Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Forestry, Wildlife, and Fisheries

First Advisor

Phillip J. Zwank


Data on behavior, distribution, and food and habitat requirements of wintering redhead ducks (Aythya americana) were collected during 2.5 winters on the lower Laguna Madre, Texas, between 1987 and 1989. Migrating redheads began arriving on the lower Laguna Madre with frontal passages in early October. They initially congregated in the central portion of the lagoon, and the majority remained there through the winter. As winter progressed, flocks dispersed to other parts of the Laguna Madre. Ninety-five percent of redhead flocks were above stands of monotypic shoalgrass (Halodule wrightii) vegetation, their primary food source. Redheads fed intermittently throughout the day; most fed by head dipping or tipping up in water between 12 and 30 cm deep. Flocks moved east and west across the lagoon as water levels changed, thereby remaining in water between 12 and 30 cm deep. Redheads consumed approximately 75% of shoalgrass rhizome biomass each winter. When rhizome biomass was grazed to below 10 g dry mass/m$\sp2$, biomass did not recover to pre-grazing levels the following summer. Thirty-three percent of the sites were grazed to below 10 g dry mass/m$\sp2$ during both winters of the study; 64% were grazed to below 10 g dry mass/m$\sp2$ during 1 of 2 years. Rhizome biomass increased inside redhead exclosures, suggesting that redhead grazing may be keeping shoalgrass below its maximum biomass. During October and November, redheads were found at freshwater sites adjacent to the lower Laguna Madre. Use of freshwater sites declined through the winter with $<$5% of redheads using freshwater sites daily after December. Within the lower Laguna Madre, redheads initially occupied areas with the lowest salinities, and they continued to favor areas with lower salinities throughout the winter. Redheads did not select areas where food was most abundant or had the highest protein content. Redhead habitat selection criteria included: areas with monotypic shoalgrass vegetation, water between 12 and 30 cm deep, rhizome biomass $>$10 g dry mass/m$\sp2$, and relatively low salinities. Because of redheads' high reliance on shoalgrass, further loss of this seagrass species from the lower Laguna Madre may adversely affect redheads.