INTRODUCTION
Omnivory implies nutrient acquisition from plant and animal sources (Agrawal & Klein, 2000; Coll & Guershon, 2002; Singer & Bernays, 2003). Dietary sources utilized by omnivores may differ widely in nutrient content and despite foraging different food types, many omnivores have a primary or preferred diet. For example, most avian omnivores are granivorous, frugivorous or herbivorous species that incorporate protein-rich insects into their diets to complement their protein-deprived primary plant diets (Coll & Guershon, 2002; Burinet al., 2016). Others are diet specialists or facultative omnivores which do not necessarily require supplementation (Griffithet al., 2021). Where omnivory is obligate, deprivation of supplementary diets will lead to nutrient limitation (Gillespie & McGregor, 2000; Gillespie et al., 2012), whereas facultative omnivores may be unaffected by such deprivation (Trichilo & Leigh, 1988; Milne et al., 1996; Milne & Walter, 1997).
Anthropogenic/natural environmental changes which may alter the availability of specific food types can lead to nutrient limitation with effects on body mass maintenance (Krieger et al., 2006; Papet al., 2008; Nwaogu et al., 2019; Filipiak & Filipiak, 2020), muscle and fat reserves (Pierce & McWilliams, 2004; Van den Burg, 2009) and physiological parameters such as Packed Cell Volume (PCV) and Haemoglobin Concentration (HBC) (Pyke et al., 2012). For example, a protein-rich diet including insects supports the maintenance of body form and function. Dietary protein deprivation may thus lead to the breakdown of protein-rich tissues including muscles and digestive organs (Piersma & Gill, 1998; Krieger et al., 2006). The ability of animals to temporally adjust body reserves in response to breeding and changes in environmental condition (Likness & Swason, 2011; Petit & Vezina, 2014; Nwaogu et al., 2017; Vezina et al., 2020; Eikenaar et al., 2021) may also be impaired by diet or nutrient limitation with effects on survival and fitness (Cooket al., 2004; see Harrison et al., 2011 for a review). Also, the effects of nutrient limitation may be sex-specific because of differences in breeding roles (e.g., Mallory et al., 2008), physiology (e.g., Filipiak & Filipiak, 2020), and morphology (e.g., Fernández‐Montraveta & Moya‐Laraño, 2007).
In the absence of diet restriction, animals may face nutrient limitations in nature due to changes in the quality of their preferred diet (see Agrawal et al., 1999; Agrawal & Klein, 2000; Bravoet al., 2019), the energetic demand of cold condition (Swanson, 2010), and the occurrence of other energy/nutrient demanding periods of the annual cycle such as breeding and moult (Murphy & Pearcy, 1993; Cherel et al., 1994; Klaassen, 1995). This may lead to temporal over-exploitation of body reserves (Milenkaya et al., 2013; Ndlovu et al., 2017; Andersson et al., 2018; Awoyemi, 2020). To maximize nutrients intake during periods of limited nutrient availability, animals may shift between diets or show preference for a given food type (McWilliams et al., 2004; Lamperti et al.,2014; Kwiecinski et al., 2017) with sex-specific variation in diet preference due to differences in morphology (Walker et al.,2014; Bravo et al., 2019), physiology, and breeding role (Kwiecinski et al., 2017; Treidel et al., 2021).
In birds, body mass, pectoral muscle and fat scores indicate physical condition in terms of energy and nutrient reserves (Milenkaya et al., 2013). PCV and HBC on the other hand, are physiological indices which may indicate overall health status (e.g., Garvin et al.,2007). Omnivorous birds in a changing Afrotropical environment are useful surrogates for investigating how diet restriction and/or nutrient limitation due to environmental changes and timing of annual cycle events can influence body condition. The Village weaver Ploceus cucullatus is an omnivorous tropical bird found throughout sub-Saharan Africa (Craig, 2010). Its omnivorous foraging behaviour makes it amenable to captive conditions and suitable for experimental diet manipulation. This allowed the effects of nutrient deprivation on body condition to be determined. In the wild, Village weavers exist in large colonies often around water bodies or human settlements, but they thrive well in captivity and are commonly kept as pets (Collias et al.,1986). Village weavers feed predominantly on grains (Collias and Collias, 1970, Collias et al., 1986), but they also forage on insects (Collias & Collias, 1970; Adegoke 1983), and fruits (Lahti, 2003; Yilangai et al., 2014). Males are polygynous. They build nests but nestlings are predominantly fed by females (Collias & Collias, 1970). Male weavers have brighter feathers compared to females especially during the breeding period (Borrow & Demey, 2004).
In this study, we tested the effect of nutrient deprivation on the body condition (body mass, pectoral muscle score, fat score, PCV and HBC) of wild-caught captive Village weavers. First, we determined that grains were the preferred diet of Village weavers, then we tested how preference for grains, insects and fruits may vary over time and between sexes. We expect weavers to show preference for grains compared to fruits and insects during periods of higher energy demands such as under colder environmental conditions. We also expected diet preference to vary between sexes due to their differences in plumage colouration, sex-roles and physiology. Secondly, we compared body condition indices between male and female weavers fed on grains and fruits and those fed on grains and insects over eight weeks. We expect; a deterioration of body condition in weavers on both diet treatments if omnivory is obligate, and a more pronounced deterioration in weavers deprived of the more essential supplementary diet between insects and fruits. We expected differences in how body condition varies over time and between sexes if nutrient demands vary differently between sexes with changes in environmental conditions over time.