Current methods and future directions in avian diet analysis

Identifying the composition of avian diets is a critical step in characterizing the roles of birds within ecosystems. However, because birds are a diverse taxonomic group with equally diverse dietary habits, gaining an accurate and thorough understanding of avian diet can be difficult. In addition to overcoming the inherent difficulties of studying birds, the field is advancing rapidly, and researchers are challenged with a myriad of methods to study avian diet, a task that has only become more difficult with the introduction of laboratory techniques to dietary studies. Because methodology drives inference, it is important that researchers are aware of the capabilities and limitations of each method to ensure the results of their study are interpreted correctly. However, few reviews exist which detail each of the traditional and laboratory techniques used in dietary studies, with even fewer framing these methods through a bird-specific lens. Here, we discuss the strengths and limitations of morphological prey identification, DNA-based techniques, stable isotope analysis, and the tracing of dietary biomolecules throughout food webs. We identify areas of improvement for each method, provide instances in which the combination of techniques can yield the most comprehensive findings, introduce potential avenues for combining results from each technique within a unified framework, and present recommendations for the future focus of avian dietary research.

Functional flexibility in a spider's orb web

ABSTRACTWeb spiders rely on vibrations propagated via their web to identify, locate and capture entangled prey. Here, we experimentally tested the robustness of the orb weaver's predation strategy when webs are severely distorted and silk tensions are drastically altered throughout the web, a common occurrence in the wild. We assessed prey identification efficiency by comparing the spider's initial reaction times towards a fruit fly trapped in the web, we measured location efficiency by comparing times and number of tugging bouts performed, and we determined capture efficiency by comparing capture times. It emerged that spiders are capable of identifying, locating and capturing prey in distorted webs, albeit taking somewhat longer to do so.

Interannual variability of diet composition and prey preference of larval redfish (Sebastes spp.) in the Gulf of St. Lawrence

Spatiotemporal overlap between fish larvae and their planktonic prey is an important source of recruitment variability. Over the past decade, one species of redfish, Sebastes mentella, from the Gulf of St. Lawrence (GSL) produced multiple strong cohorts following decades of low recruitment, which has generated strong interest in identifying potential drivers of larval survival. The present study provides the first detailed, multi-year assessment of larval redfish (Sebastes spp.) trophodynamics. Interannual variability in larval redfish diet composition and prey selectivity was assessed using high-resolution prey identification of larval gut contents and in situ prey fields. Eggs from the calanoid copepod Calanus finmarchicus represented the most frequently consumed prey in 3 of the 4 collection years, and contributed the largest proportion of carbon ingested by redfish larvae in all years. The high consumption of C. finmarchicus eggs by larvae, combined with evidence of positive selection for this taxon in some years, supports the hypothesis of a strong trophic link between larval redfish and a key calanoid copepod in the GSL ecosystem. Our results indicate that future efforts investigating GSL redfish recruitment processes should consider environment-driven variability in the reproductive phenology and abundance of C. finmarchicus.

New perspectives on trophic guilds of arthropodivorous bats in North and Central America

Trophic guilds are useful concepts for advancing our knowledge of trophic structure of communities, dynamics of species interactions, redundancy in ecosystem services, resilience to disturbances, response to climate change, conservation strategies, etc. For insectivorous bats, current literature suggests 8 trophic-related guilds. These include 3 guilds based on the openness of foraging areas, 3 based on the style of feeding, and 2 recently proposed subguilds among gleaners. Some gleaners are “passive,” using densely cluttered vegetation in which echolocation is ineffective, and others are “actively” gleaning, using echolocation to procure prey. None of these guilds is based on the actual diets of bats. We analyzed 33 reports of diet composition representing 51 species of arthropod-feeding bats inhabiting North and Central America. We wanted to determine if the classical guild structure was concordant with the actual diets of bats and to compare guild structure in the Nearctic with that in the Neotropics. Discriminant function and principle component analyses generated 5 groups of genera based on the proportion of various arthropod taxa (mainly orders) in their diets. These groups were very different from classical guilds and showed almost no overlap among bat genera between the 2 continental regions. A similar analysis based on prey flying ability and hardness of their exoskeletons suggested 4 guilds that were more consistent with classical guild concepts, had higher rates of unambiguous guild assignment, and also showed major continental differences. Our results suggest a new arrangement of 4 guilds for arthropod-feeding bats in North and Central America that are based primarily on 2 features of their prey. New molecular techniques should allow us to build on this arrangement by significantly improving the taxonomic level of prey identification.Los gremios tróficos son conceptos útiles para la mejora de nuestros conocimientos sobre la estructura trófica de las comunidades, la dinámica de las interacciones entre especies, la redundancia en los servicios de los ecosistemas, la capacidad de resistencia a las perturbaciones, la respuesta al cambio climático, las estrategias de conservación, etc. Para los murciélagos insectívoros la literatura actual sugiere ocho gremios tróficos. Estos incluyen tres gremios basados en la apertura de las zonas de alimentación, tres con base en el estilo de alimentación, es decir, la búsqueda aérea, caza de arrastre sobre superficies de agua, y recolección de presas en superficies, además de dos sub-gremios propuestos recientemente para los de hábitos recolectores. Algunos gremios son “pasivos”, los cuales forrajean en espacios excesivamente saturados de elementos de vegetación y hacen un uso de ecolocación es ineficaz, y otros recolectores “activos” los cuales utilizan la ecolocación para adquirir presas. Ninguno de estos gremios se basa en las dietas reales de murciélagos. Se analizaron 33 artículos sobre composición de la dieta, los cuales representan 51 especies de murciélagos de alimentación de artrópodos que habitan en Norte y Centro América. El objetivo del presente trabajo fue determinar si la estructura de los gremios clásicos era concordante con las dietas reales de los murciélagos, y comparar la estructura de los gremios entre las regiones Neártica y Neotrópical. En análisis de función discriminante y componentes principales se generaron 5 grupos de géneros con base a la proporción de los diferentes taxones de artrópodos (principalmente órdenes) contenidos en sus dietas. Estos grupos fueron muy diferentes de los gremios clásicos y mostraron casi ningún solapamiento entre los géneros de murciélagos de las dos regiones continentales. Un análisis similar con base en la capacidad de vuelo y la dureza de los exoesqueletos de las presas, ha apuntado a la conformación de cuatro gremios que estaban más en consonancia con los conceptos clásicos de gremio, tuvieron tasas más altas de la asignación inequívoca, y también mostró grandes diferencias continentales. Nuestros resultados sugieren un nuevo arreglo de cuatro gremios de murciélagos de alimentación de artrópodos en Norte y Centro América que se basan principalmente en dos características de su presa. Las nuevas técnicas moleculares deben permitir que construyamos sobre este acomodo, mejorando significativamente el nivel taxonómico de identificación presa.

Challenges to a molecular approach to prey identification in the Burmese python,Python molurus bivittatus

Molecular approaches to prey identification are increasingly useful in elucidating predator–prey relationships, and we aimed to investigate the feasibility of these methods to document the species identities of prey consumed by invasive Burmese pythons in Florida. We were particularly interested in the diet of young snakes, because visual identification of prey from this size class has proven difficult. We successfully extracted DNA from the gastrointestinal contents of 43 young pythons, as well as from several control samples, and attempted amplification of DNA mini-barcodes, a 130-bp region ofCOX1. Using a PNA clamp to exclude python DNA, we found that prey DNA was not present in sufficient quality for amplification of this locus in 86% of our samples. All samples from the GI tracts of young pythons contained only hair, and the six samples we were able to identify to species were hispid cotton rats. This suggests that young Burmese pythons prey predominantly on small mammals and that prey diversity among snakes of this size class is low. We discuss prolonged gastrointestinal transit times and extreme gastric breakdown as possible causes of DNA degradation that limit the success of a molecular approach to prey identification in Burmese pythons.