Collared sunbird

Abstract: 1. In contrast to hummingbirds, passerine frugivores prefer hexose sugars to sucrose and absorb sucrose poorly. It has been predicted that the sugar preferences and digestive physiology of specialized passerine nectarivores will be similar to those of frugivores. 2. We examined the preferences of 11 Lesser Double-collared Sunbirds, Nectarinia chalybea (Nectariniidae), for 20% (w/w) solutions of sucrose, glucose, fructose, a 1: 1 glucose+fructose mixture, and xylose. The order of preference was found to be sucrose = glucose + fructose = fructose > glucose > xylose. The birds rejected xylose, which has recently been found in the nectar of Protea and Faurea (Proteaceae). 3. Preferences for different sugar concentrations were also tested. In the case of sucrose and fructose, the sunbirds preferred 20% to 10% solutions, but were indifferent to 20% vs 30%. They preferred 20% to 30% glucose, but were indifferent to 10% vs 20%. 4. The concentration and sugar composition of the cloacal fluid were measured after feeding on known diets. The sunbirds absorbed sucrose, glucose and fructose from the ingested food at close to 100% efficiency, but xylose was excreted. 5. The results of this study thus contradict the belief that passerine nectarivores are inefficient at digesting sucrose and reject sucrose in favour of hexose sugars.

Abstract: The role of sunbirds (Nectariniidae) in the pollination ecology of Strelitzia nicolai (Musaceae) was studied for one year in a coastal dune forest in Zululand, South Africa. It was found that S. nicolai produced large quantities of low quality nectar (1.74 j/μl); that nectar production was highest during the day-time; and that the flowers displayed several characteristics attractive to bird-visitors. The flowers were large, conspicuous and provided the birds with a perch, facilitating easy access to the nectar. Flowers were visited by four species of sunbirds: Olive Sunbird Nectarinia olivacea, Grey Sunbird N. veroxii, Black Sunbird N. amethystina, and Collared Sunbird Anthreptes collaris. Sunbirds visited the flowers throughout the year, and apparently cued into changes in the flower angle as an indication of nectar flow rates. Sunbirds perched on the flowers in a manner which effected pollination, the pollen being transferred to the stigma via the birds' feet. Besides the sunbirds, there were other visitors (bushbabies, monkeys and insects) to the flowers, but they did not visit the flowers frequently nor did they appear to be significant pollinators. The high seed set of S. nicolai in the study area attests to the efficacy of the sunbirds as pollinators.

Abstract: The water balance of nectarivores is tightly linked to their energy balance. When nectar is dilute, consumption of a large water excess is inevitable. We investigated energy and water balance in lesser double-collared sunbirds, Nectarinia chalybea (8 g), kept at 20 °C and fed different nectar concentrations (0.4, 0.8 M sucrose or 1.2 M sucrose). The mass of sucrose consumed, body mass, day-time mass gain and night-time mass loss were the same irrespective of diet, the birds compensating energetically for changes in sucrose concentration by drinking greater volumes of the more dilute solutions. Sunbirds consumed between 0.5 times and 1.8 times their body mass in preformed water per day, depending on sucrose concentration, and excreted around 75% of the water. The difference between water gain (preformed and metabolic water) and excreted water is assumed to equal evaporative water loss, and was similar on 1.2 M and 0.8 M sucrose, but was higher on a diet of 0.4 M sucrose. The osmolalities and K+ and Na+ concentrations of the excreted fluid were extremely low, so that sunbird urine resembled that of hummingbirds and freshwater vertebrates rather than that of typical terrestrial vertebrates. N. chalybea is able to maintain energy and water balance over a range of nectar concentrations by adjusting the volume of solution consumed and by excreting copious, dilute fluid.

Abstract: Nectarivorous birds typically are small, often weighing less than 10 g, and have high basal metabolic rates. To meet their high energy needs, many nectarivorous species spend a large proportion of their time foraging on nectar. Like all birds, they also require nutrients such as protein, vitamins, essential fatty acids, minerals, and trace elements. Amino acids occur in floral nectar, and their concentrations are thought to be related to pollinator type, being low in nectar of bird-pollinated plants (Baker and Baker 1982). However, the correlation between amino acid concentration and pollinator type has been disputed by Gottsberger et al. (1984), who suggested that most of the variation in nectar amino acids results from flower damage or contamination with pollen. In any event, the amounts are too low to satisfy the protein requirements of nectarivorous birds (Martinez del Rio 1994). Other nitrogen sources are insects or pollen (Richardson and Wooller 1990, Brice 1992), both of which present birds with a number of potential problems. In insects, a large proportion of the nitrogen is in the form of chitin, a polymer that may be difficult for a small nectarivorous bird to digest. Although some seabirds are capable of producing chitinases (Jackson et al. 1992), no nectarivorous birds are known to do so. Accordingly, the major nitrogen sources obtained from insects are likely to be the proteins, peptides, and free amino acids found in their tissues. However, the amino acid composition of insects varies greatly and often is dominated by nonessential amino acids such as proline (Tomlin et al. 1993). As a result, insects are not always as good a source of protein as is widely believed. Furthermore, it may be energetically very expensive for a small nectarivorous bird to catch enough insects to meet its nitrogen requirements. Pollen has long been recognized as a major source of nitrogen for various insect pollinators, most notably honeybees (Apis mellifera; Schmidt and Buchmann 1985, Dobson and Peng 1997), and it has recently become clear that pollen also is an important element in the diet of vertebrate pollinators (Howell 1974, Richardson and Wooller 1990, Law 1992, van