1. Paper Citation
   Lauren Leischner, Scott K. Sakaluk, Charles F. Thompson, Pirmin Nietlisbach (2025): Double brooding in house wrens is repeatable but constrained by time-of-season. Behavioral Ecology.
2. Brief abstract
   Birds in some species produce two broods in a season to maximize reproductive success, but in species such as the northern house wren (Troglodytes aedon), not all individuals produce a second brood even if they have enough time to do so. We investigated whether variation in individual quality, in addition to time-of-season, explains some of the variation in the likelihood of producing a second brood. We examined the effects of individual age, body condition, and territory quality, and estimated the repeatability of producing a second brood using breeding records from a long-term dataset. We also cross-fostered eggs between earlier-nesting females (presumed high quality) and later-nesting females (presumed low quality) to delay or advance the natural nesting date, exposing an effect of quality if present. There was little evidence that the individual traits measured affected the production of a second brood. Cross-fostered, delayed females did not differ from the control in their likelihood of producing a second brood, but advanced females were more likely to produce a second brood compared with the control. However, the production of a second brood was significantly repeatable, and this repeatability was substantially higher after accounting for variation in timing of the first brood. This suggests that individual characteristics, in addition to time-of-season, have a substantial effect on the likelihood of producing a second brood, but that further studies are needed to identify the specific traits responsible for this effect.
3. Originators (of this data repository)
   Pirmin Nietlisbach
4. Contact information
   Pirmin Nietlisbach, School of Biological Sciences, Illinois State University, Campus Box 4120, Normal, IL 61790-4120, USA. pnietli@ilstu.edu, +1(309)438-8549.
5. Date of data collection
   1987-2023 (see data files)
6. Geographic location(s) of data collection
   Mackinaw Study Area in Central Illinois, USA
7. Information about funding sources that supported the collection and curation of the
   data
   This work was supported by grants from the Champaign County Audubon Society (S. Charles Kendeigh Research Grant); the Beta Lambda Chapter of the Phi Sigma Biological Sciences Honor Society (Robert D. Weigel Fund); National Institutes of Health (grant number 2R15HD076308); and the National Science Foundation (grant number 2334786). Additional support was provided by Illinois State University’s School of Biological Sciences and the Illinois State University Foundation’s Bird Study and Student Fellowship Fund (4125104).

ACCESS INFORMATION

1. Licenses/restrictions placed on the data
   CC0
2. Data derived from other sources
   N/A

DATA FILES AND VARIABLES

1. Leischneretal_2025_BehavioralEcology_Data1_TarsusMeasurements.csv

This file contains measurements used to calculate mean tarsus length, i.e. the mean.tarsus variable used in downstream analysis. Empty cells indicate missing data due to not having recorded the respective data for a variety of possible reasons, including being unable to locate, see, or capture an individual, or not recording certain measurements by a subset of field crew or (rarely) due to premature escaping or release of individuals.

• NEST: Unique identity (within a year) of the nest record associated with this individual; a unique code for each nestbox before the dash, followed by the active nest order within that box after the dash (i.e., the first active nest in a given box is identified as -1, the second nest as -2, irrespective of the identity of the adult birds responsible for the nest)
• BANDNO: Unique bandnumber of the individual bird
• DATE: Day of the year when the individual bird was recorded.
• WT: Weight (in grams) of the bird when it was recorded.
• HR: Hour of the day (in local time) when the bird was recorded; hour 6 corresponds to 6:00 through 6.59 AM, hour 7 corresponds to 7:00 through 7.59 AM, etc.
• TARSUS: The tarsus length measurement (in millimeters, to a precision of .1 millimeter).
• year: The calendar year for this record
• yearID: A concatenated variable of the unique bandnumber of the female and the year the individual was recorded, combined with an underscore for cross-referencing with other datasets
• nest.clean.year: A concatenated variable of unique nest identity (i.e., the information in NEST without spaces or dashes) and the year of the record, combined with an underscore for cross-referencing with other datasets

2. Leischneretal_2025_BehavioralEcology_Data2_BodyCondition.csv

This file contains the dataset used to estimate body condition, i.e. the conditionBLUP.first variable used in downstream analysis. To estimate body condition, we used a mixed model to account for factors that influence body mass during the first brood. We included the random effect of female identity within a year to account for multiple measurements of a female within a year, and the fixed effects of tarsus length (averaged if multiple measures were available because, unlike body mass, tarsus length is a skeletal measure that is not expected to change; raw measurements provided in Leischneretal_2025_BehavioralEcology_Data5_TarsusMeasurements.csv), hour of capture, nest stage, year (fitted as a categorical effect for each year), and the interaction between nest stage and year. Empty cells indicate missing data due to not having recorded the respective data for a variety of possible reasons, including being unable to locate, see, or capture an individual, or not recording certain measurements by a subset of field crew or (rarely) due to premature escaping or release of individuals.

• NEST: Unique identity (within a year) of the nest record associated with this individual; a unique code for each nestbox before the dash, followed by the active nest order within that box after the dash (i.e., the first active nest in a given box is identified as -1, the second nest as -2, irrespective of the identity of the adult birds responsible for the nest)
• BANDNO: Unique bandnumber of the individual bird
• SEX: The sex of the individual bird (F: Female, M: Male) determined based on presence or absence of brood patch and cloacal protuberance.
• DATE: Day of the year when the individual bird was recorded.
• WT: Weight (in grams) of the bird when it was recorded.
• HR: Hour of the day (in local time) when the bird was recorded; hour 6 corresponds to 6:00 through 6.59 AM, hour 7 corresponds to 7:00 through 7.59 AM, etc.
• mean.tarsus: The tarsus length (in millimeters) used in estimation of body condition; often only one measurement (in millimeters, to a precision of .1 millimeter) is available, but when a bird was measured multiple times, the mean value is used. The raw tarsus measurements are provided in the file Leischneretal_2025_BehavioralEcology_Data1_TarsusMeasurements.csv.
• nestbox: The identity of the nestbox without any spaces; this is a subset of the information contained in NEST
• nestbox.year: A concatenated variable of the nestbox identity and the year of this record, combined with an underscore for cross-referencing with other datasets
• nest.year: A concatenated variable of the year of this record and the unique nest identity (i.e., the information in NEST without spaces or dashes), combined with an underscore for cross-referencing with other datasets
• BD0: Brood day 0, indicating the day of the year when the first nestling hatched
• days.to.BD0: The number of days before or after BD0 of when the individual bird was recorded and measured; this is the difference of DATE minus BD0
• E1D: The day of the year when the first egg in the clutch was laid
• earliest.E1D: The day of the year when the earliest egg was laid in our study area in a given year
• days.since.earliest.E1D: This indicates how late a clutch was started relative to the first egg laid in a given year; it is the difference of E1D minus earliest.E1D
• yearID: A concatenated variable of the unique bandnumber and the year the individual was recorded, combined with an underscore for cross-referencing with other datasets
• nest.clean.year: A concatenated variable of unique nest identity (i.e., the information in NEST without spaces or dashes) and the year of the record, combined with an underscore for cross-referencing with other datasets

3. Leischneretal_2025_BehavioralEcology_Data3_DoublebroodingLongTermDataAnalysis.csv

This file contains the dataset used for the analysis of traits associated with quality. The model included the fixed effects of age, body condition (conditionBLUP.first variable, estimated with a mixed model based on data provided in the file Leischneretal_2025_BehavioralEcology_Data2_BodyCondition.csv), territory quality, and day of the season and the random effects of year (to account for environmental variation among years) and female identity (to account for repeated measures of some females; see below). Empty cells indicate missing data due to not having recorded the respective data for a variety of possible reasons, including being unable to locate, see, or capture an individual, or not recording certain measurements by a subset of field crew or (rarely) due to premature escaping or release of individuals.

• NEST: Unique identity (within a year) of the nest record associated with this individual; a unique code for each nestbox before the dash, followed by the active nest order within that box after the dash (i.e., the first active nest in a given box is identified as -1, the second nest as -2, irrespective of the identity of the adult birds responsible for the nest)
• E1D: The day of the year when the first egg in the clutch was laid
• BD0: Brood day 0, indicating the day of the year when the first nestling hatched
• FBAND: Unique bandnumber of the female attending this nest
• MBAND: Unique bandnumber of the male attending this nest; for some nests, the male remained unbanded (UNBANDED), banded with a metal band only without successful capture to read the number (R:S or L:S), or colorbanded without successful reading of the color combination (COLORBAND)
• year: The calendar year for this record
• yearID: A concatenated variable of the unique bandnumber of the female and the year the individual was recorded, combined with an underscore for cross-referencing with other datasets
• dbrooding: A binary variable indicating whether (= 1) or not (= 0) the female in this record was double-brooding this year
• year.hatch: The year in which the female of this record hatched (if age.appearance is 0) or was assumed to hatch (if year.appearance is 1); individuals that were first observed as unbanded adults were assumed to have hatched the previous year.
• age: The age of the female in the focal year; this is the difference of year minus year.hatch; age is here measured in years, with e.g. 1 year of age indicating that the bird hatched in the previous calendar year.
• nestbox: The identity of the nestbox without any spaces; this is a subset of the information contained in NEST
• nestbox.year: A concatenated variable of the nestbox identity and the year of this record, combined with an underscore for cross-referencing with other datasets
• earliest.E1D: The day of the year when the earliest egg was laid in our study area in a given year
• days.since.earliest.E1D: This indicates how late a clutch was started relative to the first egg laid in a given year; it is the difference of E1D minus earliest.E1D
• TQ: Territory quality, i.e. the number of fledglings produced in a nestbox over the six years prior.
• conditionBLUP.first: The BLUP-based estimate of body condition, estimated with a mixed model based on data provided in the file Leischneretal_2025_BehavioralEcology_Data2_BodyCondition.csv.
• age.appearance: The age of the female when first recorded in the study area; this is either 0 for females banded as nestlings, or 1 for females banded as adults and assumed to be 1 year of age at appearance; age is here measured in years, with e.g. 1 year of age indicating that the bird hatched in the previous calendar year.

4. Leischneretal_2025_BehavioralEcology_Data4_DoublebroodingRepeatability.csv

This file contains the dataset used for the analysis of repeatability of double brooding. The first model included random effects of year and female identity and no other covariates. The second model estimated the adjusted repeatability of double brooding in a model as above (i.e., with the random effects of year and female identity) plus the number of days since the beginning of the breeding season as a covariate. Empty cells indicate missing data due to not having recorded the respective data for a variety of possible reasons, including being unable to locate, see, or capture an individual, or not recording certain measurements by a subset of field crew or (rarely) due to premature escaping or release of individuals.

• NEST: Unique identity (within a year) of the nest record associated with this individual; a unique code for each nestbox before the dash, followed by the active nest order within that box after the dash (i.e., the first active nest in a given box is identified as -1, the second nest as -2, irrespective of the identity of the adult birds responsible for the nest)
• E1D: The day of the year when the first egg in the clutch was laid
• BD0: Brood day 0, indicating the day of the year when the first nestling hatched
• FBAND: Unique bandnumber of the female attending this nest
• MBAND: Unique bandnumber of the male attending this nest; for some nests, the male remained unbanded (UNBANDED), banded with a metal band only without successful capture to read the number (R:S or L:S), or colorbanded without successful reading of the color combination (COLORBAND)
• year: The calendar year for this record
• yearID: A concatenated variable of the unique bandnumber of the female and the year the individual was recorded, combined with an underscore for cross-referencing with other datasets
• dbrooding: A binary variable indicating whether (= 1) or not (= 0) the female in this record was double-brooding this year
• year.hatch: The year in which the female of this record hatched (if age.appearance is 0) or was assumed to hatch (if year.appearance is 1); individuals that were first observed as unbanded adults were assumed to have hatched the previous year.
• age: The age of the female in the focal year; this is the difference of year minus year.hatch; age is here measured in years, with e.g. 1 year of age indicating that the bird hatched in the previous calendar year.
• nestbox: The identity of the nestbox without any spaces; this is a subset of the information contained in NEST
• nestbox.year: A concatenated variable of the nestbox identity and the year of this record, combined with an underscore for cross-referencing with other datasets
• earliest.E1D: The day of the year when the earliest egg was laid in our study area in a given year
• days.since.earliest.E1D: This indicates how late a clutch was started relative to the first egg laid in a given year; it is the difference of E1D minus earliest.E1D
• conditionBLUP.first: The BLUP-based estimate of body condition, estimated with a mixed model based on data provided in the file Leischneretal_2025_BehavioralEcology_Data2_BodyCondition.csv.
• age.appearance: The age of the female when first recorded in the study area; this is either 0 for females banded as nestlings, or 1 for females banded as adults and assumed to be 1 year of age at appearance; age is here measured in years, with e.g. 1 year of age indicating that the bird hatched in the previous calendar year.

5. Leischneretal_2025_BehavioralEcology_Data5_ExperimentalDataset.csv

This file contains the dataset of the experimental corss-fostering. To assess the effect of treatment in the cross-fostering experiment, we used a generalized linear mixed model with a binary response variable of production of a second brood (i.e., yes or no) and with the independent variables of treatment (levels: control, advanced, delayed) and date (day of the year when the first egg was laid in the cross-fostered nest). Random effects of year and female identity were included to account for differences in the production of a second brood owing to environmental differences between years or the presence of an individual across multiple years, respectively. Initial models also included clutch size, the number of days advanced or delayed in the cross-fostering treatment, and territory quality to account for those effects, if present. Empty cells indicate missing data due to not having recorded the respective data for a variety of possible reasons, including being unable to locate, see, or capture an individual, or not recording certain measurements by a subset of field crew or (rarely) due to premature escaping or release of individuals.

• NEST: Unique identity (within a year) of the nest record associated with this individual; a unique code for each nestbox before the dash, followed by the active nest order within that box after the dash (i.e., the first active nest in a given box is identified as -1, the second nest as -2, irrespective of the identity of the adult birds responsible for the nest)
• E1D: The day of the year when the first egg in the clutch was laid; this refers to the clutch that was in that nest after crossfostering occurred
• BD0: Brood day 0, indicating the day of the year when the first nestling hatched
• FBAND: Unique bandnumber of the female attending this nest
• MBAND: Unique bandnumber of the male attending this nest
• year: The calendar year for this record
• yearID: A concatenated variable of the unique bandnumber of the female and the year the individual was recorded, combined with an underscore for cross-referencing with other datasets
• nest.year: A concatenated variable of the year of this record and the unique nest identity (i.e., the information in NEST without spaces or dashes), combined with an underscore for cross-referencing with other datasets
• annual.females: A concatenated variable of the year the individual was recorded and the unique bandnumber of the female, combined with an underscore for cross-referencing with other datasets
• nestbox: The identity of the nestbox without any spaces; this is a subset of the information contained in NEST
• nestbox.year: A concatenated variable of the nestbox identity and the year of this record, combined with an underscore for cross-referencing with other datasets
• TQ: Territory quality, i.e. the number of fledglings produced in a nestbox over the six years prior.
• CS: Clutch size, i.e. the number of eggs in the completed clutch.
• NY4: The count of the number of nestlings in the nest four days after the first nestling hatched
• NY12: The count of the number of nestlings in the nest at banding eleven days after the first nestling hatched
• NYDEAD: The count of the number of dead nestlings found in the nest after fledging
• treatment: The experimental treatment of delay, advance, or control as done in the cross-fostering experiment.
• distance: The number of days that the nest was advanced or delayed by experimental cross-fostering.
• doubleb: A binary variable indicating whether (= 1) or not (= 0) the female in this record was double-brooding this year