ad libitum.
Body mass was found to decrease for weavers on both diet treatments.
This implies that both diet combinations are not optimal for the Village
weavers. The decrease in body mass was more pronounced in the
insect-deprived weavers, suggesting that protein deprivation is more
detrimental than fruit deprivation. Fat, and muscles including the
digestive organs and pectoral muscles account for about 50% and 25% of
the total body mass of birds respectively (Lobocha, 2012; Ndlovuet al., 2017). Our results show that body mass loss in the
weavers fed with grains and fruits is unlikely due to fat loss, but due
to pectoral muscle breakdown. The weavers increased fat reserves over
time during the experiment probably due to decreasing temperature in the
study area. The period between Novemebr and February annually is usually
characterised by cold-dry spell with temperatures as low as
3oC in the nights and early mornings when birds need
to endure starvation. Protein-rich pectoral muscles are part of the
non-fat component of body mass which may be broken down to provide amino
acids needed for body maintenance, a plausible explanation for the
weavers not to lose fat but significantly deplete muscle tissues.
Several studies have reported the loss of body mass due to a reduction
in the size of digestive organs rather than pectoral muscles in
long-distance migratory birds because muscles are needed for
long-distance endurance flight (Piersma et al., 1999; Lindstromet al., 2000; Pierce & McWilliam, 2004; McWilliams & Karasov,
2005; Krieger et al., 2006). On the contrary, weavers in
captivity do not need to fly long distances but needed to continue
foraging, so would rather deplete pectoral muscles than shrink their
digestive tract. Moreover, a longer digestive tract may be needed to
properly digest the low nutritive fibrous fruits diet (Jordano, 1987;
Al-Dabbagh et al., 1987).
Protein deprivation in the fruit-fed weavers may be exacerbated by
moulting requirements but this may not be the case in this study becaue
moult status showed no effect on the observed loss of pectoral muscle
mass. About 90% of bird feathers are made up of keratin which is
composed of amino acids derived from protein (Stettenheim, 2000). During
moults, about 25% of protein mass is depleted and the demand for
dietary protein increases (McWilliams, 2008). Having no access to
dietary protein, the weavers fed on grains and fruits diet may have
catabolized their protein reserve to meet up with protein requirements
for other tissue repair and maintenance. Moreover, unknown to us, the
birds may have arrseted molt due to protein deprivation (Gosler, 1991)
since we took note of molt status only at the beginning of the
experiment. Females deprived of insects lost more muscle mass than males
on the same treatment. It is likely that the larger body size of males
influenced the rate at which muscle tissues are depleted since they
could have higher protein reserves than females (Piersma, 1984).
Furthermore, female weavers are responsibly provisioning nestlings;
therefore, the need to compensate for nutrient deficit incurred by
nourishing eggs, nestlings, and fledglings with protein-rich diets may
increase the protein requirement of the females (Collias & Collias,
1970; Houston, 1998) and thus could result in muscle depletion.
Variation in body fat may be due to environmental conditions over time
rather than diet treatment, but diet may have influenced the intensity
of fat accumulation in response to environmental change. We found
increased fat reserves in the weavers fed on both diet treatments, but
the male Weavers fed on grains and insects increased fat reserves more
than those fed on grains and fruits. An increase in fat reserves in
response to cold temperature has been reported in several species of
temperate birds (Goławski et al., 2015; Brodin, 2017). Energy
demands for heat production increase with decreasing temperatures
(Goławski et al., 2015). Therefore birds must accumulate fat for
both energy requirement and insulation during periods of low
temperature. The increase in fat reserve might be a response to cold
temperatures since birds were held within the cold-dry harmattan period
in West Africa (November to February) which experiences massive
fluctuations in daily temperature. Similar increases in body fat and
mass were observed in Common Bulbuls Pycnonotus barbatus held in
captivity in the same study area at the same time of year (Nwaogu,
2019). It is unclear why males fed with grains and insects gained more
fat than those fed with grains and fruits. The presence of excess simple
sugars from fruits is expected to allow de novo production and
accumulation of fat (Klassings, 1998) as reported for migrant e.g.,
White-throated Sparrows Zonotrichia albicollis (Smith &
McWilliams, 2009). Both sexes accumulated fat similarly when fed on
grains and fruits, but males accumulated fat more when fed on grains and
insects. By consuming more insects than females, the males might have
accumulated more fat than females since a combination of grains and
insects provided more fat than grains and fruits.
The observation that diet did not affect PCV, but that PCV increased
over time regardless of diet treatments suggests that factors other than
diets such as humidity, photoperiod, and temperature may be driving
variation in PCV (Fair et al., 2007). An increase in PCV can
occur when blood plasma that suspends blood cells is used to disperse
body heat in response to cold temperature (Dawson & Bortolotti, 1997).
This is consistent with the cold temperature during the study period.
Similarly, Swanson (1990) and Abelenda et al. (1993) found PCV to
increase as temperature decreases due to the need to increase heat
production in cold temperatures (Carey & Morton, 1976). The lack of a
diet effect on PCV is consistent with other studies that found no
diet-related differences in PCV in American Kestrel Falco
sparverius (Dawson & Bortolotti, 1997) and House Sparrows Passer
domesticus (Gavett & Wakeley, 1986). Sex hormones specifically
androgens which support sperm production are known to increase the
production of red blood cells in male birds (Shahani et al.,2009). This may be responsible for the higher PCV in males than females.
Variation in PCV and HBC followed the same pattern. This is not
surprising because HBC correlates positively with PCV in Passerines
(Velguth et al. , 2010). Thus, an increase in PCV in response to
cold temperature and due to androgen effect in males might have resulted
in a corresponding pattern in HBC.
This study demonstrates that seasonal or anthropogenic environmental
changes that can alter the availability of different food types
(especially insects) can be detrimental to the maintenance of body
condition and physiological response to environmental change in the
Village weaver which depends on several food sources to meet its daily
nutrient requirement and this may be the case for other omnivores.
Moreover, omnivores may be more vulnerable to environmental change
despite diverse foraging options due to their complex diet combination
(Burin et al., 2016).