These metadata provide data used to examine tree swallow incubation behavior in relation to egg mercury concentration and maternal blood mercury concentration. These data were collected from tree swallows nesting in artificial wooden nest boxes at the Cosumnes River Preserved in California's Central Valley in 2014. Included are two datasets. One dataset provides daily incubation constancy (the proportion of the day the female spent incubating her eggs), the number of incubation recesses (breaks from incubating) taken by the attending female each day and the total mercury concentration of one egg from the female's clutch, and blood total mercury concentrations collected from the mother. Additionally, we include other ecological covariates: NestID, date, julian date, nest age, full clutch size, and ambient temperature within the nestbox. The second dataset provides the length in minutes of each individual incubation recess as well as the total mercury concentration of one egg from the female's clutch, blood total mercury concentrations collected from the mother, and ecological covariates NestID, date, julian date, nest age, full clutch size, and ambient temperature within the nestbox. These data support the following publication: Hartman, C.A., Ackerman, J.T. and Herzog, M.P., 2019. Mercury exposure and altered parental nesting behavior in a wild songbird. Environmental Science & Technology. 53(9), 5396-5405. https://doi.org/10.1021/acs.est.8b07227