Effects of Weather on Breeding Ducks in North Dakota
Nest Initiation Dates
The week in which the first egg was laid by a species was estimated from backdating either broods observed or nests found. Nests or broods could be lost before they were observed, therefore the initiation dates of nesting were estimated in the field only to within a week, and subtle differences among years may have been obscured. For analysis, these dates were converted back to the Julian date of the middle day of the week.
We sought to determine if any relation existed between date of nest initiation and weather variables, as summarized by the mean temperature during the weeks when first nests are initiated. We also explored effects of arrival date on first nest initiation: If birds arrive late do they also begin to nest late? Data were available only from Salyer.
Mallard
The median week that mallards initiated nests was estimated at 16-22 April (Table 2), but ranged from 2-8 April to 30 April-6 May. The difference between median arrival date and date of nest initiation was 20 days. The initiation date was compared with mean temperatures during 2 April-6 May, the period in which mallards began to nest (Fig. 2a). The regression coefficient, -0.77, was not significant (P=0.28).Gadwall
Gadwalls initiated first nests primarily during the weeks of 7-13 May and 14-20 May, but in some years they began about 2 weeks earlier to 1 week later. Nest initiations were significantly (r=-0.488; P<0.02) delayed by low temperatures during 16 April-27 May (Fig. 2b), the period in which initial gadwall nests were detected. We attributed no additional effects to arrival date which preceded first nests by an average of 34.5 days. The regression coefficient of -2.15 suggested that the first nest was delayed by slightly more than 2 days for each Celsius degree change in average temperature during the period of initial nesting.Blue-winged Teal
Blue-winged teal began their first nest during the week of 7-13 May in most years, but variations of about a week were not uncommon. Nesting began about 24.5 days after arrival. The date of nest initiation among blue-winged teal was related (r=-0.563; P<0.01) to average temperatures during 16 April-27 May, the period of initial nesting (Fig. 2c). Again, no additional effects were attributed to arrival date. The regression coefficient of -1.97 suggested a change of nearly 2 days in nest initiation for each Celsius degree change in average temperatures.Redhead
First nests of redheads were started from the week of 16-22 April to the week of 28 May-3 June; little clumping about the mean was evident. On the basis of 11 years of data, the difference between median arrival date and first nest initiation was 25 days. First nests were recorded later when temperatures were lower during 16 April-27 May (Fig. 2d),but not significantly so (r=-0.439; P=0.18). The regression coefficient was -2.84 days per Celsius degree.
General
All species seemed to delay nest initiation under low temperature conditions, but the effect was significant only among gadwalls and blue-winged teal. For mallards the delay was 0.77 day per Celsius degree difference in mean temperatures during 2 April-6 May. Temperatures during 16 April-27 May were influential for the other species, and delays (in days per Celsius degree difference) were 2.15 for gadwalls, 1.97 for bluewinged teal, and 2.84 for redheads.Much has been written about possible influences of weather on nest initiation. Delays associated with cool springs and late ice melt have been reported for mallards (Ogilvie 1964; Nilsson 1974; Milne 1976) as well as for geese (MacInnes et al. 1974; Raveling 1978) and other waterfowl. Specific instances were described by Dane (1966), who suggested that blue-winged teal delayed nesting in 1963 because of a period of cold weather in late April and early May, and by Harris (1954), who observed that several species nested late in 1950, a year with a cold and dry spring. Coulter and Miller (1968) indicated that more American black ducks (Anas rubripes) nested early when March temperatures were above normal than when they were below normal, but the difference was only marginally significant (P=0.12).
Several investigators performed analyses comparable to ours, or provided data in sufficient detail that we could analyze them. Data from Newton and Campbell (1975) yielded a correlation between the date of first clutch and mean February temperature of r=-0.850 (P<0.05) for a population of mallards that initiated nesting in March. For later-nesting species, the onset of nesting seemed independent of weather.
Keith (1961) noticed no consistent relation between nesting phenology and the temperature and precipitation variables he considered. A reexamination of his data showed, however, that mallards, gadwalls, blue-winged teal, northern pintails (Anas acuta), and redheads all nested later in 1953-54 than in 1955-57. Temperatures during April and May averaged higher during 1955-57 than during the previous 2 years; this relation is consistent with our conclusions.
A reanalysis of Yocom's (1950) data on mallards, gadwalls, teal (blue-winged and cinnamon [Anas cyanoptera]), and redheads showed that the date of initiation of each species during the 3 years of study was negatively correlated with mean temperatures during April and May. The eight correlation coefficients averaged —0.76. We also reconsidered the data of Evans and Black (1956), and found that initiation dates during 4 years were clearly correlated with temperatures during 2 April-6 May for mallards and 16 April-27 May for gadwalls. The analogous relation for blue-winged teal was not evident. We also examined Bengtson's (1972) data for duck populations in Iceland. The dates of first initiation for mallards and gadwalls during 9 years of study were related to mean temperatures in May (r=-0.80 for mallards, r=-0.72 for gadwalls).
Several factors have been implicated in controlling the onset on breeding in waterfowl and other birds. Väisänen (1974) suggested that laying dates are partly fixed by heredity and are modified by environmental conditions and the age of the bird. The individual consistency of laying date, perhaps under genetic control, was supported by Perrins (1970), Spurr and Milne (1976), and Batt and Prince (1979). Environmental conditions include photoperiod (Marshall 1961; Raveling 1978), access to nesting and feeding sites (Marshall 1961; Perrins 1970; Krapu and Doty 1979), and weather, which may have direct effects on birds and indirect effects through changes in the nesting habitat and food resources (Marshall 1961; Väisänen 1974; Raveling 1978; Krapu and Doty 1979). The importance of age of nesting female on initiation of egg laying has been suggested by Coulter and Miner (1968), Krapu and Doty (1979), and others.
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