Introduction
Sexual partitioning of food resources is known to occur in many animal
species, but the extent and ecological significance of this phenomenon
are still poorly understood (Ruckstuhl & Neuhaus, 2006). In birds,
differences in diet indicative of resource differentiation have mostly
been studied in birds with considerable sexual dimorphism in body size
(Bravo, Ponce, Bautista, & Alonso, 2016; Catry, Alves, Gill,
Gunnarsson, & Granadeiro, 2012; Donals et al., 2007; Gonzalez-Solis,
Croxall, & Wood, 2000; Thalinger, Oehm, Zeisler, Vorhauser, &
Traugott, 2018) or in bill size or shape (Smith, 1990; Summers, Smith,
Nicoll, & Atkinson, 1990; Temeles, Mazzotta, & Williamson, 2017;
Temeles & Roberts, 1993). As a consequence, intraspecific dietary
differentiation in birds has been largely attributed to morphological
differences, with more sexually dimorphic species expected to show
higher resource differentiation (Alarcón et al., 2017; Fonteneau,
Paillisson, & Marion, 2009; Lewis et al., 2005; Phillips, McGill,
Dawson, & Bearhop, 2011; Selander, 1966). However, it is possible that
sexual food resource differentiation also occurs in monomorphic or only
slightly dimorphic birds, but this idea remains little explored (but see
Botha, Rishworth, Thiebault, Green, & Pistorius, 2017; Cleasby et al.,
2015; Elliott, Gaston, & Crump, 2010; Hedd, Montevecchi, Phillips, &
Fifield, 2014).
One of the obstacles to understand eventual sexual partitioning of food
resources is related to limitations of widely used diet analysis
methods, which often are unable to provide enough taxonomic resolution
to detect subtle differences in prey consumption (e.g., Mata et al.,
2016). This is the case, for instance, of methods widely used in avian
ecology, including for instance the morphological identification of the
remains of ingested food items (Bravo et al., 2016; Fonteneau et al.,
2009; Hunter, 1983; Hunter & Brooke, 1992), direct observation (Catry
et al., 2012), fatty acids and alcohols analysis (Owen et al., 2013), or
stable isotope analysis (Blanco-Fontao, Sandercock, Obeso, McNew, &
Quevedo, 2013; Cleasby et al., 2015; Elliott et al., 2010; Hsu, Shaner,
Chang, Ke, & Kao, 2014; Ludynia et al., 2013; Paiva et al., 2018;
Phillips et al., 2011). The advent of high-throughput DNA sequencing is
making it possible to overcome the limitations of these methods,
providing the ability to identify virtually all prey species consumed
with unprecedent taxonomic resolution (Hope et al., 2014; Nielsen,
Clare, Hayden, Brett, & Kratina, 2017; Razgour et al., 2011; Soininen
et al., 2009). As a consequence, this approach has been increasingly
used to describe the diets of a wide range of animals (Brown, Jarman, &
Symondson, 2012; Kaunisto, Roslin, Sääksjärvi, & Vesterinen, 2017;
Macías-Hernández et al., 2018; Mata et al., 2016; Soininen et al.,
2009), including birds (Coghlan et al., 2013; Deagle, Chiaradia,
McInnes, & Jarman, 2010; Jedlicka, Vo, & Almeida, 2017; Liu et al.,
2018; Sullins et al., 2018; Trevelline et al., 2018). The high taxonomic
resolution provided by high-throughput sequencing has already been used
to describe sexual dietary differences that otherwise would be almost
impossible to detect (Mata et al., 2016). However, previous studies have
focused on specialists with a relatively narrow feeding niche, while
this methodology remains underexplored in testing sexual dietary in more
generalist species such as many omnivorous passerines. Dietary
generalists are more challenging to study using metabarcoding because
they require a combination of markers to fully encompass the full
spectrum of food resources used (da Silva et al., 2019a).
Here we aim to show the power of
multi-marker metabarcoding to investigate differences in diet between
sexes, by focusing on a generalist passerine judged to have minimal
sexual dimorphism, the black wheatear (Oenanthe leucura ). To
address this general goal, the study first documents differences in
morphology (bill and body features) between sexes, and then uses a
previously developed approach for integrating metabarcoding dietary data
across multiple markers (da Silva et al., 2019a) to describe the diets
of both sexes. Using this data we then tested the hypothesis that diet
varies between sexes in terms of (i) diet diversity and (ii)
frequency of occurrence of the
main food items, and that (iii)
sexual dietary differentiation can
only be detected at the high taxonomic resolution provided by
metabarcoding. Results were used to discuss the potential of
multi-marker metabarcoding to provide a detailed understanding of
intraspecific variation in bird diets.