Diet and Foraging

Main Foods Taken

Like other hummingbirds, consumes floral nectar from several plant species and catches small insects in the air or gleans them from foliage. Nectars preferred are like those preferred by other hummingbirds—containing primarily sucrose and low quantities of other substances such as amino acids (70 Hainsworth, F. R. and L. L. Wolf. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25:101-113. Close , 71 Stiles, F. G. (1976). Taste preferences, color preferences, and flower choice in hummingbirds. Condor 78:10-26. Close , 72 Baker, H. G. (1977a). Non-sugar chemical constituents of nectar. Apidologie 8:349-356. Close ). Individual Rivoli's Hummingbirds have shown the ability to discriminate among nectars of different sugar concentrations and may use color as a cue to target flowers with higher concentration (70 Hainsworth, F. R. and L. L. Wolf. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25:101-113. Close , 73 Sandlin, E. A. (2000a). Cue use affects resource subdivision among three coexisting hummingbird species. Behavioral Ecology 11 (5):550-559. Close ). Hainsworth and Wolf (70 Hainsworth, F. R. and L. L. Wolf. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25:101-113. Close ) suggested best discrimination occurred at concentrations below about 0.7M although Sandlin (73 Sandlin, E. A. (2000a). Cue use affects resource subdivision among three coexisting hummingbird species. Behavioral Ecology 11 (5):550-559. Close , 74 Sandlin, E. A. (2000b). Foraging information affects the nature of competitive interactions. Oikos 91 (1):18-28. Close ) showed clear preference for 0.86 over 0.43M sucrose. When nectar sugar concentration is low (~5%), Rivoli's Hummingbirds will switch from a sucrose preference to a glucose preference (75 Schondube, J. E. and C. M. del Rio. (2003). Concentration-dependent sugar preferences in nectar-feeding birds: Mechanisms and consequences. Functional Ecology 17 (4):445-453. Close ). Nectar-extraction rate appears related to corolla length, with highest extraction rates measured for corollas < 30 mm (55–70 ml/s) (70 Hainsworth, F. R. and L. L. Wolf. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25:101-113. Close ). A lower extraction rate was measured from artificial feeders (35.7 μl/s) (73 Sandlin, E. A. (2000a). Cue use affects resource subdivision among three coexisting hummingbird species. Behavioral Ecology 11 (5):550-559. Close ).

Rivoli's Hummingbird (Eugenes fulgens) primarily captures small arthropods in flight (Powers et al. 2010), with prey taken both from the air and from foliage (Powers 2013).

Nectar

United States. Surprisingly little is known about the flowers preferred by this species, but some flower species that have been identified or are suspected to be visited include: century plant (Agave spp.), Red Columbine (Aquilegia triternata), Bouvardia (Bouvardia ternifolia), indian paintbrush (Castilleja spp.), Mojave Mound Cactus (Echinocereus polyacanthus), Beardlip Penstemon (Penstemon barbatus), Jacob's Ladder (Polemonium pauciflorum), Lemmon's Sage (Salvia lemmonii), and Indian Pink (Silene laciniata) (65 Baltosser, W. H. (1986a). Nesting success and productivity of hummingbirds in southwestern New Mexico and southeastern Arizona. Wilson Bulletin 93:353-367. Close , 63 Grant, K. A. and V. Grant. (1968). Hummingbirds and their flowers. New York: Columbia Univ. Press. Close , 64 Kuban, J. F. and R. L. Neill. (1980). Feeding ecology of hummingbirds in the highlands of the Chisos Mountains, Texas. Condor 82:180-185. Close ; B. Hoyer, personal communication).

In southeastern Arizona visits Giant Trumpet (Macromeria viridiflora; Boraginaceae), which grows in mountainous areas between 1,500–3,000 m (76 Boyd, A. (2002). Morphological analysis of Sky Island populations of Macromeria viridiflora (Boraginaceae). Systematic Botany 27 (1):116-126. Close ). Flowers of M. viridiflora in southeastern Arizona (where they are used by E. fulgens) are larger than in northern U.S. populations, suggesting pollinator-mediated selection (76 Boyd, A. (2002). Morphological analysis of Sky Island populations of Macromeria viridiflora (Boraginaceae). Systematic Botany 27 (1):116-126. Close ).

Mexico. In Volcán de Colima, southern Mexico, flowering plants available to Rivoli's Hummingbird between 1,500 and 2,500 m in the arid pine–oak forest include: thistle (Cirsium spp.), Fuchsia (Fuchsia parviflora), Lion's-ear (Leonotis nepetaefolia), Vervain (Lippia umbellata), Bellflower (Lobelia laxiflora), Rose of Sharon (Malvaviscus arboreus), bean (Phaseolus formosus), and sage (Salvia mexicana). Rivoli's Hummingbird is rarest during summer in this region, but more common when thistle and bellflower come into bloom. Above 2,500 m, flowers include: Butterfly Bush (Buddleia cordata), Nightblooming Jessamine (Cestrum terminale), thistle (Cirsium spp.), Beard-tongue (Penstemon roseus), current (Ribes ciliatum), and Snowberry (Symphoricarpos microphyllus). Rivoli's Hummingbird is permanent resident in this habitat where it forages as a trapliner primarily on Cirsium thistles, although it frequently feeds on beard-tongue if present (47 Des Granges, J. L. (1978). Organization of a tropical nectar-feeding bird guild in a variable environment. Living Bird 17:199-236. Close ). Des Granges (47 Des Granges, J. L. (1978). Organization of a tropical nectar-feeding bird guild in a variable environment. Living Bird 17:199-236. Close ) describes Rivoli's Hummingbird as a specialist on long-corolla flowers.

In Parque Nacional La Malinche, Tlaxcala (2,900 m elevation), where vegetation is a mosaic of pine-oak forest, pasture, and second-growth vegetation, individuals regularly visit Bouvardia (B. ternifolia) between May-August (77 Torres, I., L. Salinas, C. Lara and C. Castillo-Guevara. (2008). Antagonists and their effects in a hummingbird-plant interaction: Field experiments. Écoscience 15 (1):65-72. Close ).

In areas surrounding Mexico City and Distrito Federal, Rivoli's Hummingbird commonly defends Century Plant (Agave salmiana); this plant forms many flowers, each of which produces about 3.5 ml/12 h of nectar containing 12–13% sugar (49 Martinez Del Rio, C. and L. E. Eguiarte. (1987). Bird visitation to Agave salmana: comparisons among hummingbirds and perching birds. Condor 89:357-363. Close ). Although sugar concentration of this plant's nectar is relatively low for a hummingbird flower, the tightly clumped arrangement of the flowers probably increases foraging efficiency (49 Martinez Del Rio, C. and L. E. Eguiarte. (1987). Bird visitation to Agave salmana: comparisons among hummingbirds and perching birds. Condor 89:357-363. Close ). In Hildago, Mexico (~92 km north of Mexico City), Rivoli's Hummingbirds feed on “honeydew” produced by scale insects in the genus Strigmacoccus (Margarodidae) whose secretions are ~35–45% sugar (78 Lara, C., V. Martinez-Garcia, R. Ortiz-Pulido, J. Bravo-Cadena, S. Loranca and A. Cordoba-Aguilar. (2011). Temporal-spatial segregation among hummingbirds foraging on honeydew in a temperate forest in Mexico. Current Zoology 57 (1):56-62. Close ). In Tlaxcala, Mexico (~62 km southeast of Mexico City), Rivoli's Hummingbirds feed from and pollinate the mistletoe Psittacanthus calyculatus (Loranthaceae) that flowers from April–October with each flower producing 2.3 µL of nectar containing 22% sucrose (79 Azpeitia, F. and C. Lara. (2006). Reproductive biology and pollination of the parasitic plant Psittacanthus calyculatus (Loranthaceae) in central Mexico. Journal of the Torrey Botanical Society 133 (3):429-438. Close ). In areas around Tlaxcala Rivoli's Hummingbirds also feed on Bouvardia, indian paintbrush (C. scorzonerifolia and C. tenuiflora), sage (S. elegans and S. mocinoi), penstemon (P. getianoides and P. roseus), and common heal-all (Prunella vulgaris) (80 Lara, C. (2006). Temporal dynamics of flower use by hummingbirds in a highland temperate forest in Mexico. Écoscience 13 (1):23-29. Close ).

On Cerro San Felipe, Oaxaca, flowers commonly used by Rivoli's Hummingbird from June to September are beard-tongue (Penstemon kunthii) and iris (Rigidella orthantha). The iris occurs in dense scattered stands near streams and open areas and is an important food source in May, less so after mid June. Beard-tongues' peak blossoming period is early August through October; like the iris, it is confined to open areas. Used by Rivoli's Hummingbirds as well as other hummingbirds during this period: indian paint-brush (Castilleja spp.), thistle (Cirsium mexicanum), Cuphea (Cuphea jorullensis), Lamourouxia viscosus, lobelia (Lobelia laxiflora), Macromeria doscolor, sage (Salvia stolonifera), Satejura mexicana, and Scarlet Betony (Stachys coccinea). These species, however, were simply occasional food sources and not important to territorial dynamics (50 Lyon, D. L. (1976). A montane hummingbird territorial system in Oaxaca, Mexico. Wilson Bulletin 88:280-299. Close ).

Near Xalapa City, Veracruz, E. fulgens is an infrequent visitor of Palicourea padifolia, which grows at mid-level elevations in cloud forests (81 Hernández, A. and J. F. Ornelas. (2007). Disassortative pollen transfer in distylous Palicourea padifolia (Rubiaceae), a hummingbird-pollinated shrub. Écoscience 14 (1):8-16. Close ).

Rivoli's Hummingbird is nectarivorous, feeding on nectar from plant species such as Passiflora membranacea, Tillandsia vicentina, and Cirsium subcoriaceum at the Huitepec Ecological Reserve in highlands of Chiapas (Partida 2011). P. membranacea was the most important species in Huitepec Reserve during June for the community of hummingbirds.

Insects

Rivoli's Hummingbirds may be more insectivorous than other North American hummingbirds (e.g., 43 Marshall, J. T., Jr. (1957). Birds of pine-oak woodland in southern Arizona and adjacent Mexico. Pacific Coast Avifauna no. 32. Close , 82 Pough, R. H. (1957). Audubon Western Bird Guide: Land, Water, and Game Birds. Doubleday & Company, Inc., Garden City, NY, USA. Close ). Marshall (43 Marshall, J. T., Jr. (1957). Birds of pine-oak woodland in southern Arizona and adjacent Mexico. Pacific Coast Avifauna no. 32. Close ) suggests that Rivoli's Hummingbirds can inhabit pine-oak woodlands away from riparian areas because the species “can dispense with both moist habitat and flowers.” In the Chiricahua Mountains of southeastern Arizona, birds placed in aviaries survived > 2 h without drinking nectar while actively catching insects (E. Sandlin, personal communication; DRP). Behavior of free-living males is consistent with high-arthropod consumption (83 Powers, D. R., J. A. Van Hook, E. A. Sandlin and T. J. McWhorter. (2010). Arthropod foraging by a southeastern Arizona hummingbird guild. Wilson Journal of Ornithology 122 (3):494-502. Close ).

Crops of captives often have full and diverse arthropod loads (84 Weymouth, R. D., R. C. Lasiewski and A. J. Berger. (1964). The tongue apparatus in hummingbirds. Acta Anatomica 58:252-270. Close ). Of 12 wild birds collected for stomach analysis, all contained arthropods (85 Coues, E. (1874). Birds of the Northwest: A handbook of the ornithology of the region drained by the Missouri River and its tributaries. U.S. Department of the Interior, U.S. Geological Survey, Miscellaneous Publication 3. Washington, DC, USA. Close , 43 Marshall, J. T., Jr. (1957). Birds of pine-oak woodland in southern Arizona and adjacent Mexico. Pacific Coast Avifauna no. 32. Close , 86 Remsen, Jr., J. V., F. G. Stiles, and P. E. Scott (1986). Frequency of arthropods in stomachs of tropical hummingbirds. Auk 103(2):436-441. Close ); three individuals examined by Cottam and Knappen (85 Coues, E. (1874). Birds of the Northwest: A handbook of the ornithology of the region drained by the Missouri River and its tributaries. U.S. Department of the Interior, U.S. Geological Survey, Miscellaneous Publication 3. Washington, DC, USA. Close ) had consumed a variety of insects and spiders; 6 males examined by Powers et al. (83 Powers, D. R., J. A. Van Hook, E. A. Sandlin and T. J. McWhorter. (2010). Arthropod foraging by a southeastern Arizona hummingbird guild. Wilson Journal of Ornithology 122 (3):494-502. Close ) consumed 2 classes and 4 orders of arthropods, including Hymenoptera, Homoptera, Diptera, and Araneae.

Rivoli's Hummingbird lives in many areas that have a distinct dry season, where free-standing nectar can be in short supply. Nevertheless, Powers et al. (83 Powers, D. R., J. A. Van Hook, E. A. Sandlin and T. J. McWhorter. (2010). Arthropod foraging by a southeastern Arizona hummingbird guild. Wilson Journal of Ornithology 122 (3):494-502. Close ) found no morphological specialization that suggested adaptation for increased arthropod foraging relative to other hummingbirds. The maintenance nitrogen requirement of Rivoli's Hummingbirds is 4.03 mg N/d (only 23.6% the predicted value), suggesting their protein requirement is low as has been shown with other hummingbird species (87 McWhorter, T. J., D. R. Powers and C. M. del Rio. (2003). Are hummingbirds facultatively ammonotelic? Nitrogen excretion and requirements as a function of body size. Physiological and Biochemical Zoology 76 (5):731-743. Close ).

Laboratory studies suggest that Rivoli's Hummingbird makes food choices based on sugar concentration, and secondarily on nectar extraction efficiency (70 Hainsworth, F. R. and L. L. Wolf. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25:101-113. Close ). In the Chiricahua Mountains, Arizona, Rivoli's Hummingbird may select sugar-poor food source when Blue-throated Hummingbird territorial activity is high (88 Pimm, S. L., M. L. Rosenzweig and W. Mitchell. (1985). Competition and food selection: field tests of a theory. Ecology 66:798-807. Close ).

Nectar is stored only in the crop. Crop volume ranges from 0.9–1.1 ml (mass 7–10 g) (89 Hainsworth, F. R. and L. L. Wolf. (1972). Crop volume, nectar concentration and humming-bird energetics. Comparative Biochemistry and Physiology 42A:359-366. Close ).

Nectars used by hummingbirds are generally simple, containing primarily carbohydrate (primarily sucrose) and water. Small amounts of amino acids and electrolytes may be present but are generally not sufficient to meet all nutritional needs (72 Baker, H. G. (1977a). Non-sugar chemical constituents of nectar. Apidologie 8:349-356. Close , 90 Brice, A. T. and C. R. Grau. (1991). Protein requirements of Costa's Hummingbird (Calypte costae). Physiological Zoology 64:611-626. Close ).

No direct measurement of daily energy expenditure. Using time budgets, Wolf et al. (91 Wolf, L. L., F. G. Stiles and F. R. Hainsworth. (1976). Ecological organization of a tropical highland hummingbird community. Journal of Animal Ecology 45:349-379. Close ) estimated total daily energy expenditure of Talamanca Hummingbirds in Costa Rica to be about 64 kJ/d when feeding on Cirsium thistles (45 kJ/d when using torpor at night) and about 65 kJ/d when feeding on Centropogon (46 kJ/d when using torpor). These estimates are based on numerous assumptions and are probably low. For comparison, field metabolic rate of the similarly sized Blue-throated Hummingbird in the Chiricahua Mountains of southeastern Arizona determined by doubly labeled water is 82 kJ/d (92 Powers, D. R. and T. M. Conley. (1994). Field metabolic rate and food consumption of two sympatric hummingbird species in southeastern Arizona. Condor 96:141-150. Close ), 74% higher than the time-budget estimate for Rivoli's Hummingbird.

Hainsworth et al. (93 Hainsworth, F. R., M. F. Tardiff and L. L. Wolf. (1981). Proportional control for daily energy regulation in hummingbirds. Physiological Zoology 54:452-462. Close ) measured feeding rate of Rivoli's Hummingbird from the Chiricahua Mountains (sex not reported, but probably males) in the laboratory and determined that they consumed 11.8 ± 2.8 meals/h (mean ± 95% confidence limit) of 17% sucrose, with a meal size of 190 ± 19.0 µl (< 20% of predicted crop volume [1.05 ml]) (89 Hainsworth, F. R. and L. L. Wolf. (1972). Crop volume, nectar concentration and humming-bird energetics. Comparative Biochemistry and Physiology 42A:359-366. Close ). No measurement of feeding frequency has been made on free-living Rivoli's Hummingbirds. Blue-throated Hummingbirds in the Chiricahua Mountains consumed about 8 meals/hour (92 Powers, D. R. and T. M. Conley. (1994). Field metabolic rate and food consumption of two sympatric hummingbird species in southeastern Arizona. Condor 96:141-150. Close ), 32% fewer than Rivoli's Hummingbirds in the laboratory. If feeding frequency and meal size are representative of free-living Rivoli's Hummingbirds in the Chiricahua Mountains, then the foraging pattern that might be expected for this traplining population is frequent small meals that reduce flight costs (94 Calder, W. A., L. L. Calder and T. D. Fraizer. (1990). The hummingbird's restraint: a natural model for weight control. Experientia 46:999-1002. Close ). Small, rapidly metabolized meals are consistent with lower respiratory quotient in this species (0.767 ± 0.012) compared to sympatric Blue-throated Hummingbird (0.828 ± 0.068) and Black-chinned Hummingbird (0.851 ± 0.194) (83 Powers, D. R., J. A. Van Hook, E. A. Sandlin and T. J. McWhorter. (2010). Arthropod foraging by a southeastern Arizona hummingbird guild. Wilson Journal of Ornithology 122 (3):494-502. Close ).

Rivoli's Hummingbird maintains a constant body temperature of about 40°C (95 Wolf, L. L. and F. R. Hainsworth. (1972). Environmental influence on regulated body temperature in torpid hummingbirds. Comparative Biochemistry and Physiology 41A:167-173. Close ). Basal metabolic rate (BMR) of Rivoli's Hummingbird in the Chiricahua Mountains of southeastern Arizona is 0.054 kJ g^-1h^-1, within the zone of thermal neutrality (32–34°C) (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ), which is 10.2% higher than that predicted by Aschoff and Pohl (97 Aschoff, J. and H. Pohl. (1970). Der Ruheumsatz von Vögeln als Funktion der Tageszeit und der Körpergrösse. Journal of Ornithology 111:38-47. Close ) for a nonpasserine bird of the same size. From 30° to 5°C, metabolic rate increases linearly to maintain constant body temperature (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ). The slope of this line is the rate of heat loss (thermal conductance) and is 6.03 kJ g^-1h^-1°C^-1for birds in the Chiricahua Mountains (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ). This high rate of heat loss is consistent with the fact that small animals must work harder (i.e., have a higher rate of heat production) to maintain homeothermy. Hovering metabolic rate of Rivoli's Hummingbirds in the Chiricahua Mountains of southeastern Arizona is 0.62 kJ g^-1h^-1, which is 11.5 x BMR (D. Powers, unpublished data).

Small size also effects evaporative water loss (EWL), an important mechanism for dissipating heat. EWL in Rivoli's Hummingbirds ranges from 6.7 to 12.5 mg H₂O g^-1h^-1 between 15 and 25°C, respectively, in relatively dry air (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ). At 25°C, EWL is 17% lower than that measured for Anna's Hummingbird (Calypte anna; about 4.5 g, the highest value measured for any vertebrate) under similar conditions (98 Powers, D. R. (1992). Effect of temperature and humidity on evaporative water loss in Anna's Hummingbird (Calypte anna). Journal of Comparative Physiology B Biochemical Systemic and Environmental Physiology 162:74-84. Close ). During hovering, respiratory evaporative water loss (REWL) in Rivoli's Hummingbirds increases with operative temperature (Te) (males: log REWL [mg/min] = 1.600Te^-1.517; females: log REWL = 1.486Te^-1.495 (99 Powers, D. R., P. W. Getsinger, B. W. Tobalske, S. M. Wethington, S. D. Powers and D. R. Warrick. (2012). Respiratory evaporative water loss during hovering and forward flight in hummingbirds. Comparative Biochemistry and Physiology A-Molecular & Integrative Physiology 161 (2):279-285. Close ).

The high metabolic requirement of Rivoli's Hummingbird is consistent with the fact that it has one of the highest recorded heart rates of any vertebrate (range 420–1,200 beats/min) (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ). This does not differ substantially from that of other hummingbirds, even those of smaller size, indicating that the heart of a Rivoli's Hummingbird may be working at maximum capacity for its cardiac system.

Like most hummingbirds studied, Rivoli's Hummingbird has the ability to enter torpor at night to reducing nighttime energy expenditure but will avoid torpor when energy intake is high (100 Powers, D. R., A. R. Brown and J. A. Van Hook. (2003). Influence of normal daytime fat deposition on laboratory measurements of torpor use in territorial versus nonterritorial hummingbirds. Physiological and Biochemical Zoology 76 (3):389-397. Close ). Wolf and Hainsworth (95 Wolf, L. L. and F. R. Hainsworth. (1972). Environmental influence on regulated body temperature in torpid hummingbirds. Comparative Biochemistry and Physiology 41A:167-173. Close ) observed individuals regularly entering torpor below 30°C. In experiments where birds from the Chiricahua Mountains were exposed to natural nighttime temperatures, birds entered torpor about half the time; torpor appears to be used when total body fat falls below 4% (100 Powers, D. R., A. R. Brown and J. A. Van Hook. (2003). Influence of normal daytime fat deposition on laboratory measurements of torpor use in territorial versus nonterritorial hummingbirds. Physiological and Biochemical Zoology 76 (3):389-397. Close ). It is unclear from these experiments if time or temperature influence torpor use.

Torpor metabolism of Rivoli's Hummingbird in the Chiricahua Mountains ranges from 0.1 ml O₂g^-1h^-1at 14.9°C to 1.18 ml O₂g^-1h^-1 at 26.9°C (96 Lasiewski, R. C. and R. J. Lasiewski. (1967). Physiological responses to the Blue-throated and Rivoli's hummingbirds. Auk 84:34-48. Close ). These values support Wolf and Hainsworth's (95 Wolf, L. L. and F. R. Hainsworth. (1972). Environmental influence on regulated body temperature in torpid hummingbirds. Comparative Biochemistry and Physiology 41A:167-173. Close ) view that Rivoli's Hummingbird enters torpor even at relatively warm temperatures (< 30°C).

Steady-state oxygen consumption of torpid birds decreases with temperature in an exponential fashion. When ambient temperature drops below 14°C for birds in the Chiricahua Mountains (101 Hainsworth, F. R. and L. L. Wolf. (1978a). Regulation of metabolism during torpor in "temperate" zone hummingbirds. Auk 95:197-199. Close ), oxygen consumption increases, presumably to defend some minimum body temperature.

Free-living hummingbirds consume, on average, 1.6–1.7 times their body weight in fluids via nectar each day (102 Powers, D. R. and K. A. Nagy. (1988). Field metabolic rate and food consumption by free-living Anna's Hummingbirds (Calypte anna). Physiological Zoology 61:500-506. Close , 92 Powers, D. R. and T. M. Conley. (1994). Field metabolic rate and food consumption of two sympatric hummingbird species in southeastern Arizona. Condor 96:141-150. Close ), but captive Rivoli's Hummingbirds have been shown to consume as much as ~3.5 times their body weight in fluids (87 McWhorter, T. J., D. R. Powers and C. M. del Rio. (2003). Are hummingbirds facultatively ammonotelic? Nitrogen excretion and requirements as a function of body size. Physiological and Biochemical Zoology 76 (5):731-743. Close ). This more than meets the birds' physiological needs, thus eliminating the requirement of drinking free water. If its excretory system has the same characteristics found in several smaller species, then it produces cloacal fluids that are more dilute than blood plasma (103 Calder, W. A., III, and S. M. Hiebert. (1983). Nectar feeding, diuresis, and electrolyte replacement of hummingbirds. Physiological Zoology 56:325-334. Close ). Total nitrogen loss in Rivoli's Hummingbirds is 1.98 mg N/d, most of which (84.6%) is excreted as urates (87 McWhorter, T. J., D. R. Powers and C. M. del Rio. (2003). Are hummingbirds facultatively ammonotelic? Nitrogen excretion and requirements as a function of body size. Physiological and Biochemical Zoology 76 (5):731-743. Close ).