The data for this paper is contained in five (5) separate files: 1: The file entitled "Nests" contains data relevant to the nests in the study. 2: The file entitled "Young" contains data on the nestlings produced in the study. 3: The file entitled "Cuckolds" contains data on the age and body mass of the males rearing a given brood of young and, if the brood contained extra-pair young, the age and body mass of the extra-pair sires. 4: The file entitled "WPM_EPM" contains data on the age and body mass of the males rearing a given brood of young and also for extra-pair sires (this dataset is structured differently from "Cuckolds"). 5: The file entitled "Occupancy" contains data related to territory quality so that nests in the current study could be assigned a metric of territory quality. These data were collected by Charles Thompson, Scott Sakaluk, Anna Forsman, Brian Masters, Bonnie Johnson, Scott Johnson, and fellow Wren Crew members at Illinois State University. The data were collated and analyzed by Emerson Keith Bowers in 2015 (contact: ekbowers@ilstu.edu). Column headings (variable names) in the files are described below. Missing data are indicated by empty cells and not by a symbol (e.g., a period). Dataset "Nests": YEAR: The year in which the nest was created NEST_NUM: Nest number; an arbitrarily assigned number attributable to each individual clutch/brood. NEST: Nestbox and clutch/brood number. For example, "A 11-2" represents nestbox A 11, and "-2" indicates that clutch/brood was the second one attempted in box A 11 during the current year. FBAND: The unique U.S.F.W.S. band number of the female producing the clutch/brood. MBAND: The unique U.S.F.W.S. band number of the male rearing the brood with his social mate. MALE_AGE: The minimum age of the male rearing the brood. MALE_MASS: The body mass (g) of the male rearing the brood. EPM: A binary response indicating whether or not the brood contained at least one extra-pair young (1=yes, 0=no). CS: Clutch size. E1D: Egg-1 date. The day of year (ordinal calendar) on which the first egg of the clutch was laid. NUM_EGGS: The number of eggs in a nest that could be expected to hatch (sometimes this differs from clutch size, e.g. if an egg is lost during the course of incubation). NY4: The number of hatchlings alive in the nest on brood-day 4 (brood-day zero [BD0] is the day of the year on which hatching in the nest began). NE4: The number of eggs unhatched in the nest on brood-day 4. PROP_HATCHED: The proportion of eggs that hatched successfully. PROP_FLEDGED: The proportion of eggs that produced fledglings. PROP_HATCH_FLEDGE: The proportion of young that hatched and survived to fledging. FLEDGED: The number of young that survived to fledge. TERRITORY_QUAL: The quality of the breeding territory, quantified as the number of broods produced in a given nestbox (e.g., A 11) over the ten years prior to this study. Dataset "Young": YEAR: The year in which the nest was created NEST_NUM: Nest number; an arbitrarily assigned number attributable to each individual clutch/brood. NEST: Nestbox and clutch/brood number. For example, "A 11-2" represents nestbox A 11, and "-2" indicates that clutch/brood was the second one attempted in box A 11 during the current year. FBAND: The unique U.S.F.W.S. band number of the female producing the brood. BANDNUM: The unique U.S.F.W.S. band number of the individual nestling in the brood. MASS11: The body mass (g) of the nestling on brood-day 11. TARSUS11: The length of the nestling's tarsus (mm) on brood-day 11. PHAX: The PHA-induced skin swelling (mm) on brood-day 12, approximately 24 h post-injection. MBAND: The unique U.S.F.W.S. band number of the male rearing the brood with his social mate. MALE_AGE: The minimum age of the male rearing the brood. PATERNITY: Whether the nestling was a within-pair young (sired by the male rearing the brood) or sired by an extra-pair male. SIRE: The genetic sire of the nestling (U.S.F.W.S. band number). SIRE_AGE: The minimum age of the nestling's genetic sire. SEX: The sex of the nestling (F=female, M=male). LONGEVITY: The number of years that a nestling bred as an adult in the local population. Dataset "Cuckolds": YEAR: The year in which the nest was created NEST_NUM: Nest number; an arbitrarily assigned number attributable to each individual clutch/brood. NEST: Nestbox and clutch/brood number. For example, "A 11-2" represents nestbox A 11, and "-2" indicates that clutch/brood was the second one attempted in box A 11 during the current year. FBAND: The unique U.S.F.W.S. band number of the female producing the clutch/brood. MBAND: The unique U.S.F.W.S. band number of the male rearing the brood with his social mate. MALE_AGE: The minimum age of the male rearing the brood. MALE_MASS: The body mass (g) of the male rearing the brood. PATERNITY: Whether a given nestling in the brood was a within-pair or extra-pair young. Nests in which extra-pair young occured have multiple records/rows in the dataset so that the age and body mass of the within-pair and extra-pair males can be compared. IDENTITY: The ID (U.S.F.W.S. band number) of the genetic sire of the nestling. SIRE_AGE: The minimum age of the nestling's genetic sire. SIRE_MASS: The body mass of the nestling's genetic sire. EPM: A binary response indicating whether or not the brood contained at least one extra-pair young (1=yes, 0=no). Dataset "WPM_EPM": YEAR: The year in which the nest was created NEST_NUM: Nest number; an arbitrarily assigned number attributable to each individual clutch/brood. NEST: Nestbox and clutch/brood number. For example, "A 11-2" represents nestbox A 11, and "-2" indicates that clutch/brood was the second one attempted in box A 11 during the current year. FBAND: The unique U.S.F.W.S. band number of the female producing the clutch/brood. MBAND: The unique U.S.F.W.S. band number of the male rearing the brood with his social mate. MALE_AGE: The minimum age of the male rearing the brood. SIRE: The ID (U.S.F.W.S. band number) of the genetic, extra-pair sire that cuckolded the male rearing the brood. SIRE_AGE: The minimum age of the nestling's genetic sire. Dataset "Occupancy": NESTBOX: The nestbox available for use by the birds. NUM_BROODS: The number of broods initiated in the nextbox during the ten years preceding this study. OCCUPIED: A binary response indicating whether or not the nextbox was occupied in the current study (1=yes, 0=no) (see Figure 1 in the paper).