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Sex-specific foraging behaviour might be influenced by digestive constraints. However, evidence for sex differences in digestive performance is limited. Various physiological traits are known to be body size dependent. Therefore, we hypothesized that body size differences between male and female birds may lead to differences in their digestive characteristics. We predicted that if food intake and digestive functions are only governed by body mass, then males that are heavier than females would have higher food intake, food assimilation efficiency and gut transit time, but not after controlling for the effect of body mass. We fed a diet of equicaloric solutions of sucrose and a 1:1 mixture of glucose and fructose (hexose mixture) solutions to Palestine sunbirds (Nectarinia osea). When fed sucrose solutions, males had longer transit times but similar absorption efficiencies as females. Transit times, corrected for differences in body mass and food intake, were still longer in males than in females when fed on sucrose solutions. The sex-specific differences in transit time disappeared when the birds were fed the hexose mixture. Our results suggest that males take longer to digest than females when fed on sucrose-rich nectars as opposed to hexose-rich nectars, and therefore can allow themselves a relatively lower digestive capacity. This may suggest sex-specific co-evolution of sunbirds within mixed plant communities, which have both sucrose- and hexose-rich nectar-producing plants. Furthermore, future studies on digestion in birds may pay attention to sex-specific differences. Key words: absorption efficiency, foraging, Palestine sunbird, Nectarinia osea, sexual dimorphism, transit time.

The removal of males in pairs of orange-tufted sunbirds increased the provisioning effort of lone females. However, this compensatory response is incomplete as evidenced by the reduction in total visit rate in the male-removed group. The females' inability to fully compensate for the lack of partner may be caused by time constraints on the rate of food collection or due to efforts done at a physiological maximum.

An analysis of parental care in the orange-tufted sunbird revealed that males showed more mobbing efforts than their female partners and the nest guarding activity increased both in males and females with increased brood size. For provisioning at higher rates, males failed to guard nests. Visit rates decreased in experimental nests and larger broods as evidenced by the lower chick growth rates.

The observation that Orange-tufted Sunbirds do not feed flower nectar to their young was verified by feeding parents a sugar solution colored red by a food dye. The color of the parental feces were distinguished from the fecal sacs of chicks by placing paper sheets below feeders. The parental feces were colored but not the fecal sacs. While it is possible that chicks are able to digest the food dye, other similar experiments were conducted, and it was concluded that the parental Sunbirds feed on nectar but it is not given to their young.