DOI: 10.5061/dryad.05qfttfg4
title: Predominantly terrestrial foraging and reproductive gains from a high trophic level diet in roof-nesting Herring Gulls (<em>Larus argentatus</em>)

description: Wild animal species often use human-modified environments for foraging and
 reproduction, but this may require dietary diversification with fitness
 consequences. The extent to which colonising species successfully exploit
 such habitats is poorly understood. We used stable isotope analysis of egg
 yolk to investigate the association between foraging choices and
 reproductive success in 102 female Herring Gulls (Larus argentatus) over 3
 years in a roof-nesting, pericoastal breeding colony. Stable isotopes of
 egg yolk predominantly reflect maternal diet during egg production. We
 measured δ13C as an indicator of foraging habitat, and δ15N as an
 indicator of trophic level. We predicted diverse foraging choices across
 marine, terrestrial, and urban environments due to gulls’ generalist
 foraging strategy and the variety of nearby foraging opportunities. We
 also predicted higher reproductive success associated with marine feeding
 compared to terrestrial feeding or feeding on human food and refuse,
 because marine food has historically been gulls’ natural food type and has
 been previously associated with greater reproductive success.
 Surprisingly, δ13C values indicated predominantly terrestrial foraging for
 egg production. Egg mass increased significantly with lower δ13C,
 indicative of more terrestrial feeding. These findings may reflect the
 availability of habitats and foods nearby or indicate adaptive dietary
 choices. Fledging success increased significantly with elevated δ15N,
 indicating that mothers feeding at higher trophic levels before laying
 produced higher quality eggs and/or had superior offspring-rearing
 capacity. A high trophic level maternal diet may nutritionally benefit
 offspring or improve parental condition. Egg stable isotope ratios of δ13C
 and δ15N were highly repeatable within clutches, enabling us to predict
 stable isotope values of unsampled eggs from sampled sibling eggs. Our
 results highlight high usage of terrestrial foods for egg production,
 whereas marine and anthropogenic feeding were rare. The reasons for this
 preference warrant further investigation to advance understanding of
 species that use human-modified environments.
description: # Predominantly terrestrial foraging and reproductive gains from a high
 trophic level diet in roof-nesting Herring Gulls (*Larus argentatus*)
 Dataset DOI: [10.5061/dryad.05qfttfg4](10.5061/dryad.05qfttfg4) ##
 Description of the data and file structure These data were collected to
 investigate the association between foraging choices and reproductive
 success in 102 female Herring Gulls (*Larus argentatus*) over 3 years in a
 roof-nesting, pericoastal breeding colony. Stable isotopes of egg yolk
 were used as an indicator of maternal foraging habitat and trophic level
 during egg production. ### Files and variables #### File:
 Pred_Terr_For_Code.R **Description:** R code used to perform statistical
 analyses on the data. #### File: Pred_Terr_For_Data.csv
 **Description:** Table containing the data used for the statistical
 analyses. ##### Variables * NestID: Unique identifiers for each clutch
 surveyed. * Egg: Egg's position in the clutch laying sequence (A, B,
 or C). * Clutch_Size: Total number of eggs in clutch. * Year: Year the egg
 was laid (2022, 2023, or 2024). * Julian: Date the egg was laid, converted
 to Julian format, where 1 = 1st January for that year. Missing values
 where the lay date is unknown are indicated by NA. * LayDate: Date the egg
 was laid in DD/MM/YYYY format. Missing values where the lay date is
 unknown are blank cells. * Mass: Mass of egg in grams on day of laying. *
 Pipped: Date the egg pipped in DD/MM/YYYY format. Missing values where the
 egg did not pip are blank cells. * HatchDate: Date the egg hatched in
 DD/MM/YYYY format. Missing values where the egg did not hatch are blank
 cells. * Hatched: Binary indicator of whether the egg hatched, where 1 =
 hatched and 0 = did not hatch. Eggs that were removed for sampling are
 indicated by NA. * d15N_Sampled: δ^15^N value of lipid-removed yolk in ‰
 from sampled eggs. Eggs that were left in the nest, and so not sampled,
 are indicated by NA. * d15N: δ^15^N value of lipid-removed yolk in ‰, with
 values for unsampled eggs inferred from sampled eggs in the same clutch.
 Eggs with missing values, where there were no sampled eggs within the same
 clutch to infer values from, are indicated by NA. * d13C_Sampled: δ^13^C
 value of lipid-removed yolk in ‰ from sampled eggs. Eggs that were left in
 the nest, and so not sampled, are indicated by NA. * d13C: δ^13^C value of
 lipid-removed yolk in ‰, with values for unsampled eggs inferred from
 sampled eggs in the same clutch. Eggs with missing values, where there
 were no sampled eggs within the same clutch to infer values from, are
 indicated by NA. * Survival_days: Number of days the chick survived
 post-hatching. After 35 days, the chick was considered to have fledged and
 was no longer monitored. Eggs with missing values, which were removed for
 sampling or failed to hatch, are indicated by NA. * Sampled: Indicates
 whether the egg was removed from the nest for sampling (SAMPLED) or left
 in the nest (LEFT). * Fate_Egg: Indicates whether the egg was removed from
 the nest for sampling (SAMPLED), left in the nest but failed to hatch
 (FAILED), successfully hatched but the chick died before fledging (DEAD),
 or successfully hatched and survived to fledging (FLEDGED). * Fate_EggII:
 Details the fate of eggs that failed to hatch and chicks that failed to
 fledge. * Fledged: Indicates whether the chick survived to fledging (1) or
 did not (0). Eggs that were removed for sampling are indicated by NA. *
 Mass_d0: Mass of the chick on day of hatching in grams. Eggs with missing
 values, which were removed for sampling or failed to hatch, are indicated
 by NA. * Sex: Sex of the chick determined by molecular sexing analysis of
 blood samples collected on day 3 (± 1 day) post-hatching. F = female, M =
 male. Missing values are indicated by NA. ## Code/software R version 4.4.0
 and RStudio version 2024.04.0 are required to run the code provided.
 Required packages are as follows: * lme4 * lmerTest * MuMIn * dplyr *
 ggplot2 * sjPlot * rptR * tidyr * lmtest * ggeffects Line 20 requires
 editing to include the correct file path to the data file before the code
 will run successfully. Code should be run line by line rather than all at
 once. ## Access information Data for reference ranges of δ^13^C from
 terrestrial, anthropogenic, and marine food sources (lines 118 - 126 in
 the code) were derived from the following sources: * Fletcher, J. W. J.,
 Tollington, S., Cox, R., Tolhurst, B. A., Newton, J., McGill, R. A. R.,
 Cropper, P., Berry, N., Illa, K., & Scott, D. M. (2025). Utilisation
 of Anthropogenic Food by Red Foxes (Vulpes vulpes) in Britain as
 Determined by Stable Isotope Analysis. Ecology and Evolution, 15(3).
 [https://doi.org/10.1002/ece3.70844](https://doi.org/10.1002/ece3.70844) *
 Hobson, K. A. (1995). Reconstructing Avian Diets Using Stable-Carbon and
 Nitrogen Isotope Analysis of Egg Components: Patterns of Isotopic
 Fractionation and Turnover. The Condor, 97(3), 752–762.
 [https://doi.org/10.2307/1369183](https://doi.org/10.2307/1369183) *
 O’Connell, T. C., & Hedges, R. E. M. (1999). Investigations into the
 effect of diet on modern human hair isotopic values. American Journal of
 Biological Anthropology, 108(4), 409–425. * Ronconi, R. A., Steenweg, R.
 J., Taylor, P. D., & Mallory, M. L. (2014). Gull diets reveal dietary
 partitioning, influences of isotopic signatures on body condition, and
 ecosystem changes at a remote colony. Source: Marine Ecology Progress
 Series, 514, 247–261.
 [https://doi.org/10.2307/24894791](https://doi.org/10.2307/24894791) *
 Swan, G. J. F., Bearhop, S., Redpath, S. M., Silk, M. J., Goodwin, C. E.
 D., Inger, R., & McDonald, R. A. (2020). Evaluating Bayesian stable
 isotope mixing models of wild animal diet and the effects of trophic
 discrimination factors and informative priors. Methods in Ecology and
 Evolution, 11(1), 139–149.
 [https://doi.org/10.1111/2041-210X.13311](https://doi.org/10.1111/2041-210X.13311)