This data package includes data on the foraging-associated movement and nutrient deposition of large nesting colonies of Western Gulls (Larus occidentalis), a generalist carnivore known to forage on human refuse. The study took place on the Channel Islands of California. Specifically, we equipped gulls with GPS loggers to assess the frequency of urban foraging alongside bird density data to estimate wild and urban-derived guano deposition on the island sites. We also conducted nest counts and vegetation surveys to assess potential nutrient sources, in addition to collecting soil samples at different sites throughout the islands to measure nutrient differences in the soil between the early and late part of the Western Gull nesting season. 

This study was conducted on Anacapa Island (ANIS) and Santa Barbara Island (SBIS), the two smallest of the Channel Islands off the coast of California (USA), which are currently managed as part of the Channel Islands National Park. Currently, they currently host large breeding colonies of various seabird species, including L. occidentalis (Carter et al. 1992).

Soil sampling

We sampled soil on ANIS and SBIS in 2019 to test for concentrations of available nitrogen and phosphorus, as these soil characteristics are associated with nutrient deposition in seabird colonies (Ellis 2005; Young et al. 2010). Sampling locations throughout the two islands were selected based on nesting L. occidentalis densities from previous breeding seasons (personal communication with National Park Service) to approximate an equal distribution of high and low bird density sites at each island. Eight sites were sampled on East Anacapa Island and eleven sites were sampled throughout Santa Barbara Island.

In order to assess the nutrient deposition throughout the nesting season, we collected soil at two separate time points: early in the nesting season when gulls are establishing territories and starting to build nests (“pre-nesting”, April 2019) and once most chicks had hatched (“post-nesting”, August 2019). At each site on each island (Fig S1), we collected four random samples of topsoil (10 cm) within a 5 m radius of site origin. We also conducted nest counts (five 10 m x 4 m belt transects covering a 200 m² area around site origin) at each site in August 2019 to account for bird density in soil nutrient differences. Additionally, we conducted five vegetation survey quadrats at each site to assess presence and abundance of nitrogen-fixing plant species (family: Fabaceae) that could be driving nitrogen inputs to study site soil.

Soil samples were dried at the field site, homogenized (by site), and sieved (<2mm) upon return to lab facilities at UC Santa Barbara. Soil samples were then sent to Brookside Laboratories (New Bremen, OH, USA) for analysis. At Brookside Laboratories, each sample was evaluated for concentrations of available nitrogen (NO₃- and NH₄-N) [cadmium reduction method (Dahnke and Johnson 1990)] and phosphorus (Mehlich 1984).

Site coordinates for soil samples can be found in chis.soil.meta.csv

Foraging behavior

We studied L. occidentalis foraging behavior during the incubation period of 2016 (April–May). We collected feather and blood samples from 137 adult birds (53 on SBIS and 84 on ANIS)  for stable isotope analysis, as well as outfitting a subset of birds with GPS trackers. All sampled and tagged birds were captured from nests containing 2–3 eggs with a noose carpet or single-foot snare. Birds that were sampled but not tagged were only captured once and sampled for feathers (two breast feathers and two underwing coverts) and blood (between 0.5 and 2.0 ml of blood from the brachial vein) for stable isotope analysis. We also measured body mass, culmen, tarsus, and skull length. Nine of the sampled gulls regurgitated when captured, and these opportunistic diet samples were also collected in the field.

A subset of 52 birds (21 on SBIS and 31 on ANIS) was outfitted with a GPS logger (IGotU GT-120, Mobile Action Technology Inc.), which recorded location every 30–60 s with an accuracy of 2–4 m. Upon initial capture, the logger was attached to the bird’s three central tail feathers using TESA cloth tape. The loggers were removed from their original casing to decrease total weight and wrapped in waterproof heat shrink wrap. Loggers were deployed for a minimum of 24 h before attempting recovery using the capture methods mentioned above. GPS loggers were recovered from 17 gulls on SBIS (80%) and 26 gulls on ANIS (83%). In addition to removing the GPS logger upon recapture, we conducted the sampling protocol for feathers, blood, and morphometric data described above. All blood samples were kept cold and centrifuged blood within two hours of collection, then kept frozen in liquid nitrogen until return to the laboratory at UC Santa Barbara.

Stable Isotope Analysis

We conducted stable isotope analysis of ¹³C and ¹⁵N isotope ratios in feather and blood samples to explore foraging differences across the breeding and non-breeding season. We used isotopic signatures from plasma to infer diet from the breeding season, as the integration rate for plasma is high and isotope ratios represent a feeding period of ~3 days (Hobson 2005). We also used isotopic signatures from feathers to infer longer time periods as feathers have a much slower integration rate and isotopic ratios reflect diet at the time of feather growth after a molting period (Hobson and Clark 1992; Hobson 2005). For L. occidentalis, molting occurs twice a year in the early fall and late winter and last at least two months, thus feather isotopic rations represent the non-breeding season and shoulder of the breeding season (McCaskie 1983; Howell and Corben 2000). Prior to isotopic analysis, we centrifuged the sampled blood was centrifuged soon after collected, and plasma was then freeze-dried, ground, and ~0.5 mg were loaded into tin capsules. Feathers for each bird were first cleaned of surface lipids and contaminants using a 2:1 chloroform and methanol solution followed by two methanol rinses, dried, and then cut into small fragments and ~0.5mg were loaded into tin capsules. Tin capsules were sent to the University of California, Davis Stable Isotope Facility for analysis. Samples were analyzed for ¹³C and ¹⁵N isotopes using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK).

Metadata for methods and datasets can be found in file "Guerra_WEGU_subsidies_metadata.rtf". 

Code for analysis can be found in file: wegu.subsidies.ms.code_f.Rmd