SREL Reprint #0008




Notes on Fall Plumages, Weights, and Fat Condition in the Ruby-throated Hummingbird


R. A. Norris, C. E. Connell, and D. W. Johnston



In early September, 1955, an estimated 100 to 150 Ruby-throated Hummingbirds foraged, waged battle, and roosted in about four acres of Crotalaria in the Savannah River Plant area, Aiken County, South Carolina.  Much of their food was nectar taken from the Crotalaria flowers.  The birds’ numbers declined after mid-September, and the last individuals were seen on September 24.  Ten hummingbirds caught in mist nets were color-marked, but there were no repeat records.  Both the shape of the sixth primary and the throat coloration provided criteria for distinguishing in the hand males and females among immatures.  An adult female lacked the ventral buffiness characteristic of young females.  The sexes were present in about the same numbers, but adults (male and female) seemed to comprise only about 7 per cent of the aggregation.  Body weights for summer and (especially) fall-collected hummingbirds (including some from Georgia and Florida) averaged about 3.8 grams for females and about 3.5 grams for males.  Weights increased markedly after mid-September, as did fat content.  Heart ratios, in contrast, decreased with increase in body weight and fatness; heart weight per se was relatively constant and was significantly greater in males than in females.  Fat content, expressed as per cent of wet weight, ranged from about 11 to 15 per cent in June to about 41 to 46 per cent in heavy, premigratory individuals.  The heaviest birds, each carrying about two grams of fat, were thought to have sufficient fuel to travel nonstop some 800 miles.



Norris, R.A., C.E. Connell, and D.W. Johnston. 1957. Notes on fall plumages, weights, and fat condition in the ruby-throated hummingbird. The Wilson Bulletin 69:155-163.




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