Relation of Avian Egg Weight to Body Weight

Abstract

The relation of egg weight as a function of adult body weight was examined in more than 800 species of birds. Plotting the individual values separately for various orders and families on a log-log scale provides a satisfactory description for the allometric equation where egg weight a body weight. The power function, b, appears to be common for all the orders and families and is equal to 0.67 or %. The proportionality constant, a, varies and is characteristic for each group. Relative egg sizes can be compared among different orders, families, and tribes at a specific body weight common to. each particular group. Thus the relative egg weight for 13 orders at 100 g body weight varies from 21% for the Procellariiformes to 4.5% for the parasitic Cuculiformes. At 30 g body weight the relative egg size among 14 families of Passeriformes ranges from 20% in the Fringillidae to 8.7% in the Hirundinidae and for 9 tribes of the family Anatidac from 10% for the Oxyurini to 5% for the Cairinini at 1000 g body weight. The relation of clutch weight and number of eggs to. body size is described for the families Anatidae and Phasianidae. In these two families the increase in clutch weight is proportional to the square root of body weight. When clutch weight is expressed as percent of body weight this value is inversely proportional to body weight. The maximal value is equal to. 100% where clutch weight equals or slightly exceeds the body weight of the female. A similar value was obtained for eight species of the family Fringillidae and may represent a physiological limit of energy production prior to incubation for these birds. Incubation time can also be expressed as a function of body weight and is equal to nearly 9 times the body weight raised to the 0.167 power. On the other hand, egg weight as noted above is proportional to the power of the body weight and when expressed as percent of body weight the power function is (% - 1.0) or -%. Thus as a general rule, a 10- fold increase in body weight is associated with a 46% increase in incubation time, a 4.7 times larger egg weight, or a reduction of the percent egg weight to 0.59 of the previous value.