( Columba livia ) in cities requires strategies for population control, such as limiting pigeon productivity by providing food treated with a contraceptive agent. The local development of bait is a reasonable alternative to use of imported ones, which are often less affordable in developing countries. We aimed to indirectly infer the reproductive inhibition induced in free‐roaming feral pigeons exposed to a custom‐developed bait made from whole maize grains coated with a waterproof, natural, and biodegradable biopolymer carrier containing 1,900 ppm nicarbazin (NCZ) by monitoring blood plasma levels of 4,4′‐dinitrocarbanilide (DNC), the active residue of NCZ. The experiment lasted from 6 to 27 December 2023. We placed 500 g of maize kernels daily inside a live‐capture trap and a camera on the rooftop of a building in Córdoba, Argentina. All 21 captured pigeons were individually marked, weighed, blood sampled for baseline values (T 0 ) of DNC, and released. The pigeon population on the rooftop was previously estimated (47 individuals) via a Schnabel multiple capture‐mark‐recapture method. We subsequently provided 10 g of NCZ bait per bird daily for eleven days, during which pigeons were captured, blood sampled, and released. 4,4′‐dinitrocarbanilide plasma levels were determined by HPLC, and was undetectable in the eight captured pigeons at T 0 . Conversely, after NCZ was supplied, the average daily DNC levels in the 25 pigeons sampled throughout the treatment phase ranged from 5.91 to 7.41 mg.kg −1 , with individual concentrations (4.93 to 11.43 mg.kg − 1 ) that fell within the reported range (4 to 6 mg.kg − 1 ) for contraceptive effects in chickens and captive feral pigeons. Upon reintroducing the untreated maize, DNC was no longer detected in 3 pigeons after five days, whereas the other 2 captured birds still presented with DNC (5.12 and 6.34 mg.kg − 1 ). Our custom‐developed NCZ bait was attractive and species‐specific in the study environment, and its continuous provision may inhibit pigeon reproduction, with a potentially reversible effect after cessation of treatment. The local development and systematic application of such bait may result in an effective nonlethal long‐term strategy for managing free‐roaming feral pigeons in urban environments, particularly in those colonies living in buildings.