DOI: 10.5061/dryad.n2z34tmv4
title: The dynamic trophic niche of an island bird of prey

description: Optimal Foraging Theory predicts an inverse relationship between the
 availability of preferred prey and niche width in animals. Moreover, when
 individuals within a population have identical prey preferences and
 preferred prey is scarce, a nested pattern of trophic niche is expected if
 opportunistic and selective individuals can be identified. Here, we
 examined intraspecific variation in the trophic niche of a resident
 population of striated caracara (Phalcoboenus australis) on Isla de los
 Estados (Staten Island), Argentina, using pellet and stable isotope
 analyses. While this raptor specializes on seabird prey, we assessed this
 population’s potential to forage on terrestrial prey, especially invasive
 herbivores as carrion, when seabirds are less accessible. We found that
 the isotopic niche of this species varies with season, age, breeding
 status and, to a lesser extent, year. Our results were in general
 consistent with classic predictions of the Optimal Foraging Theory, but we
 also explore other possible explanations for the observed pattern.
 Isotopic niche was broader for groups identified a priori as opportunistic
 (i.e., non-breeding adults during the breeding season and the whole
 population during the non-breeding season) than it was for individuals
 identified a priori as selective. Results suggested that terrestrial input
 was relatively low, and invasive mammals accounted for no more than 5% of
 the input. The seasonal pulse of rockhopper penguins likely interacts with
 caracara’s reproductive status by constraining the spatial scale on which
 individuals forage. Niche expansion in spatially flexible individuals did
 not reflect an increase in terrestrial prey input, rather it may be driven
 by a greater variation in the types of marine prey items consumed.
description: <i>Blood and feather
 collection</i> Blood samples
 (~1 ml) were collected from the brachial vein of ~20-day old chicks (43
 individuals from 17 nest sites; 1-3 chick/nest*year) captured manually,
 and from juveniles, immatures and adults during the breeding (n= 8) and
 non-breeding (n= 8) seasons captured with walk-in and noose traps, and
 later stored in 70% ethanol (Hobson, Gloutney, & Gibbs, 1997). Age
 of individuals was determinate by plumage cues (Strange 1996).  We used
 the mean value of each nest for those with more than one chick, obtaining
 8-10 independent samples/year. Also, as in some cases we collected samples
 from the same nests in multiple years, when we estimate overall isotopic
 niche parameters for chicks, we use the mean isotopic values for each one
 of the 17 nest sites. All 59 captured birds were banded with plastic rings
 (Ecotone, Poland) and no individual was sampled twice during the study
 period. To obtain floater and breeding adult samples, we collected moulted
 wing feathers, and classified them in relation to their distance from the
 nests. When collected from nest sites, we assumed it was moulted by a
 breeding adult (n= 13, one feather/nest); and when collected &gt;300 m
 apart from any active nest, by a floater (n= 63). Caracaras nest in a
 nearly colonial arrangement with very small breeding territories (Strange,
 1996). The
 &gt;300 m threshold was assumed not likely to represent breeding adult
 samples because observed foraging of the breeding adults was mainly
 associated with the nearest penguin patch (i.e., median &lt;50 m and
 in all cases &lt;200 m) and floaters are two to five-fold more
 abundant than breeding adults (UB unpublished). Therefore, we assume a
 distance of &gt;300 m from any known nest site is an area unlikely to
 be used by a breeding adult. Feathers were identified as belonging to
 adult birds (i.e., &gt;5 years old) following Strange (1996). Floater
 abundance was 92 (95% CI 62-139) individuals in 2018, and since we
 obtained 11-29 samples/year, we assume no double sampling in this part of
 the population either. Moulting of feathers in the study area was only
 observed during the breeding season for both floaters and breeding adults,
 and thus we assumed that feathers are synthetized during the period of
 rockhopper penguin presence. Samples used are summarized in Table
 1.   <i>Prey sample
 collection</i> To describe
 the potential prey resources for caracaras for building mixing models, we
 collected tissue samples of representative prey based on prey remains
 observed in pellets, published literature and field observations (Catry et al., 2008;
 Rexer-Huber & Bildstein, 2013). From 2017 to 2018, we
 collected samples from marine and terrestrial prey on Isla de los Estados
 (Table 2). Mussels were collected manually from the intertidal during low
 tide. Birds and invasive mammal samples were collected from fresh dead
 animals in our systematic surveys along the shores and at seabird
 colonies. Recently abandoned eggs were collected manually. Rodents were
 collected using Sherman-like traps, and insects were collected using
 pitfall traps. Sea lion feaces observed to be eaten by caracaras were
 collected in the non-breeding season Observatorio Island, 40 km to the NE
 of our study area. All other samples were collected in Franklin Bay during
 the breeding season.  
 <i>Stable isotope analysis</i> To prepare our samples, we rinsed feathers with a 2:1 chloroform:methanol solution to remove surface lipids and dried them at room temperature. Blood samples were first dried at 60 °C for 24 h and then freeze-dried for another 24 h. We weighed ~0.60 mg of each sample into tin capsules, which were flash combusted in a Costech ECS4010 elemental analyser coupled to a Thermo-Fisher Delta Plus XP continuous-flow stable isotope ratio mass spectrometer. Stable isotope values were normalized using a two-point system with glutamic acid reference material (USGS-40 and USGS-41). Measurement precision based on reference material was 0.1 ‰ for both δ<sup>13</sup>C and δ<sup>15</sup>N. Stable isotope values were calculated with the following equation and are expressed in standard delta (δ) notation in per mil units (‰): where X is <sup>13</sup>C or <sup>15</sup>N and R is the corresponding ratio <sup>13</sup>C/<sup>12</sup>C or <sup>15</sup>N/<sup>14</sup>N. The R<sub>standard</sub> values were based on Vienna Pee Dee Belemnite (VPDB) for δ<sup>13</sup>C and atmospheric N<sub>2 </sub>(AIR) for δ<sup>15</sup>N values.