DOI: 10.5061/dryad.573n5tbg4
title: Red-footed and masked boobies stable isotope data

description: Animals that co-occur in a region (sympatry) may share the same
 environment (syntopy), and niche differentiation is expected among closely
 related species competing for resources. The masked booby (Sula
 dactylatra) and smaller congeneric red-footed booby (Sula sula) share
 breeding grounds. In addition to the inter-specific size difference,
 females of both species are also larger than the respective males
 (reversed sexual size dimorphism). Although both boobies consume similar
 prey, sometimes in mixed-species flocks, each species and sex may
 specialize in terms of their diet or foraging habitats. We examined inter-
 and intra-specific differences in isotopic values  (δ13C and
 δ15N) in these pelagically feeding booby species during the incubation
 period at Clarion Island, Mexico, to quantify the degrees of inter- and
 intra-specific niche partitioning throughout the annual cycle. During
 incubation, both species preyed mainly on flyingfish and squid, but masked
 boobies had heavier food loads than red-footed boobies. There was no
 overlap in isotopic niches between masked and red-footed boobies during
 breeding (determined from whole blood), but there was a slight overlap
 during the non-breeding period (determined from body feathers). Female
 masked boobies had a higher trophic position than conspecific males during
 breeding; however, no such pattern was detected in red-footed boobies.
 These results provide evidence of inter- and intra-specific niche
 partitioning in these tropical seabird species, particularly during the
 breeding period and in the more dimorphic species. Our results suggest
 that these closely related species use different strategies to cope with
 the same tropical marine environment.
description: This study was conducted at Clarion Island, Revillagigedo
 Archipelago, Mexico (18°21’7.53” N, 114°43’18.61” W)
 Individual masked and red-footed boobies were captured at their
 nest by hand or using a hand net from a distance of 1–2 m.
 A few drops of blood (~ 0.15 mL) were collected from the brachial
 vein of individual birds using a 25 G needle and non-coated capillary
 tubes. The blood samples were placed on glass microscope slides and
 air-dried. Whole blood reflects the diet assimilated during the previous
 3–4 weeks (Vander Zanden et al., 2015), and whole blood samples therefore
 provide information on the bird’s diet during the incubation and/or
 pre-laying period. Body feathers were collected from the ventral part of
 adult birds and stored in individual paper bags. Body feathers were
 considered optimal given that they are easy to collect and do not impair
 the flight ability of the sampled birds (Jaeger et al., 2009; Bighetti et
 al., 2022). In contrast to whole blood, body feathers integrate
 information about the diet during the period when the feather was formed
 (from weeks to months; Grecian et al., 2015, Petalas et al., 2024), which
 is usually during the non-breeding period for these boobies (Grace et al.,
 2020; Schreiber et al., 2020). As mentioned before, no signs of molting,
 such as missing primary feathers, worn feathers of feather growth were
 observed while manipulating the individuals, supporting that molting
 occurs outside the breeding season. Dried whole blood
 samples (0.2–0.6 mg) were scraped from the slides and placed in tin cups
 in the laboratory. The feathers were immersed in a 2:1 chloroform and
 methanol solvent to remove surface oils and associated contaminants
 (Hobson et al., 2014). The samples did not undergo lipid extraction, and
 the low C:N (all &lt; 4) mass ratios indicated that mathematical
 correction for high lipid content was not required (Post et al., 2007).
 The isotope values of all samples were analyzed at the Leibniz Institute
 for Zoo and Wildlife Research, Berlin, Germany using a Flash elemental
 analyzer (Thermo Fisher Scientific, Bremen, Germany) connected in sequence
 via a ConFlo (Thermo Fisher Scientific, Bremen, Germany) to a stable
 isotope ratio mass spectrometer (Delta V; Thermo Fisher Scientific,
 Bremen, Germany). The instrument was flushed with chemically pure helium
 gas for measurements. Stable isotope ratios were expressed in delta
 notation indicating the deviation from international standards (in air
 nitrogen for nitrogen and V-PDB for carbon), according to the equation:
 δX=[(Rsample/Rstandard) −1], where X is <sup>13</sup>C or
 <sup>15</sup>N and R is the ratio
 <sup>13</sup>C/<sup>12</sup>C or
 <sup>15</sup>N/<sup>14</sup>N, respectively.
 Secondary isotopic reference materials were tyrosine
 (δ<sup>13</sup>C: -23.96 ± 0.02 ‰;
 δ<sup>15</sup>N: 4.36 ± 0.04 ‰) and leucine
 (δ<sup>13</sup>C: -30.15± 0.05‰; δ<sup>15</sup>N:
 10.82 ± 0.08 ‰). The analytical precision of both
 δ<sup>13</sup>C and δ<sup>15</sup>N was of
 &lt; 0.2 ‰.
description: # Red-footed and masked boobies stable isotope data
 [https://doi.org/10.5061/dryad.573n5tbg4](https://doi.org/10.5061/dryad.573n5tbg4) The data is composed of δ13C and δ15N values obtained from blood and feathers from red-footed and masked boobies breeding on Clarion Island, Revillagigedo Archipelago. ## Description of the data and file structure The values presented are carbon values and nitrogen values from blood samples (PO) and feathers, collected from female and male, red-footed and masked boobies. * Date of study: March 2017 and 2018 * Project name: Variations in inter-specific and sex-related niche partitioning in pelagic boobies during their annual cycle * There are 2 different species of boobies in the dataset collected from 1 island (Isla Clarion) in the Revillagigedo Archipelago. Contains **112** values from stable isotopes of red-footed and masked boobies. Carbon (δ13C) and nitrogen (δ15N) values are in ‰. The data set has three relevant **factor** variables: **Species:** Dactylatra, SulaSula * Dactylatra refers to masked boobies (*Sula dactylatr*a) * SulaSula refers to red-footed boobies (*Sula sula*). **Type:** PO, Feathers * PO refers to whole blood samples * Feathers referes to feather samples **Sex:** Female, Male * Female refers to those individuals which were female * Male refers to those individuals which were male The data set also includes two relevant **numeric** variables: **Carbon**: refers to the 13C stable isotopes values obtained from the lab **Nitrogen:** refers to the 15N stable isotopes values obtained from the lab * Carbon (δ13C) and nitrogen (δ15N) values are in ‰. **Additional columns include:** * ID: the ID to separate between individuals * Plate: the plate during the stable isotope analyses * Ring: the ID to separate between individuals * Plate.1: to refer to the plate name during the stable isotope analyses * Line: to refer to the line number during the stable isotope analyses This information is not relevant for analyses, but are useful for **reference to the lab.**