This dataset accompanies: Reif J., Reifova R., Skoracka A. and Kuczyński L. Competition-driven niche segregation on a landscape scale: evidence for escaping from syntopy toward allotopy in two coexisting sibling passerine species. Journal of Animal Ecology, in press The data file contains information on nightingale’s abundance, their co-occurrence status and geographic coordinates of sampling plots. Variable names plot: sampling plot identifier count: maximum no. nightingales counted each year on each plot during both (early/late) visits co-occ: combination of species and co-occurrence status (CN – common nightingale, TN – thrush nightingale, alop – allopatry, alot – allotopy, synt – syntopy) year: year of the study (range: 2000-2014) env.grad: linear environmental gradient perpendicular to the species border obs: observer identifier lon: longitude (WGS84) lat: latitude (WGS84) Bird data were collected under the Common Breeding Bird Survey in Poland, which is a country-wide scheme for monitoring the population changes for widespread and common breeding bird species. We are grateful to all volunteers who contributed to this project. Bird monitoring data were provided by the General Inspectorate of Environment Conservation in Poland (http://monitoringptakow.gios.gov.pl/about-project). For questions contact lechu@amu.edu.pl