Annual Changes in Gizzard Size and Function in a Frugivorous Bird

Abstract

Frugivory often entails substantial adaptations of the digestive tract, associated with the presence of indigestible seeds and a comparatively low concentration of nutrients in the diet. Species consuming mistletoe berries face the added complication of manipulating the viscous, adhesive pulp characteristic of this birddispersed fruit and commonly exhibit extreme adaptations of digestive anatomy and function (Forbes 1880, Wetmore 1914, Desselberger 1931, Steinbacher 1935, Walsberg 1975). An example is the Phainopepla (Phainopepla nitens), a small songbird resident in the Sonoran Desert from October through April. During this period, the bulk of the Phainopepla's diet consists of berries of the desert mistletoe (Phoradendron californicum) (Walsberg 1975, 1977). The pericarp of this fruit (its pulp) is mucilaginous, which aids in adhering the seed to host plants but also makes it more difficult for a consuming animal to manipulate. The Phainopepla's digestive apparatus is highly specialized to process large numbers of these berries by rapidly removing the exocarp (the berry's skin) within the digestive tract (Walsberg 1975). A foraging bird typically fills its crop with berries, then passes them singly to the gizzard (ventriculus). The small gizzard averages only 1.5% of body mass and will contain a single 3to 5-mm diameter mistletoe fruit. The gizzard apparently contracts and extrudes the seed and semiliquid pulp out of the exocarp into the small intestine. The exocarp is retained in the distal portion of the gizzard. This process is repeated for 12-24 berries, then the accumulated mass of exocarps is ejected from the gizzard into the small intestine. As little as 12 min elapse between ingestion and egestion and at least 1,100 berries may be consumed per day with caloric digestive efficiencies averaging 49% (Walsberg 1975, 1977). Desert mistletoe, however, is consumed only during the period between October and May. From late spring until fall, Phainopeplas occupy semiarid habitats to the north, east, and west of the Sonoran Desert (e.g., oak woodland, riparian woodland in chaparral areas) (Walsberg 1977). Compared to the period in the Sonoran Desert, the diet is more generalized and a wide variety of small berries are consumed (e.g., Lycium spp., Rhamnus crocea, Rhus spp., and Sambucus mexicana). We report here a major seasonal change in gizzard size and function correlated with this dietary shift. Data were collected incidental to a larger analysis of this species' annual cycle of reproduction, molt, and body composition. Because the annual shift in gizzard structure was noted only late in our study, comparisons are limited to samples collected between August and December.