David L. Swanson

Seasonal Acclimatization of Thermoregulation in the Black-Capped Chickadee

Black-capped Chickadees (Parus atricapillus) show behavioral adaptations (food caching, cavity roosting) and can undergo regulated bouts of nocturnal hypothermia, both of which reduce costs associated with wintering in temperate latitudes. These adjustments could reduce the need for the seasonal metabolic adjustments found in other small passerines that must deal with severe winter cold. We have examined this possibility by determining the extent of seasonal variation evident in chickadees concerning standard metabolic rate, metabolic response to temperature, cold resistance, and maximal thermogenic capacity (Vo2sum). Regression equations relating metabolism to air temperature below thermoneutrality did not differ significantly in either slope or Y-intercept between summer and winter, and neither did minimum thermal conductance for normothermic birds vary seasonally. Seasonal constancy in these parameters demonstrates the minor role that seasonal changes in insulation play in acclimatization in chickadees. However, winter birds tolerated cold stress far better than summer birds. This improved cold tolerance was associated with a significant increase in Vo2sum (36%) in winter relative to summer. Standard metabolic rates (SMR) were also significantly increased in winter birds compared to summer birds. Thus, Black-capped Chickadees show seasonal metabolic acclimatization similar to, or greater than, other temperate wintering passerines in addition to behavioral adaptations and nocturnal hypothermia.

Seasonal Metabolic Variation in Birds: Functional and Mechanistic Correlates

The influence of seasonal changes in temperature and climate on metabolic rates in birds has been a topic of interest to ornithologists and ecophysiologists for decades (e.g., Hart 1962; Dawson 1958; Miller 1939). Because metabolic rates increase linearly with temperature in endotherms outside the thermal neutral zone, comparisons of metabolic rates among seasons or species require standardized measurements of metabolic rates. The most common of these standardized metabolic rates used for comparisons of energetics among seasons or species is basal, or resting, metabolic rate. It often serves as a baseline for comparisons of metabolic costs of activities within species, and for comparisons of the “rate of living” among species or species groups (e.g., Wiersma et al. 2007a; White et al. 2007; McKechnie et al. 2006; McKechnie and Wolf 2004; Trevelyan et al. 1990; McNab 1988; Bennett and Harvey 1987; Kersten and Piersma 1987). Theoretically, basal metabolic rate (BMR) is the minimum metabolic rate required for maintenance in endotherms. BMR is measured within the thermal neutral zone under postabsorptive digestive conditions during the resting phase of the daily cycle on resting, nongrowing, nonreproductive animals (McNab 1997). It is doubtful whether truly BMRs can ever be achieved in the laboratory, so the term resting metabolic rate (RMR) is often used to refer to such measurements, even when the standard conditions for BMR have been met. Here, I will revert to the standard terminology and consider BMR as the metabolic rate measured under the standard conditions listed above, recognizing that this may not, in fact, represent truly basal rates.

Evidence for a proximate influence of winter temperature on metabolism in passerine birds.

The roles of ultimate and proximate factors in regulating basal and summit metabolic rates of passerine birds during winter have received little study, and the extent to which winter temperatures affect these variables is unknown. To address this question, we measured basal and summit (maximum cold‐induced) metabolic rates in black‐capped chickadees (Poecile atricapillus), dark‐eyed juncos (Junco hyemalis), and American tree sparrows (Spizella arborea) during winters from 1991/1992 to 1997 in southeastern South Dakota. Both temperature and these metabolic rates varied within and among winters. Least‐squares regression revealed significant negative relationships for normalized basal and summit metabolism against mean winter temperature for all species pooled ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A comparative analysis of thermogenic capacity and cold tolerance in small birds

R^{2}=0.62

Seasonal variation in cold tolerance, basal metabolic rate, and maximal capacity for thermogenesis in White-breasted Nuthatches Sitta carolinensis and Downy Woodpeckers Picoides pubescens, two unrelated arboreal temperate residents

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Are summit metabolism and thermogenic endurance correlated in winter-acclimatized passerine birds?

P\leq 0.001