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Effects of Weather on Breeding Ducks in North Dakota

Methods


Weather Data

We obtained weather data from the monthly and annual publications of the National Oceanic and Atmospheric Administration, National Climatic Center, Asheville, North Carolina. Weather information pertinent to Salyer was recorded at Upham; that for Woodworth was recorded at the station or at nearby towns.

Weather data were tabulated as weekly averages of daily maximum, minimum, and mean temperatures, and of total precipitation during the week. We discovered during preliminary analyses that the average daily mean temperature adequately represented that aspect of weather, so minimum and maximum temperatures were not considered further.

Duck Arrival Dates

The first ducks of each species to arrive in spring were recorded at both study areas. Although a more representative measure of arrival might have been the date on which the first major influx appeared, this measure is more subjective and, in most years, the first ducks seen and the first waves to arrive were separated by only a few days.

Breeding Pair Populations

During 1949-54, pair counts were made at Salyer on a sample of wetland areas and were nonstatistically projected to the entire refuge. During 1955-63, ducks were censused on virtually the entire refuge; methods used were similar to those described by Hammond (1969).

Pair counts were systematically made at Woodworth during May 1965-77. All wetland basins containing water were surveyed.

Nesting Studies and Brood Counts

Nesting studies were performed at Salyer and Woodworth to obtain information on dates of first nest initiation and peak hatching, and span of nesting.

Brood surveys were the most important source of data for brood size analysis, nesting chronology and, in conjunction with estimates of the breeding population, productivity rate. Guides by Gollop and Marshall (1954) and Hammond (1970) were used for conducting brood surveys.

Broods at Salyer were censused from a boat or canoe powered by an outboard motor through open water channels of the marshes. Time of day, date, weather requirements, and scanning points were standardized. During the 14-year span of taking brood counts at Salyer, marsh vegetation gradually encroached on some of the better brood areas, making observation difficult. We suspect that estimates of productivity during 1958-62 were low compared with those made during 1949-57.

Brood counts were made at Woodworth each July by performing a "beat-out," in which many people simultaneously plodded through each wet pond or walked shorelines to record all broods or hens exhibiting broody behavior.

Broods of different species vary in time spent on the water and in their preference for emergent vegetation. For these reasons, greater proportions of broods were observed for some species (e.g., gadwalls and redheads) than for others (e.g., mallards and blue-winged teal).

We constructed nesting chronology curves by backdating broods observed. These curves contain data representing only successful nests. Any cause of nest loss, such as predation, that was both appreciable and variable among years would modify the curve and possibly mask effects of weather. The long span of the study made such variability likely.

We used nesting chronology curves to estimate date of first nest initiation, date of peak hatching, and span of the nesting period, but recognized that an observed curve of nesting chronology might not reflect the actual curve because of losses that occur. We were unable to determine the date of first nest initiation at Woodworth because of limited samples.

Ducks at Salyer were subjected to two influences, botulism and flooding, that did not markedly affect those at Woodworth; predator reduction occurred in both areas. Botulism outbreaks occurred in one or two marsh units in 1949, 1959, and 1960. Productivity data were gathered from other areas whenever possible. Flooding on the Souris River and its tributaries occasionally inundated low-lying nesting habitat and depressed nesting success and productivity. Predation was fairly intense at Salyer and probably was more so at Woodworth. It was countered by various predator control methods; in some years, strychnine-treated eggs were used at Salyer, and shooting and trapping occurred on both areas.

Data Analysis

We employed the simplest methods of analysis adequate for the task. For example, we treated each species individually—although a multivariate approach might yield more powerful hypothesis tests, we chose the single-species approach to highlight important distinctions among the species.
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