DOI: 10.25349/d9j31x
title: Western Gull movement and nutrient deposition on Channel Islands, CA

description: 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. 
description: 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
 <em>L</em>. <em>occidentalis</em> (Carter et al.
 1992). <strong>Soil
 sampling</strong> 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
 <em>L</em>. <em>occidentalis</em> 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 m<sup>2</sup> 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 (&lt;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 (NO<sub>3</sub>- and NH<sub>4</sub>-N)
 [cadmium reduction method (Dahnke and Johnson 1990)] and phosphorus
 (Mehlich 1984). Site coordinates for soil samples can
 be found in <em>chis.soil.meta.csv</em>
 <strong>Foraging behavior</strong>
 We studied <em>L</em>.
 <em>occidentalis</em> 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.
 <strong>Stable Isotope Analysis</strong>
 We conducted stable isotope analysis of
 <sup>13</sup>C and <sup>15</sup>N 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 <em>L</em>.
 <em>occidentalis</em>, 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 <sup>13</sup>C
 and <sup>15</sup>N isotopes using a PDZ Europa ANCA-GSL
 elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass
 spectrometer (Sercon Ltd., Cheshire, UK).
description: 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