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Effects of Weather on Breeding Ducks in North Dakota

Brood Size


We used two measurements of brood size: average numbers of ducklings in broods of age Class I and of age Class II. Class I ducklings ranged in age from 1 day to about 18 days in mallards and gadwalls,13 days in blue-winged teal, and 24 days in redheads (Gollop and Marshall 1954). Age ranges in days of Class II ducklings were 19 to 45 for mallards, 19 to 44 for gadwalls, 14 to 36 for blue-winged teal, and 25 to 54 for redheads. These data, from Salyer, span 7 to 16 years depending on species and age class. Brood size averages are based on a minimum of five broods; most averages involved much larger sample sizes. The Class I brood size reflects the number of eggs in a clutch, the proportion of those that hatch, the proportion of hatched ducklings that reach water, and the survival of those ducklings until time of census. The Class II brood sizes reflect these factors, as well as survival from Class I and Class II.

Although it is easy to conceive of numerous hypotheses relating brood size to various aspects of weather, we were unable to detect any relation between precipitation and brood size. Correlations between mean temperatures and brood size were at least suggestive. Additionally, brood size was correlated with breeding population and with year, the correlation with year suggesting a trend in average brood size.

We considered mean temperatures during 21 May-1 July, when eggs in most nests were hatched and young brought to water. Correlation coefficients between brood size and pair population size, mean temperature, and year are given in Table 5. Intercorrelations exist between certain of the explanatory variables (e.g., population size and year) because most species generally increased in number during the course of the study. These intercorrelations confounded our analysis of effects on brood size.

Table 5. Correlation coefficients between brood size and breeding pair population, mean temperature during 21 May-1 July, and year.
Class and species Pair population Mean temperature (°C) Year
I
Mallard -0.29 0.07 -0.49
Gadwall -0.64b -0.15 -0.83c
Blue-winged teal -0.46a -0.47a -0.72c
Redhead -0.36 -0.04 -0.14
II
Mallard -0.62b -0.40 -0.56b
Gadwall -0.74c -0.45a -0.50b
Blue-winged teal -0.34 -0.63c -0.81c
Redhead -0.53 +0.77b +.040
aP<0.10.
bP<0.05.
cP<0.01.

Mallard

Average brood sizes for mallards were 6.8 in Class I and 6.5 in Class II (Table 6). Class I brood size was not significantly correlated with population size, mean temperature, or year (Table 5). Class II brood size was correlated with population size (Fig. 6a) and year (Fig.7a). The effect of population size was more marked, and the effect of year, after accounting for the effect of population size, was no longer significant (P=0.31).

Table 6. Summary statistics of average Class I and Class II brood sizes at Salyer.
Class and species na Mean SDb
I
Mallard 11 6.8 0.8
Gadwall 17 7.3 0.7
Blue-winged teal 17 7.0 1.0
Redhead 14 6.5 1.1
II
Mallard 15 6.5 1.3
Gadwall 16 7.2 0.6
Blue-winged teal 16 6.6 1.4
Redhead 7 6.5 0.7
an=number of years.
bSD=standard deviation.

Gadwall

Gadwall brood sizes averaged larger than those of mallard—7.3 in Class I and 7.2 in Class II. Class I brood size was correlated with population size (Fig. 6b) and, especially, with year (Fig. 7b). The effect of population size was not significant after accounting for the trend, which suggested a decline in brood size of 0.11 duckling per year. Class II brood size was correlated, in decreasing order, with population size (Fig. 6c), year (Fig. 7c), and mean temperature. The effect of population size dominated, and marginal effects of other variables were not significant.

Blue-winged Teal

Brood sizes of blue-winged teal were intermediate between those of mallard and gadwall—7.0 in Class I and 6.6 in Class II. Class I brood size trended downward (Fig. 7d), and correlated to lesser extents with mean temperature and population size. Class II brood size was significantly associated with year (Fig. 7e) and mean temperature (Fig. 7f). The decline with year averaged 0.23 Class II duckling annually.

Redhead

Data on redhead brood size were limited; both classes averaged 6.5 ducklings. None of the correlations with Class I brood size was significant, and the number of Class II broods was inadequate to examine any hypotheses.

GIF-Average brood size in relation to breeding pair population

GIF-Average brood size by year and in relation to average temperature

General

Brood size tended to be largest and least variable among gadwalls. Blue-winged teal had the next largest broods; mallards and redheads had the smallest.

Intercorrelations among explanatory variables, notably year and breeding pair populations, obscured our analysis. Class I brood sizes declined for most species during the period of study. The decline can be attributed only partly to increased breeding pair populations. Weather was not strongly associated with Class I brood size; only blue-winged teal demonstrated even a marginal relation with mean temperature.

Class II brood sizes were negatively correlated with breeding pair populations among all species; correlations attained significance among mallards and gadwalls. Average brood sizes of all dabbling species declined during the course of the study, but only in blue-winged teal was this effect not accounted for by concurrent population increases. All dabblers tended to have smaller Class II broods during years when temperatures during 21 May-1 July were above normal; the significance of this effect varied among species.

Many investigators have identified cold and wet weather as stressful to ducklings, especially young ones (Hildën 1964; Koskimies and Lahti 1964; Boyd and Campbell 1967; Bengtson 1972; Seymour 1982). Our data did not confirm this association, but the weather data we used—average temperature over extended periods—may have been too crude to detect short-term inclement spells that could be fatal to young ducklings. Instead, we detected some associations between above-normal temperatures during late May and June and reductions in brood size, particularly in Class II. An analysis of Stoudt's (1971) mallard and blue-winged teal data also showed negative correlations between average temperatures in June and Class I and Class II brood size:

Mallard (10 years) Blue-winged teal (9 years)
Class I r=-0.478 (P=0.17) r=-0.312 (P=0.43)
Class II r=-0691 (P=0.02) r=-0.477 (P=0.20)

Several investigators (Dane 1966; Perrins 1970; Batt and Prince 1979; Krapu 1981) have shown that clutches laid later in the season are smaller than those laid earlier, and that clutches laid during years with cool springs average smaller than those of normal years (Krapu and Doty 1979). We anticipated that this effect might carry through to brood size, but in our analysis we found no association between brood size and date of peak hatch, a measure of the phenology of the season.

In addition, we detected a significant decline in average brood sizes during 1947-62. Some of the decline can be attributed to increased breeding pair populations of the dabblers. The dependency on population size could reflect interference either among breeding pairs or among ducklings. Dzubin (1969) suggested that large pair populations might result in many nests located at considerable distances from brood water, and thereby lower brood survival. Makepeace and Patterson (1980) reported that the daily mortality of shelduck (Tadorna tadorna) ducklings increased with the density of broods, as did aggressive interactions. The additional variability due to year is difficult to explain, but could result either from changes in methods used to count ducklings or, more likely, from alterations in the habitat used by breeding adults and ducklings or in the predator composition. In addition, increased predation might have resulted in greater contributions of renests, which contain fewer eggs than do initial clutches. Brood sizes of the redhead were not related to pair populations and did not decline during the study period.


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