Size of digestive organs in breeding Brown-headed Cowbirds, Molothrus ater, relative to diet

Abstract

We weighed gizzards and livers and measured small intestines of male and female Brown-headed Cowbirds (Molothrus ater) collected before, during, and after the egg-laying season in southwestern Ontario in 1976. Previously published data about the diets of these birds were used to assess if their digestive organs changed in size in response to changes in diet quality (diets high in animal matter were considered high in quality). Gizzard weight decreased when the birds fed primarily on arthropods and increased when they fed primarily on seeds. Intestine length, particularly of females, was highest when the diet included the least amount of plant material. This result was unexpected, based on the limited data from other passerines. However, diets of females were lowest in plant material when they were laying eggs and we argue that the increased intestine length at that time reflects an increased rate of food consumption. Changes in liver weight were more equivocal but generally support the idea that rate of food consumption and nutrient demand can be more important than diet quality in determining changes in size of this organ. We conclude that attempts to assess changes in diet quality of wild passerines by looking at changes in size of their digestive organs should be done cautiously, particularly if baseline data are from captive birds.