DOI: 10.5061/dryad.47d7wm3dk
title: Isotopic analysis reveals landscape patterns in the diet of a subsidized predator, the common raven

description: 1. Anthropogenic subsidies to native predators can have cascading effects
 on sensitive prey populations, but the spatial mechanisms behind these
 effects are often unknown. 2. We used a stable isotope mixing model to
 reconstruct spatially naïve assimilated diets of common raven (Corvus
 corax) chicks and then used regression analysis to investigate landscape
 patterns in assimilated chick diet, with particular respect to the eggs
 and chicks of greater sage-grouse (Centrocercus urophasianus). 3.
 Assimilated raven diets were primarily composed of mammal carrion,
 followed by anthropogenic food and sage-grouse eggs and chicks. 4. Raven
 diets showed landscape gradients, whereby raven chicks in nests near
 active greater sage-grouse breeding leks consumed a higher proportion of
 sage-grouse eggs, sage-grouse chicks, and insects in their diet and less
 mammal carrion. A majority of raven nests on anthropogenic nesting
 structures (78.7%) were within 5 km of the nearest sage-grouse lek. Ravens
 nesting in high-probability greater sage-grouse nesting habitat consumed
 more insects and plants and less mammal carrion. 5. In landscapes devoid
 of natural raven nesting substrates, such as our study area, anthropogenic
 nesting substrates can ‘anchor’ breeding ravens nearer to greater
 sage-grouse leks, with concomitant increases in raven predation on greater
 sage-grouse nests. Curtailment of anthropogenic nesting substrates within
 5 km of a sage-grouse lek may have a disproportionately positive impact on
 sage-grouse populations. More generally, these findings highlight that the
 spatial arrangement of anthropogenic subsidies can result in indirect
 interactions between humans and predators with direct implications for
 predators and prey.
description: Stable isotope sampling
 Feathers are keratinized tissues that preserve the isotopic
 record indefinitely and can represent data spanning the period for which
 the feathers were grown (Pearson et al. 2003).  We collected and combined
 two scapular feathers from 179 raven nestlings (usually two nestlings per
 nest) via clipping for stable isotope analysis of carbon
 (<i>δ</i><sup>13</sup>C) and nitrogen
 (<i>δ</i><sup>15</sup>N).  Feathers were collected
 between May 9<sup>th</sup> and July 3<sup>rd</sup>
 in 2013 and May 2<sup>nd</sup> and July
 17<sup>th</sup> in 2014.  Only fully-grown or emerged portions
 of pin feathers were collected (i.e., dry feathers only) and clippings
 measured approximately 50 mm on average but varied depending on the age of
 chicks and stage of feather development.  Feathers were placed in
 individually labeled and sealed paper envelopes and stored at room
 temperature until shipment to the analytical lab.  The sampling was
 non-lethal and minimally invasive.  All raven chicks from which feathers
 were collected were banded with USFWS leg bands and returned to the nest
 (Federal Bird Banding Permit #23780). We also collected
 samples from potential diet items within the study area. These items
 included hair samples collected from opportunistic road-killed herbivore
 mammals (n=6; three domestic cows [<i>Bos taurus</i>], one
 domestic sheep [<i>Ovis aries</i>], one jackrabbit
 [<i>Lepus townsendii</i>], and one pronghorn
 [<i>Antilocapra americana</i>]), greater sage-grouse egg shell
 membranes from already hatched or depredated nests (n=13), sage-grouse
 feathers from already hatched or depredated nests (n=5), and sage-grouse
 feathers from a depredated hen (n=1) found within the study area.  We also
 collected anonymous human hair samples that were pooled from floors of
 hair salons and barber shops in Rawlins, Wyoming.  These human hair
 samples were used to generally reflect composite anthropogenic diets
 (e.g., composite isotopic signatures of anthropogenic food sources such as
 dumps and transfer stations often based on C<sub>4</sub>
 plants; O’Connell and Hedges 1999).  All samples were stored in
 individually labeled paper envelops and sent to the analytical lab for
 analysis.       Laboratory
 analysis All samples were sent to the
 University of Wyoming Stable Isotope Facility (Laramie, Wyoming, USA) for
 sample preparation and analysis following each field season.  To clean the
 samples, a 2:1 mixture of chloroform:methanol was used to rinse each
 sample, followed by 3 rinses of deionized water.  Samples were loaded into
 tins with a range from 0.75 to 0.85mg.  The standard uncertainty for the
 lab’s instruments for carbon is 0.15 permil and for nitrogen is 0.2
 permil, although the precision was less than or equal to 0.1 permil for
 all runs of our samples.  Samples were ground into extremely fine powder
 using a ball mill.  Isotopic analyses were conducted using a
 Carlo Erba 1110 or Costech 4010 Elemental Analyzer coupled
 to a Finnigan Delta+XP
 continuous flow inlet isotope ratio mass spectrometer (Thermo Finnigan,
 Bremen, Germany).  Repeated measurements with laboratory
 CO<sub>2</sub>-in-air working
 standards had a precision of &lt;0.1‰. The <em>δ</em><sup>13</sup>C values of
 CO<sub>2</sub> in respiration
 samples were corrected to the international standards, Vienna Pee Dee
 Belemnite for <sup>13</sup>C and the atmospheric nitrogen for
 <sup>15</sup>N.  We calculated <i>δ</i> values
 (per mil, ‰) using <i>δ</i>X =
 [(R<sub>sample</sub>/R<sub>standard</sub>) -
 1] where X
 is the element of interest (<sup>13</sup>C
 or <sup>15</sup>N), R<sub>sample</sub> is for the
 ratio of the heavier to lighter isotope
 (<sup>13</sup>C/<sup>12</sup>C
 or <sup>15</sup>N/<sup>14</sup>N) of the sample,
 and R<sub>standard</sub> is for the ratio
 (<sup>13</sup>C/<sup>12</sup>C
 or <sup>15</sup>N/<sup>14</sup>N) of the
 international standard. Reporting of isotope data and
 measurement results follow the guidelines provided by Coplen
 (2011). <i>Tissue and fractionation
 adjustments</i> We post-processed laboratory
 results of the source diet item in two ways.  First, we combined mammals
 into a single ‘mammal carrion’ category because: (1) we lacked sufficient
 species-specific sample sizes to estimate diet contribution of each mammal
 species, (2) all mammals shared similar ecological niches compared to
 other potential diet sources, and (3) ravens likely feed on mammal
 carcasses opportunistically.  We did not consider mammalian predators as
 potential diet items because Kristan et al. (2004) found evidence of
 predators in only 0.2% of raven pellets.  Second, we used data from the
 peer-reviewed literature to adjust the stable isotope values from our
 tissue samples to reflect fractionation within the body of each source
 species.  We did this because adult ravens were likely bringing
 energy-rich tissues (e.g., muscle) of mammals to the nest rather than our
 sampled tissue (e.g., fur).  We subtracted 1.6‰ from the
 <i>δ</i><sup>13</sup>C value for all mammal fur
 samples to reflect mammal muscle (hereafter ‘mammal carrion’; Roth and
 Hobson 2000, Caut et al. 2009).  We subtracted 4.15‰ from the
 <i>δ</i><sup>15</sup>N value and subtracted 1.85‰
 from the <i>δ</i><sup>13</sup>C value for all
 human hair samples to reflect human diet in the study area (hereafter
 ‘anthropogenic food’; Schoeller 1986, Minagawa 1992).  We subtracted 3.41‰
 from the <i>δ</i><sup>13</sup>C value for all
 sage-grouse egg membranes to reflect egg yolk and albumen but did not
 adjust the <i>δ</i><sup>15</sup>N values for eggs
 (Hobson 1995).  We subtracted 2.14‰ from the
 <i>δ</i><sup>15</sup>N value and 1.24‰ from the
 <i>δ</i><sup>13</sup>C value for all sage-grouse
 feather samples to reflect sage-grouse muscle (hereafter ‘sage-grouse
 chick’).  We used previously published data from the literature to define
 the isotopic values of sagebrush steppe C<sub>3</sub> plants
 (<i>δ</i><sup>15</sup>N = 1.54‰,
 <i>δ</i><sup>13</sup>C = -26.60‰; Kelly 2000,
 Mowat and Heard 2006, Feranec 2007, Kohn 2010) and sagebrush steppe
 insects during spring (<i>δ</i><sup>15</sup>N =
 5.65‰, <i>δ</i><sup>13</sup>C = -25.51‰; Blomberg
 et al. 2013).