Breaking bonds: maternal and offspring state shape constraint-based brood adoption in a seaduck

Öst, Markus 1 ; Steele, Benjamin2; Jaatinen, Kim3

Published Nov 03, 2025 on Dryad. https://doi.org/10.5061/dryad.d51c5b0hb

Data files

Nov 03, 2025 version files 72.38 KB

Data1_BEHECO-2025-0052_clean.csv

16.47 KB
Data2_BEHECO-2025-0052_clean.csv

43.69 KB
README.md

12.22 KB

Nov 03, 2025 version files 72.50 KB

Data1_BEHECO-2025-0052_clean.csv

16.47 KB
Data2_BEHECO-2025-0052_clean.csv

43.69 KB
README.md

12.34 KB

Abstract

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

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)

Dataset overview

This repository contains the raw data underlying two logistic regression analyses reported in the manuscript:

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)
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.

Contact

Corresponding author: Markus Öst (markus.ost@abo.fi).