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COPYRIGHT 2005 Wilson Ornithological Society
ABSTRACT.--Current models to estimate daily energy expenditure (DEE) for free-living birds are limited to either those that use fixed thermoregulatory costs or those that more accurately estimate thermoregulatory costs, but require extensive and often logistically difficult measurements. Here, we propose a model based on basal metabolic rate (BMR), activity budgets, and site-specific energetic costs of thermoregulation that requires only simple measures of ambient temperature and wind speed to provide estimates of DEE. We use the model to calculate the DEE of Buffleheads (Bucephala albeola) wintering at six habitats that afford differing degrees of protection from exposure within Narragansett Bay, Rhode Island. Bufflehead activity budget data collected during the winters of 2001-2002 and 2002-2003, along with average temperatures and wind speeds at the sites, were used to calculate DEE that ranged from 46.9 to 52.4 kJ/hr and increased with increasing wind speed. The energetic cost of thermoregulation composed as much as 28% of total DEE and increased with wind speed. Our DEE values were 13.4% higher, and thermoregulatory costs were up to 2x higher than those calculated using an existing model that incorporates fixed thermoregulatory costs. We also saw an increase in feeding activity with increasing wind speed; sensitivity analysis of the effects of wind speed and feeding activity showed that a 1 m/sec increase in wind speed at our sites increased DEE by 2.5%, whereas a corresponding increase in feeding activity increased DEE by 4.5%. This suggests that in temperate winter habitats, increased feeding activity may have a greater impact on Bufflehead DEE than wind exposure. Site-specific model estimates of DEE could also provide additional insight into the relative contribution of environmental conditions and changes in waterfowl behavior to DEE. Received 27 May 2004, accepted 12 January 2005.
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The daily energy expenditure (DEE) of a species is the sum of basal metabolic rate (BMR), thermoregulatory requirements, and the energetic cost of daily activities such as feeding, locomotion, and social behaviors. Quantitative assessments of the daily activities of wintering waterfowl have been used both to identify important habitats for these species and to assess their response to changes in habitat quality (Fredrickson and Drobney 1979, Brodsky and Weatherhead 1985a, Baldassarre et al. 1988, Paulus 1988). Waterfowl activity budgets may be influenced by habitat type (Turnbull and Baldassarre 1987, Rave and Baldassarre 1989) and site characteristics such as food abundance, protection from exposure, and level of disturbance (Nilsson 1970, Jorde et al. 1984, Paulus 1984, Quinlan and Baldassarre 1984, Brodsky and Weatherhead 1985b, Miller 1985). Changes in waterfowl activity may also be tied to changes in DEE that result from the influence of habitat characteristics. For example, increased exposure to cold and wind may increase thermoregulatory energy costs, and therefore require increased feeding to offset higher energetic costs (Bennett and Bolen 1978, Hickey and Titman 1983). Models that allow comparison between the energetic costs of thermoregulation and specific waterfowl behaviors could be used to determine the relative magnitude of these costs, and may also provide insight into the effects of habitat quality on the DEE of resident waterfowl.
Traditional measures of DEE for birds from time-activity budgets use multiples of BMR to estimate energetic costs of activities, but may differ in how the thermoregulatory component of DEE is estimated (Weathers et al. 1984). Early estimates of DEE included either a fixed cost of thermoregulation or one based solely on ambient temperature (Kendeigh 1949, Schartz and Zimmerman 1971, Koplin et al. 1980). Models subsequently evolved to include a means to more accurately estimate thermoregulatory costs, but only by the extensive measurement of many variables (e.g., whole-body thermal resistance, forced-convective resistance), some of which may be logistically difficult to obtain for free-living wildlife (Pearson 1954, Stiles 1971, Walsberg 1977). Weathers et al. (1984) proposed the use of standard operative temperature, or indices that allow single-number representations of complex thermal environments, to overcome some of these difficulties. However, while providing a much more rigorous estimate of thermoregulatory costs, this approach is limited by the need for the construction and calibration of taxidermic mounts, and may be best suited for aviary or well-controlled field applications. To date, researchers estimating DEE for free-living birds using published activity-based models are limited to either those that use fixed thermoregulatory costs or those that more accurately estimate thermoregulatory costs, but at the expense of extensive and often logistically difficult measurements of many variables.
Previous studies estimating DEE for wintering waterfowl have employed models that use factorial increases of BMR and that assume a fixed cost of thermoregulation (Wooley and Owen 1978, Albright et al. 1983, Morton et al. 1989, Parker and Holm 1990). For wintering waterfowl in northern areas exposed to low temperatures and high winds, thermoregulation may compose as much as 80% of daily energetic costs (Ettinger and King 1980, Walsberg 1983). These costs may vary between wintering habitats because of differing degrees of protection from exposure to wind and cold (Porter and Gates 1969, Goldstein 1983, Bakken 1992). If estimates of DEE are to be useful in assessing habitat quality for wintering waterfowl, they need to include some measure of the energetic cost of thermoregulation based on local environmental conditions.
Here, we present an activity-based model that includes habitat-specific measures of thermoregulatory costs to estimate DEE of waterfowl in different habitats. Our model requires only simple measures of ambient temperature and wind speed, along with waterfowl activity budgets and morphological measurements. Thermoregulatory costs are calculated by using heat loss via conduction and convection as a function of temperature and wind speed to estimate the metabolic heat production required to maintain body temperature (Birkebak 1966, Goldstein 1983). Because of the ability to estimate site-specific DEE based on local conditions, the model may be useful in evaluating habitats that provide differing degrees of protection from high winds and extreme temperatures. Model estimates could also be used to provide insight into the relative contribution of environmental conditions and differences in waterfowl behavior to changes in DEE.
In this study, we used our model to estimate the DEE of Buffleheads (Bucephala albeola) at six wintering habitats in Narragansett Bay, Rhode Island, that afford differing degrees of protection from exposure to wind and cold temperatures. Our specific objectives were to (1) compare estimates of DEE obtained using our model with those obtained using a previously published model that incorporates a fixed cost of thermoregulation, and (2) examine changes in DEE across the sites and determine the relative contribution of wind speed and waterfowl feeding behavior to changes in DEE.
METHODS
DEE site-specific thermoregulation model.--Our model incorporating site-specific thermoregulatory costs into DEE for wintering Buffleheads (hereafter, SST model) consists of (1) a thermoregulatory component (E[E.sub.Thermoreg])--an estimate of the metabolic heat production required to balance heat loss from the bird to the environment through conduction and convection, and (2) an activity component (E[E.sub.Activity])--an estimate of additional energetic costs resulting from specific daily activities of wintering Bufflehead expressed as multiples of basal metabolic rate (BMR). We sum these components to arrive at an estimated DEE. In our model, metabolic heat production includes resting energy expenditure in a thermoneutral environment (i.e., BMR) and the additional energy expenditure required to maintain thermal equilibrium. The model uses average temperatures and wind speeds that coincide with activity budget sampling at the sites; DEE is reported in kJ/hr.
Basal metabolic rates were estimated from those of 16 North American duck species...
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