Sex-Biased Provisioning: A Test for Differences in Field Metabolic Rates of Nestling Eastern Bluebirds

• Published in: The Condor (Los Angeles, Calif.) Calif.), 1991-11, Vol.93 (4), p.793-798
• Main Authors: Droge, Dale L., Gowaty, Patricia Adair, Weathers, Wesley W.
• Format: Article
• Language: English
• Subjects: Animal ethology; Biological and medical sciences; Bird nesting; Daughters; Fathers; Female animals; Food; Fundamental and applied biological sciences. Psychology; Isotopes; Male animals; Metabolism; Psychology. Psychoanalysis. Psychiatry; Sialia sialis; Sons; Weather
• ISSN: 0010-5422; 1938-5129
• DOI: 10.2307/3247713

Male Eastern Bluebirds (Sialia sialis) feed their daughters more frequently than their sons. One hypothesis which accounts for this pattern of paternal care is that daughters require more food because they have a higher energy expenditure than their male siblings. We measured the field metabolic rate of one male and one female nestling from 14 different nests using the doubly labeled water technique. Male and female CO2production did not differ over a 24-hr period (mean ± SD: 3.77 ± 0.59 ml/g hr and 3.51 ± 0.47, respectively). In 7 of the 14 broods male CO2production was more than 5% higher than female production. The lack of a significant difference in male and female field metabolic rates allows us to eliminate two potential explanations for sex-biased provisioning in Eastern Bluebirds: (1) one sex has a higher daily energy expenditure; and (2) one sex requires more energy to compete with the other sex for food. Sex-biased provisioning may be unrelated to the energetic demands of the offspring.