Visceral Morphology of Redheads (Aythya americana) in Louisiana, Texas, and South Dakota

Abstract

We collected redheads (n = 872) from three locations over a three-year period, as follows: (1) Chandeleur Sound, Louisiana (n = 287, winter 1987-88, 1988-89); (2) Laguna Madre, Texas (n = 551, winter 1988-89, 1989-90); and (3) near Waubay, South Dakota (n = 34, spring 1990). We measured the mass of the esophagus, proventriculus, gizzard, intestines, ceca, liver, spleen, pancreas, and kidneys, the length of the intestines and ceca, and the distance from the ceca to the rectum. All were analyzed for differences due to sex, age, state, year, and time of year (season). In the upper gastrointestinal tract, Texas redheads had heavier esophagus and proventriculus mass, but Louisiana birds had heavier gizzards; all birds showed a general increase in gizzard mass from early to late winter. For the lower gastrointestinal tract, birds collected in Louisiana had consistently longer and heavier intestines and ceca than the Texas redheads, possibly to facilitate water reabsorption for osmoregulation, necessary because the Louisiana birds did not use a supplementary fresh water source for drinking like the Texas birds did. The mass of ceca and intestines generally decreased over the winter, especially in the Texas redheads, possibly because of their gradual acclimation to high salinities over that time period, and thus the reduced need for intestinal water reabsorption. Kidney mass in juveniles and pancreas mass in adults and juveniles showed a linear increase over the wintering period, whereas liver mass showed a decrease, in birds from Louisiana and Texas, perhaps due to variation in the rate of food consumption among groups. Adults generally had larger kidneys than juveniles, but only in early winter. We found no significant differences among treatment groups for spleen mass or ceca-to-rectal distance. We found that redheads collected in South Dakota had significantly heavier livers and kidneys than the wintering birds, but lighter gizzards; intestinal and cecal differences were generally inconsistent or absent. The shift from estuarine wintering habitat to inland freshwater wetlands in the spring likely contributed to the observed differences. We believe that variations in diet quality, salinity, and physiological demands account for most or all of the differences that we found between the different treatment groups.