Shiny Cowbird ( Molothrus bonariensis ) brood parasitism occurrence and impact increases with decreasing forest cover

ABSTRACTCowbirds brood parasitism has a detrimental effect on the breeding success of their hosts. The occurrence of parasitism observed may be related to environmental features at landscape or nest-site scales. Such relationships have been assessed for the Brown-headed Cowbird (Molothrus ater) in the large forests of the northern hemisphere. Here, we present a study conducted on the Shiny Cowbird (M. bonariensis) in native forests of the southern hemisphere in Argentina. These forests are characterised as semi-open woodlands, with forest fragments of variable sizes immersed in native grasslands. We aimed to evaluate the effect of Shiny Cowbird brood parasitism on the breeding success of a native passerine, the Masked Gnatcatcher (Polioptila dumicola), and to assess the relationship of its occurrence with environmental features at landscape and nest-site scales. During three breeding seasons (2015–2018) we monitored 207 gnatcatcher nests of which 70 were parasitised. Brood parasitism was the cause of nest failure in 60% of the parasitised nests. In addition, breeding success measured by apparent nest success, egg survival, hatching success and nestling survival were significantly lower for parasitised nests than for non-parasitised nests. Brood parasitism occurrence was negatively associated with forest cover, where nests located in sites with less forest cover experienced a higher occurrence of parasitism than those with greater cover. As these native semi-open forests face a continuous decline, our results add to the evidence of negative impacts of deforestation.KEYWORDS: Argentinanesting biologyPolioptilidaereproductive parameterssouth temperate foresttalares AcknowledgmentsWe would like to thanks María Luisa Shaw for allowing us to conduct this study at the ‘Luis Chico’ ranch. We also thank J. Paxman, S. Naegl, S. Stöckli, M. Honeyman, M. Ospina, E. Grim, D. Haegedus, M. Fontaine, C. Tiernan, A. Wolf, B. Vidrio, A. Valencia, T. Lansley and C. Dudley for help with fieldwork. We appreciate the improvements in English usage made by Peter Lowther through the Association of Field Ornithologists’ program of editorial assistance. This paper is Scientific Contribution N° 1241 of the Institute “Dr. Raúl A. Ringuelet” (ILPLA, CCT-La Plata CONICET, UNLP). This study was partially supported by the ‘Fondo para la Investigación Científica y Tecnológica (Agencia Nacional de Promoción Científica y Tecnológica)’, under Grant # 2014-3347. This study was conducted with research permits from the regional nature conservation authority (Organismo Provincial para el Desarrollo Sostenible, OPDS #17717, Dirección de Areas Naturales Protegidas, Buenos Aires province, Argentina). LNS is a CONICET Research Fellow.Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary materialSupplemental data for this article can be accessed at https://doi.org/10.1080/01584197.2023.2262497.