Effects of Weather on Breeding Ducks in North Dakota
Productivity
The brood to pair ratio, despite various drawbacks, is the most widely known and used index to productivity, and is perhaps the best currently available. The following analysis is based on brood to pair ratios for 14 years at Salyer and 13 years at Woodworth (Table 3). For comparability, ratios were multiplied by 1,000 at Salyer and by 100 at Woodworth.
To determine what effect weather had on the productivity rates, we examined the data relative to mean temperatures and precipitation totals during various intervals in the breeding season.
Mallard
Mallard productivity indices averaged 10.3 at Salyer and 27.1 at Woodworth. The different methods of counting broods rendered between-area comparisons impossible. Yearly variation was considerable; coefficients of variations (CV) were 72% at both areas.Rates of mallard productivity at Salyer varied with mean temperature during 23 April-3 June (Fig. 5a). The expected increase in the brood index was 2.4 for every Celsius degree increase in temperature (P<0.02). The effect of precipitation was nonsignificant. No relation between mallard productivity rates and weather variables was detected at Woodworth.
Gadwall
Gadwall productivity indices averaged 65.3 (CV=110%) at Salyer and 36.0 (CV=59%) at Woodworth. We found no correlation between gadwall productivity rates and weather variables at either study area (Fig. 5b).Blue-winged Teal
Productivity rates for the blue-winged teal averaged 31.9 (CV=135%) at Salyer and 39.4 (CV=53%) at Woodworth. At Salyer, these rates were significantly (P<0.05) related to mean temperatures during 23 April-3 June (Fig. 5c), but much of the correlation was due to an extreme value in 1952, a warm year with an unusually large brood to pair index. With that year omitted from our data the regression was marginally significant (P=0.06) and the rate of change was 3.7 per Celsius degree. Precipitation had no noticeable effect on the brood to pair counts.No association between blue-winged teal productivity and weather variables was noted in the Woodworth data
Redhead
At Salyer, redhead productivity indices averaged 38.5 (CV=93%). Woodworth data were not examined because the pair populations were too small to base a productivity rate on.The Salyer data displayed a relation to mean temperatures during 23 April-3 June (Fig. 5d). The productivity index of this species varied 10.0 for each Celsius degree change in mean temperature (P=0.04). No effect of precipitation was discernible.
General
The productivity rate, unlike the other characteristics we investigated, yielded far different results from the two study areas. We attribute the absence of noticeable weather effects at Woodworth to a masking by high but variable rates of predation. It is widely believed that predation had intensified from the period when Salyer data were gathered to that when Woodworth data were gathered.At Salyer, all species except the gadwall exhibited a significant correlation between brood to pair ratios and mean temperatures during 23 April-3 June. The regression coefficients, expressed as a percentage of the average index, were 0.68% for mallard, 0.32% for blue-winged teal (1952 excluded), and 0.72% for redhead. These values were highest for the mallard and redhead, which began nesting first, lower for the teal, which initiated next, and nonsignificant for the gadwall, the latest to nest.
This analysis suggests that productivity at Salyer was higher during years with warm mid-season weather, with the exception of the late-nesting gadwall.
Other researchers have associated reduced productivity with climatic features, notably delayed, cool springs. Nilsson (1974) found mallards more productive in years with warm April weather. Makepeace and Patterson (1980) reported higher survival of shelduck (Tadorna tadorna) ducklings in years with more warm days in the season, and in years with fewer windy days. The number of rainy days was not significantly associated with duckling survival. Lensink (1973) found, as we did, that delayed, cool springs affected the productivity of early-nesting ducks more than later-nesting species. Milne (1976) suggested that, among Arctic-nesting waterfowl, dabbling ducks were more sensitive to low spring temperatures than were diving ducks.
Two published reports contained sufficient detail for us to examine their data in a comparable manner. Evans and Black (1956) provided productivity data and temperatures for 1950-53. Three years had similar temperatures during 23 April-3 June, but 1950 was somewhat cooler. Productivity rates of mallards were noticeably lower that year, but those of blue-winged teal and gadwall were not appreciably different from the other years. These results are generally consistent with ours. A reanalysis of Smith's (1971) data showed that brood to pair ratios during 1953-58 were positively correlated with minimum temperatures during April and May for mallard, blue-winged teal, and all species combined. Regression coefficients were largest for the mallard, suggesting greater sensitivity of that early-nesting species.
Low temperatures during the breeding season can reduce productivity by affecting breeding adults or young. Effects on the breeding pair include nonnesting by some of the population (Lensink 1973; Milne 1976), late nesting (discussed earlier), and reduced clutch size, which can depress brood to pair ratios if smaller broods are less likely to survive or to be seen than larger ones.
Direct effects of low temperatures include death from exposure, increased vulnerability to predators, and reduced opportunity to feed adequately. Boyd and Campbell (1967) observed greater duckling losses during periods of cold, wet weather, and Hildën (1964) and Bengston (1972) found that newly hatched ducklings were particularly susceptible. Seymour (1982) observed a 1- or 2-day-old mallard duckling become separated from its mother and broodmates, and die within 60 min of exposure to 4°C air and 3°C water temperatures.
Increased susceptibility to predation during periods of cold has not been reported; however, reduced opportunity to feed was noted by Hildën (1964), who stated that surface feeding becomes more difficult in cold, windy weather. This phenomenon could partly account for the greater susceptibility of dabbling ducks compared with that of divers.
Koskimies and Lahti (1964) found differences among species with respect to cold-hardiness of newly hatched ducks, and postulated that cold-hardiness is an uneconomical strategy except in a relatively narrow zone of environmental conditions. They claimed that a low metabolic rate, associated with reduced adaptation to cold, is advantageous during normal weather in most breeding areas. Thus, the less cold-hardy species tend to be more adaptable and to have larger breeding ranges.
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