Natural History Notes and Diet Multi-Tissue Isotope Analysis of an Understudied Neotropical Highland Bird

Abstract

Many montane species with limited dispersal capacity and rigid habitat requirements are threatened by climate change. However, the extent of this threat is difficult to evaluate for those species whose basic life history, behavior, and resource-use information is lacking. I begin to provide this information for the sexually monomorphic Volcano Junco (Junco vulcani, Family: Passerellidae), a relatively unstudied sparrow that inhabits Isthmian páramo above the tree line of Costa Rica and western Panama. Isthmian páramo is threatened by climate change-mediated upslope vegetation shifts, but anecdotal evidence indicates that Volcano Juncos may perform downslope range expansions into clearcut areas that mimic the vegetation structure of páramo. This suggests a possible short-term resilience to anthropogenic disturbance that warrants further investigation of Volcano Junco ecology, behavior, and life history. Over three field seasons, I tracked Volcano Juncos at three sites in Costa Rica, collecting the first detailed observations of their breeding behavior, vocal behavior, and site fidelity. I used linear discriminant analysis to establish best practices for sexing Volcano Juncos in the field and performed multi-tissue stable isotope analysis to characterize individual, seasonal, ontogenetic, and geographic variation in diet. I also discuss novel evidence of greater than previously documented dispersal capacity, as well as apparent long-term social bonds, and their implications for the persistence of Volcano Juncos in a rapidly changing world. In Chapter One, alongside natural history notes, wing chords of 76 mm or greater were established as a threshold for assigning a male sex determination in Volcano Juncos. Linear discriminant analysis revealed that Volcano Junco sex determination was most influenced by bill depth and wing chord, with models excluding bill depth decreasing the certainty of low probability sex assignments, and models excluding wing chord decreasing the certainty of low and high probability sex assignments. In Chapter Two, I established that individual variation was likely not present at Cerro Buenavista, a site frequented by birding ecotourists and disturbed by telecommunications infrastructure. Individual variation appeared to be present at Cerro La Asunción, a nearby relatively undisturbed site, as well as at Volcán Irazú, a higher latitude site where Volcano Juncos were consistently able to exploit trash cans. However, this is likely an artifact of smaller sample size for these two sites. Feather samples were significantly more enriched in Carbon at Volcán Irazú than at Cerro La Asunción, suggesting greater relative abundance of C4 or CAM photosynthesizing plant resources across the landscape, or alternatively, a diet including human snack food from trash cans. Volcano Junco feathers from Buenavista, where tour guides sporadically feed Volcano Juncos corn-based snacks to lure them into the open - but where Volcano Juncos do not have consistent access to trash cans - displayed intermediate values of Carbon enrichment. Patterns of nitrogen enrichment in feathers showed the opposite pattern, suggesting that Volcano Juncos at Volcán Irazú, followed by Cerro Buenavista, had the highest trophic position of the three sites. Claw samples from all three sites reinforced these patterns, but with greater resolution, suggesting that claw samples collected during the same sampling period, despite averaging isotope enrichment over a longer period due to continuous growth, may more reliably reflect dietary information than feathers, which incorporate isotope ratios during the 1–2-week period of molt. In Chapter Three, after leveraging ellipse geometry and circular statistics to explore additional aspects of the Volcano Junco isotopic niche, sampling period played the greatest role in establishing patterns of ellipse eccentricity (i.e., the consistency of isotope resource-use). Conversely, patterns of Bayesian ellipse area, a proxy for trophic niche width, were most influenced by tissue type. Differences among the magnitude and direction of ellipse centroid shifts reinforced findings from Chapter Two, which indicated that, while seasonality likely occurred and was measurable in claw samples, this variation may also have been caused by a year effect, detectable in feather samples and therefore making it impossible to attribute to either cause.Though many of our findings were not conclusive, they point towards generalist feeding strategies in more comprehensively sampled Volcano Junco populations and justify the further study of less sampled populations. Though we have only begun the process of describing Volcano Junco ecology and behavior, a flexible dietary phenotype bodes well for their persistence in scenarios of increased anthropogenic disturbance, though this flexibility may be insufficient to protect Volcano Juncos from the threat of upslope vegetation creep.