DOI: 10.5061/dryad.d51c5b0hb
title: Breaking bonds: maternal and offspring state shape constraint-based brood adoption in a seaduck

description: The adaptiveness of adoption for donors, recipients, and offspring remains
 unsettled despite long-standing interest. Using decade-long data on
 individually marked female and duckling common eiders (Somateria
 mollissima), displaying frequent alloparenting, we examined factors
 influencing the likelihood of females donating young and ducklings being
 adopted. We explored how donor traits, including maternal body condition,
 relative head size (a proxy of relative brain size), and relative hatching
 date, and offspring characteristics such as body condition relative to
 siblings, affect these processes. At least one offspring was permanently
 adopted in 34.7% of brood observations. Females in better body condition
 and larger relative head size were less likely to donate offspring, while
 the likelihood of offspring transfer was higher in larger natal broods.
 The odds of offspring donation peaked just before the population’s
 hatching peak, suggesting that recipient availability influences adoption.
 Ducklings in poorer body condition than their natal broodmates and those
 whose mothers were in lower body condition were, respectively,
 significantly and marginally significantly more likely to be adopted.
 Offspring transfers may thus result from physiological and cognitive
 constraints, rather than reflecting a fitness-maximizing strategy.
 Donating young and becoming adopted ultimately align with a salvage
 strategy for poor-condition donors and poor-condition offspring, but are
 likely driven by constraints rather than active tactics. Multiple tenders
 in broods prevented unique recipient identification, yet prior research
 suggests that recipients may accrue fitness benefits. Future research
 quantifying the fitness consequences for all parties in different
 environmental contexts is required for a more comprehensive understanding
 of alloparental behavior. Keywords: alternative reproductive tactics, body
 condition, cognitive constraints, common eider, conspecific brood
 parasitism, salvage strategy
description: # Breaking bonds: maternal and offspring state shape constraint-based
 brood adoption in a seaduck **Article title:** Breaking bonds: maternal
 and offspring states relate to constraint-based brood donation in a
 seaduck\ **Journal:** Behavioral Ecology\ **Manuscript ID:**
 BEHECO-2025-0052.R2\ **Authors:** Markus Öst; Benjamin Steele; Kim
 Jaatinen\ **Corresponding author:** Markus Öst
 ([markus.ost@abo.fi](mailto:markus.ost@abo.fi)) ## Dataset overview This
 repository contains the raw data underlying two logistic regression
 analyses reported in the manuscript: 1. **Female-level analysis** —
 factors associated with whether a female was a *donor* (lost at least one
 duckling to another brood) vs. a *reference* (no permanent loss).\
 **File:** `Data1_BEHECO-2025-0052_clean.csv` (cleaned, standardized
 variable names) 2. **Duckling-level analysis** — factors associated with
 whether an individual duckling was *adopted* into another brood vs.
 *stayed* in the natal brood.\ **File:** `Data2_BEHECO-2025-0052_clean.csv`
 (cleaned, standardized variable names) ## File formats and encodings * All
 tabular data are UTF-8 encoded comma-separated values (CSV) files with one
 header row and no BOM. * Missing values are represented by empty
 cells/`NA`. * Decimal separator is a period. * No thousands separators are
 used. ## Variable dictionary ### 1) `Data1_BEHECO-2025-0052.csv`
 (female-level) | Variable | Type | Units / Coding | Description | |
 ----------------------------------------------- | ------- |
 ------------------- |
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `year` | integer | year | Study year (2008–2017). | | `female_id` | integer | id | Unique identifier of the focal female (steel band number), referring to the potential donor. | | `female_age_years` | integer | years | Minimum age since banding in breeding population (0 = banding year/first recorded breeding; 1 = the following year, etc.). | | `female_breeding_experience_firsttimer0_prior1` | integer | 0/1 | 0 = first recorded breeder; 1 = previously banded breeder. | | `viable_brood_size_at_hatching` | integer | number of ducklings | Number of live, viable ducklings at hatch. | | `female_body_condition_index` | float | index (unitless) | Size-corrected body condition: standardized residuals from a linear regression predicting female projected weight at hatching from structural size (radius–ulna length). Positive values indicate better condition (see Methods). | | `donor_status_1donor_0reference` | integer | 0/1 | 1 = female donated/lost ≥1 duckling to another brood (permanent); 0 = no permanent loss (reference). | | `female_relative_head_size_index` | float | index (unitless) | Size-corrected head volume residuals (proxy for relative brain size). Higher values = relatively larger head. | | `hatch_date_offset_from_annual_median_days` | integer | days | Hatch date minus the population’s annual median hatch date (negative = before peak; positive = after). | ### 2) `Data2_BEHECO-2025-0052_clean.csv` (duckling-level) | Variable | Type | Units / Coding | Description | | ----------------------------------------------- | ------- | ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `year` | integer | year | Study year (2008–2017). | | `female_id` | integer | id | ID of the natal (donor) female associated with the duckling. | | `duckling_weight_g` | integer | g | Weight of individual duckling at hatch. | | `duckling_tarsus_mm` | float | mm | Tarsus length of individual duckling at hatch. | | `adopted_status_1adopted_0stayed` | integer | 0/1 | 1 = duckling permanently adopted into another brood; 0 = remained in natal brood. | | `female_body_condition_index` | float | index (unitless) | Natal female’s body condition index: standardized residuals from a linear regression predicting female projected weight at hatching from structural size (radius–ulna length). Positive values indicate better condition (see Methods). | | `duckling_tarsus_mm_centered_within_brood` | float | mm | Within-brood deviation of the focal duckling’s tarsus length from the mean tarsus length of all ducklings in its natal brood. | | `duckling_weight_g_centered_within_brood` | float | g | Within-brood deviation of the focal duckling’s weight from the mean weight of all ducklings in its natal brood. | | `duckling_body_condition_centered_within_brood` | float | index | Within-brood deviation of the focal duckling’s body condition from the mean body condition of all ducklings in its natal brood. Duckling body condition was calculated as standardized residuals from a linear regression of hatch weight on structural size (tarsus length); higher values indicate better condition. | | `female_age_years` | integer | years | Minimum age since banding in breeding population (0 = banding year/first recorded breeding; 1 = the following year, etc.). | | `female_breeding_experience_firsttimer0_prior1` | integer | 0/1 | 0 = first recorded breeder; 1 = previously banded breeder. | | `viable_brood_size_at_hatching` | integer | number of ducklings | Number of live, viable ducklings at hatch for natal female. | | `hatch_date_offset_from_annual_median_days` | integer | days | Hatch date minus the population’s annual median hatch date (negative = before peak; positive = after). | ## Study site, data collection, and derived variables (summary) Fieldwork took place in the Tvärminne archipelago (59°50′N, 23°15′E), Western Gulf of Finland, from 2008 to 2017. Data were collected at two time points: (i) capture of incubating females in late incubation and (ii) nest revisits at the estimated hatching of each brood. Refer to the variable dictionary above for variable definitions and calculations, and to the manuscript for full details. ## Code & analyses Statistical analyses were performed in R 4.0.4 using binomial GLMs with model selection and model averaging. Non-base packages: * **MuMIn 1.43.17** (model selection and averaging) * **varTestnlme 1.3.5** (variance component tests) * **rptR 0.9.23** (repeatability estimates) ## Ethical approvals Fieldwork complied with Finnish regulations; permits: ESLH-2009-02969/Ym-23; ESAVI/1697/04.10.03/2012; ESAVI/2831/04.10.07/2015. ## File naming conventions Filenames and column names use ASCII, lowercase_with_underscores; indicator variables are coded 0/1 as documented in the variable dictionary. ## How to cite this dataset Öst M, Steele B, Jaatinen K (2025). Breaking bonds: maternal and offspring states relate to constraint-based brood donation in a seaduck. Dryad Digital Repository. [https://doi.org/10.5061/dryad.d51c5b0hb](https://doi.org/10.5061/dryad.d51c5b0hb). ## Contact Corresponding author: Markus Öst ([markus.ost@abo.fi](mailto:markus.ost@abo.fi)).