In most dietary studies of fruit-eating birds, sampling methods consist of direct observations of foraging and fecal sample analyses, and the consumption of resource types is measured as the percent of occurrence in diet samples, or as counts (frequency) of ingested items. Since these measures are usually biased towards the overestimation of small and abundant resources, the amount of ingested mass is a more accurate measure of the crude energy and nutrients provided by a given food source. In this study, we use direct observations of foraging behavior to describe the diet of four frugivorous-insectivorous bird species during the breeding season in Southern Yungas forests, and we compare the number of ingested items and ingested dry mass as measures of fruit and arthropod consumption. In terms of ingested food dry mass, fruit consumption represented over 95% of the diet of the four bird species. The estimated proportion of fruit in the diet differed significantly among methods in three bird species. The comparison of two methods to quantify food consumption by birds allowed us to determine that, when gross amount of ingested matter is considered, the proportion of arthropods in the diet of frugivorous-insectivorous species is much smaller than previously reported. Our study suggests that the use of food dry mass is more appropriate than methods based on item counting to determine the importance of food items in the diet, since it avoids overestimation of resources which contribute comparatively little to the total energy and nutrients ingested. The comparison of these methods highlights the importance of using more reliable measurements of the contribution of different types of food to characterize the diet of frugivorous-insectivorous bird species.
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