Northern Prairie Wildlife Research Center
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.
|Class and species||Pair population||Mean temperature (°C)||Year|
|Class and species||na||Mean||SDb|
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.