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An Analysis of Body Mass, Wing Length, and Visible Fat Deposits of Dark-Eyed Juncos Wintering at Different Latitudes

Val Nolan Jr. and Ellen D. Ketterson
The Wilson Bulletin
Vol. 95, No. 4 (Dec., 1983), pp. 603-620
Stable URL: http://www.jstor.org/stable/4161832
Page Count: 18
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An Analysis of Body Mass, Wing Length, and Visible Fat Deposits of Dark-Eyed Juncos Wintering at Different Latitudes
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Abstract

Many temperate-zone bird species accumulate fat stores during winter. These stores are commonly assumed to serve as energy reserves for the longer nights, colder temperatures, and increased probability of fasting during periods of snow cover; but little is known about what regulates fat storage at the proximate level. If seasonal fattening is a response to winter climate, then a latitudinal cline in degree of fattening would be expected in many parts of the world. This paper reports such latitudinal variation in the body mass of Dark-eyed Juncos (Junco hyemalis) captured at six locations under a variety of environmental conditions during several winters. Juncos at higher latitudes (Michigan and Indiana) were significantly heavier than those from more southern sites (Tennessee, South Carolina, Alabama, and Mississippi). This was true of young and adult individuals of both sexes and was not a function of latitudinal variation in body size (as measured by wing length). Visible fat stores were also greater at higher latitudes, and we conclude that northern juncos were fatter than their southern counterparts. Multiple regression of body mass on measures of temperature, day length, and snow cover as well as on wing length, latitude, and hour of day indicates that latitude was by far the best predictor of mass; but all the variables produced significant partial regression coefficients. Thus, juncos were heavier when recent temperatures had been colder, when days were longer, and when snow was present. Additionally, they were heavier if they had longer wings or were caught later in the day. Two explanations of latitudinal variation in fattening are considered. Northern and southern juncos may represent genetically differentiated populations varying in their regulatory physiology. Alternatively, juncos may simply respond in a graded manner to conditions that vary in their proximate physical environments. Results to date can be taken to support either view.

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