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Daytime Energy Management by Tropical Hummingbirds: Responses to Foraging Constraint
Author(s) -
Tiebout Harry M.
Publication year - 1991
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1940586
Subject(s) - foraging , hummingbird , energy expenditure , perch , ecology , energy budget , daytime , biology , nectar , food intake , energy requirement , environmental science , zoology , fishery , atmospheric sciences , fish <actinopterygii> , physics , pollen , psychology , regression , psychoanalysis , endocrinology
Because hummingbirds are extremely sensitive to energy stress, yet often face conditions when energy intake is reduced or energy expenditures must be increased, they should have flexible energy budgets. I experimentally measured the ability of two syntopic tropical hummingbird species to manage daytime energy budgets under conditions simulating natural foraging constraint, defined here as low nectar secretion rate, low flower density, or both. I tested a territorial species, Amazilia saucerottei, and a traplining species, Chlorostilbon canivetii, individually in a large flight cage at two rates of food delivery (HIGH = ad libitum, LOW = 64% of ad libitum) and two perch—to—feeder distances (NEAR = 4 m, FAR = 20 m). Both species increased flight time when distance was increased (FAR > NEAR by 44%) and when rate of food delivery was reduced (LOW > HIGH by 23%). However, birds on the HIGH food delivery rate did not increase food intake to compensate for increased flight expenditures to the FAR feeder. Birds on LOW food maintained total rates of energy expenditure that were less than birds on HIGH food, and LOW food birds experienced no effect of feeder distance on expenditure. These results appear to be due primarily to short—term reductions in perching metabolic rates (PMRs) by birds on LOW food or with the FAR feeder. Reduced daytime PMRs helped to mitigate the potentially negative impacts of foraging constraint: rates of mass loss of the LOW food birds were an estimated 48% less than if birds had maintained normal PMRs. Reduced PMRs also enabled birds visiting the HIGH FAR feeder to maintain rates of mass gain equal to unconstrained birds (HIGH NEAR feeder). Relative differences between species in energy management reflected their foraging models. The low—reward trapliner paid the greatest energy costs at LOW food, regardless of distance, due to its overall tendency for high flight time. In contrast, the territorialist did poorly in the HIGH FAR treatment, due to flight expenditures associated with defending a rich but dispersed resource. Although individual birds exhibited considerable flexibility in energy management, the foraging mode represented by each species appeared to be energetically specialized for particular levels of food availability and dispersion.