1. Paper citation: María Espinosa-Colín, Carlos de la Cruz, and Jorge S. Gutiérrez (2026) Experimental evidence for flexibility in provisioning behaviour in a cooperatively breeding bird. Behavioral Ecology (in press). 2. Summary: The flexible-investment hypothesis posits that parents should allocate their resources according to offspring needs. The available evidence suggests that this is valid in most avian biparental care systems, yet experiments designed to quantify the degree to which caregivers respond to the demands of offspring are rare in cooperative breeding systems, where offspring are reared by individuals additional to the breeding pair (i.e., helpers). By cross-fostering Iberian magpie (Cyanopica cooki) chicks of different ages, we tested whether breeders and helpers adjust their reproductive investment to meet the demands of their current brood. We show that both breeders and helpers (all males) prolonged their nestling care period when raising chicks younger than their own and shortened it when raising chicks older than their own, with no difference in reproductive success. Moreover, parents and helpers generally adjusted their feeding rates according to chick developmental stage: provisioning increased when broods were experimentally shortened (receiving younger chicks) and decreased when broods were prolonged (receiving older chicks). Planned contrasts indicated that breeders—particularly females in the shortening experiment and males in the prolonging experiment—adjusted their care more strongly than helpers, suggesting that carers of different sexes differ in their flexible responses to brood age. Altogether, these findings support the flexible-investment hypothesis and suggest that helpers’ feeding rules are less flexible than those of breeders. Combined with previous observational work, our results are consistent with the idea that helping behaviour may function as a signal to gain direct benefits through reciprocity. 3. Originator: Jorge S. Gutiérrez. 4. Contact information: Jorge S. Gutiérrez (E-mail addresses: jorgesgutierrez@unex.es), Department of Anatomy, Cell Biology and Zoology, Faculty of Sciences, University of Extremadura, Badajoz, Spain. 5. Date of data colletion: This experiment was performed during the 1998, 1999 and 2018 breeding seasons (from March to July). 6. Geographic location(s) of data collection: 39º 4' 17.67'' N, 6º 51' 6.39'' W 7. Information about funding sources that supported the collection and curation of the data: NA ACCESS INFORMATION 1. Licenses/restrictions placed on the data Creative Commons Attribution 4.0 International 2. Data derived from other sources: NA DATA FILES AND VARIABLES Data files and the variables that they contains: 1. data_provisioning_clean.csv: contains invididual provisioning (# hourly feeds) data from experimental (prolonged or shortened) and control broods. These data are used in the main analysis on chick provisioning. Variables included in this file: year = year of study nest = nest ID laying_date = laying date of a given nest laying_date_std = standardised laying date (i.e., days after 1 April) eggs = number of eggs chicks = number of chicks fledgings = number of fledgings feed_date = date of a given feed feed_day = standardised feed date (i.e., days after 1 April when a give feed occurred) feed_day day_since_hatching = days after hatching for a given feed status = female, male or helper ring = ring code euring_age = EURING age (see https://www.bto.org/sites/default/files/u17/downloads/about/resources/agecodes.pdf). Note that in the analyses we defined two classes of age: first-year birds and adults helpers = number of helpers help = yes/no feeds = number of feeds/h feeds_per_chick = feeds per chick in a given brood experimental_group = control, prolonged or shortened broods period = before/after swap selected_days = For control broods, we selected the same days to be compared with the respective experimental broods and thus make observations fully comparable group = control, prolonged or shortened broods (accounting for observations fully comparable, see above) nest_val = filter "nest_val = 1" select only one (fixed) value per individual nest; this is just to check whether the number of chicks and fledglings differed among shortened, prolonged and control broods before swapping. 2. data_morphometrics.csv: contains fledgings morphometric measurements from experimental (prolonged or shortened) and control broods. This data are used to assess the effect of treatment on chick development and body condition before fledging. Variables included in this file: year = year of study nest = nest ID laying_date = laying date of a given nest laying_date_std = standardised laying date (i.e., days after 1 April) fledgings = number of fledgings experimental_group = control, prolonged or shortened broods period = before/after swap selected_days = for control broods, we selected the same days to be compared with the respective experimental broods and thus make observations fully comparable group = control, prolonged or shortened broods (accounting for observations fully comparable, see above) help = yes/no chick_ID = chick ring code wing = wing cord length (mm) tarsus = tarsus length (mm), from the middle of the mid-tarsal joint to the distal end of the tarsometatarsus mass = body mass (g) sex = female or male days_measure = days old when measured mass_valid = 0/1 (1 if the individual was measured on days 11-13) 3. data_stay_clean.csv: contains data on rearing period length to test for differences in parental care period between treatments. Variables included in this file: year = year of study nest = nest ID laying_date = laying date of a given nest laying_date_std = standardised laying date (i.e., days after 1 April) eggs = number of eggs chicks = number of chicks fledgings = number of fledgings period = before/after swap selected_days = For control broods, we selected the same days to be compared with the respective experimental broods and thus make observations fully comparable group = control, prolonged or shortened broods (accounting for observations fully comparable, see above) help = yes/no hatching_date = hatching date fledging_date = fledging date stay_at_exp_nest = number of days at the experimental nest hatching_date_original_nest = hatching date at the original nest fledging_age = fledging age (d) CODE SCRIPT code_BEHECO.R: R code to reproduce the analyses presented in the paper cited above. R version 3.6.2 (2019-12-12) Platform: x86_64-apple-darwin15.6.0 (64-bit) Running under: macOS 10.16 Matrix products Attached packages: rstatix_0.7.0; cowplot_1.0.0; EnvStats_2.4.0; viridis_0.6.2; viridisLite_0.4.0; forcats_0.4.0; stringr_1.4.0; dplyr_1.1.4; purrr_0.3.3; readr_2.1.4; tidyr_1.1.3; tibble_3.2.1; tidyverse_1.3.0; ggsignif_0.6.0; car_3.0-6; carData_3.0-3; DHARMa_0.4.5; performance_0.9.1; interactions_1.1.3; emmeans_1.4.6; multcomp_1.4-15; TH.data_1.0-10; MASS_7.3-51.4; survival_3.1-8; mvtnorm_1.1-1; glmmTMB_1.0.2.1; nlme_3.1-142; lmerTest_3.1-1; lme4_1.1-21; Matrix_1.2-18; ggeffects_1.5.0; ggplot2_3.3.6; boot_1.3-23; MuMIn_1.43.15; pscl_1.5.5; reshape_0.8.8