Co-Occurrences of Foods in Stomachs and Feces of Fruit-Eating Birds

Publisher Website
Google Scholar
Abstract
Short-term dietary mixing by American Robbins (Turdus migratorius) and other fruit-eating birds in eastern North America was examined using 5,697 records of stomach contents for 11 bird species and 3,618 avian fecal droppings from New Jersey. Avian seed budgets were estimated by using fruit morphological data to relate foraging observations to seed counts from stomachs and feces. Remnants of multiple taxa of foods were found commonly in individual feces and stomachs although these samples held only 0.25-2 times the seeds consumed during a typical feeding bout. Depending on bird taxon, seeds from different fruit species were mixed in 1.5-39.6% of feces and 4.2-41.6% of stomachs, and fruit and animal material were mixed in 24-59% of stomachs. Frequency of mixed seeds was positively correlated with proportion of fruit in the stomach and with seed concentration of fruit in the diet. For birds overwintering in the United States, seed mixing in stomachs peaked in winter when birds were most dependent on fruit, not in fall when fruit abundance and diversity were greatest. Thus, estimates of dietary mixing may be biased by seasonal or habitat-related trends in avian fruit dependence or fruit morphology. Furthermore, mixing was no less common in feces than in comparable stomachs even though contained three to four times more seed mass. This finding along with our observations of seed treatment by birds suggested that mixing was amplified by shuffling of seeds and fruits in the upper gut and by variability of seed transit times through the intestines.