This dataset is from the article: Yanyan Zhao, Yu Liu, Elizabeth SC Scordato, Myung-Bok Lee, Xiaoying Xing, Xinyuan Pan, Yang Liu, Rebecca J Safran, Emilio Pagani-Núñez (2020), The impact of urbanization on body size of Barn Swallows Hirundo rustica gutturalis. Ecol. Evol. Comments and requests should be addressed to Yanyan Zhao (zhaoyy55@mail2.sysu.edu.cn). Description: This README file describes the datasat accompanying the above publication. Methods of data collection and definition of key variables: We selected 128 sites within 13 provinces of China. All the sites were clustered around 15 main urban areas, simply labelled as "city" in the dataset. The built-up area within a 1-km pixel grid around the nest was extracted for each individual bird using ArcGIS 10.1 from the dataset of Global 1-km Consensus Land Cover (http://www.earthenv.org/), simply labelled as "urbanization" in the dataset. We captured adult Barn Swallows during the breeding season from April to June 2014 and May to June 2015 by installing mist nets close to their nests after the first-brood nestlings hatched, and banded them in order to avoid duplicates. We recorded body mass of 359 breeding adult individuals using a digital scale to the nearest 0.01g (Pesola, Schindellegi, Switzerland) and maximum-chord wing length using a metal ruler with an end stop to the nearest mm. We calculated the regression residuals of body mass and wing length as another dependent variable, simply labelled as "body size index" in the dataset. We considered including climatic variables -average annual temperature and precipitation from 2011 to 2015 as explanatory variables, which were obtained from Loess plateau science data center, National Earth System Science Data Sharing Infrastructure, National Science & Technology Infrastructure of China (http://loess.geodata.cn). More details of data processing and analysis can be found in the article.