Aspects of northern pintail wintering ecology on the southern high plains of Texas



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Texas Tech University


Waterfowl may spend more time on wintering grounds than at any other location during their annual cycle. The Southern High Plains of Texas (SHP) are an important wintering area for Central Flyway waterfowl. Up to one million waterfowl use the SHP during winter including as many as 300,000 northern pintails (Anas acuta). This study investigated 1) nutrient reserve dynamics of wintering northern pintails, 2) models of condition for wintering northern pintails, 3) condition and diet biases associated with hunter-killed northern pintails and 4) molt patterns of wintering northern pintails.

Total precipitation was 58% higher in 1985-86 than in 1984-85 (April through March periods). Body, fat and gizzard weights and percent fat were higher in the wet year (1985-86). Age classes (adult and immature) did not differ with the exception of ash weight. Males had higher body, protein (ash-free lean dry), ash, gizzard and liver weights than females. Percent fat was higher for females. Changes in nutrient levels may be endogenously regulated to increase survival during stressful periods of the annual cycle. Changes in gizzard weight probably reflect changes in diet (fiber content) between years and across the wintering period.

Pair status was not consistently associated with an improvement in nutrient reserve status. A threshold of condition may regulate timing of pair bond formation. Initiation of courtship and pair bond formation were advanced in the wet year.

Four models of condition were developed for wintering northern pintails. Models explained more than 65% of the variation in their dependent variable (fat weight, the logarithm of fat weight or a condition index incorporating fat weight) in all cases. All models accounted for greater than 70% of the variation in fat weight in the validation data set. Bias was low and negative.

Aggregate percent volume and aggregate percent dry weight of all plant foods except green material differed between decoy-shot (huntershot) and jump-shot northern pintails. Corn was more prevalent in hunter-shot birds while nonagricultural seeds and total animal food were higher for jump-shot pintails. Food habits studies conducted in areas where waterfowl make regular field-feeding flights should acknowledge the distinct contributions of both natural foods and waste grains. Fat weight was higher for jump-shot than decoy-shot immature pintails in winter 1984-85 but this was the only evidence of a condition bias associated with collection method.

Prealternate molt scores were highest in October and declined to January. Males essentially complete prealternate molt and females initiate prebasic molt in January. Pintails advanced their molt schedule(s) and attained alternate plumage sooner (and for females, began prebasic molt sooner) in the wet year (1985-86). More than 95% of females undertaking prebasic molt were paired, indicating that pair bond formation may regulate initiation of prebasic molt. Prebasic molt scores increased each month through spring departure. Adult birds completed prealternate molt before immatures. Male pintails did not undergo a supplemental molt. Feather replacement was concentrated in mid molt for both sexes.

In the wet year (1985-86), above-average prewinter (April-September) precipitation flooded playa basins on the SHP and resulted in an abundance of high-quality habitats for wintering waterfowl. The difference in precipitation between study years markedly influenced body condition, molt pattern and pairing chronology.