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.

Blood and feather collection

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 >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 >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 <50 m and in all cases <200 m) and floaters are two to five-fold more abundant than breeding adults (UB unpublished). Therefore, we assume a distance of >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., >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.

 

Prey sample collection

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.

 

Stable isotope analysis

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 δ¹³C and δ¹⁵N. Stable isotope values were calculated with the following equation and are expressed in standard delta (δ) notation in per mil units (‰):

where X is ¹³C or ¹⁵N and R is the corresponding ratio ¹³C/¹²C or ¹⁵N/¹⁴N. The R_standard values were based on Vienna Pee Dee Belemnite (VPDB) for δ¹³C and atmospheric N₂ (AIR) for δ¹⁵N values.