Discussion
Our results confirmed all our hypothesis and showed that although black wheatears exhibit only minor sexual size dimorphism there was dietary differentiation between both sexes, by (i) males having an overall higher diet diversity and (ii) females preying more often on some ant species than males. This is the first time such intra-specific differences are either studied or found in birds using metabarcoding techniques. As expected, the differences found in the diet composition and estimated richness were smaller or not significant using higher taxonomic ranks, suggesting that if lower taxonomic resolution methodologies had been used, these differences would not have been detected. This methodology could be particularly relevant for birds as passerines and near passerines, that feed on hyper diverse taxonomic groups that are often difficult to identify, as insects and other arthropods, and in which diets are often evaluated to the order or family level through conventional techniques (Araújo, Lopes, da Silva, & Ramos, 2016; Catry et al., 2019; Hodar, 1995).
The morphometric differences between sexes observed in our study were related to the thicker bill and longer wings and tail of males. In previous studies conducted in Alicante (Pérez-Granados & Seoane, 2018) and Hoya de Guadix (Møller, Lindén, Soler, Soler, & Moreno, 1995), Spain, males were described not only as having longer wings (wing length and 3rd primary) and tail, but also as being heavier and with a longer tarsus than females. This indicates that sexual size dimorphism on this species may differ across its distribution. The fact that our males showed longer wings and tail, but similar body mass and tarsus, a proxy for body size (Freeman & Jackson, 1990; Pérez-Granados & Seoane, 2018; Rising & Somers, 1989), suggests a higher flight capability of males compared to females. It has been suggested that the larger wings and tail of male black wheatear’s could be related to their stone-carrying behaviour (Pérez-Granados & Seoane, 2018; Soler, Soler, Møller, Moreno, & Lindén, 1996) that is mainly done by males (Aznar & Ibáñez-Agulleiro, 2016; Moreno, Soler, Møller, & Linden, 1994). Males also move more often in their territories than females, especially for territory defence, not only against conspecifics, but also against other birds of different sizes (Møller, 1992; Prodon, 1985). Regarding the thicker bill of males, it could also be an adaptation to the stone-carrying behaviour and higher aggressivity.
The dietary composition of black wheatear observed in our study was largely similar to that documented elsewhere. In particular, the large dietary spectrum of arthropod groups and the ability to hunt relatively large prey such as reptiles was already reported from natural habitats of Spain, where the most frequent prey were also ants (Hodar, 1995; Richardson, 1965). The highest difference found between previous dietary studies of this species and our work, is the high frequency of berries detected in our study. To some extent this could be due to the different methods used for the identification of the droppings remains (da Silva et al., 2019a). However, it is more likely related to differences in habitat, since the Portuguese population occurs mainly in traditional agricultural habitats (vineyards and olive groves) where Solanum nigrum is a very widespread and abundant herb, providing a high number of ripe fruits, while the studied Spanish populations were located in shrub steppe areas, presumably with a lower availability of berry-bearing plant species during the wheatear’s breeding season (Hodar, 1995).
The differences in diet composition observed in our study are likely more related to sexual behavioural differences during the breeding season than to the morphometric differences observed. Although males have a more robust bill than females, its length and width is similar, which in principle allows both sexes to capture and swallow similar prey items. In some birds it has been reported that females tend to forage closer to their offspring than males (Sunde, Bølstad, & Møller, 2003). This behaviour could lead females to prey more often on abundant and predictable prey like ants, even if these are smaller and less nutritious (Dean & Milton, 2018). On the other hand, the higher mobility of males within territories could explain the lower frequency of some less nutritious prey (e.g., ants), and the wider range of other prey, likely less predictable and abundant.
As far as we could find, this is the first example of a monomorphic (or minor dimorphic) passerine species exhibiting dietary differences between sexes, during the breeding season.
Usually, the more sexually dimorphic a bird species is a higher resource differentiation is expected (Fonteneau et al., 2009; Lewis et al., 2005; Phillips et al., 2011; Selander, 1966). Nevertheless, on some monomorphic seabirds species, different foraging areas have been described between sexes, especially in the beginning of the breeding period (Cleasby et al., 2015; Hedd et al., 2014; Pinet, Jaquemet, Phillips, & Le Corre, 2012). On two New Guinean whistlers, passerine species with little sexual dimorphism, vertical segregation was also found between sexes and attributed to male territory defence and intersexual food resource differentiation (Freeman, 2014). Nonetheless, it is not clear how spatial segregation translates into dietary segregation, and there seems to be little evidence of dietary segregation in monomorphic species (Catry et al., 2019; Phillips et al., 2011), despite some exceptions (Cleasby et al., 2015). Regardless of the main cause for the dietary differentiation found in our study, it shows a sexual dietary differentiation during the breeding period, which may help lowering intraspecific competition, which can be especially important in the (semi-)arid landscapes where black wheatears occur.
Overall, our study shows how even minor dimorphic bird species can have subtle differences in diet during their breeding season. The differences found were most likely related to sexual differences in behaviour rather than morphology, which means that this pattern might be far more common than what is currently recognized in birds. Moreover, this pattern was only possible to detect thanks to the high taxonomic resolution offered by metabarcoding, as analyses at higher taxonomic ranks were not able to identify such differences. At a time when metabarcoding is starting to be used to re-visit and assess the diet of many species, as well as to study other species interactions like pollination, it becomes increasingly important to understand the impact of taxonomic resolution in ecological studies (Renaud, Baudry, & Bessa‐Gomes, 2020). Finally, this study is an example of how the development of new techniques, such as metabarcoding, can help ecological studies go a bit further and gain better insights into fine ecological patterns that could otherwise go unnoticed.