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We define the central span of nesting to be the interval from the date on which 10% of observed nestings began to the date on which 90% of them began. The span thus includes the central 80% of observed nestings.

We explored three hypotheses in our analysis of these data:

(1) That warm weather early in the nesting season will prompt nesting and thereby result in relative synchrony and a short span of nesting. Conversely, cooler weather will inhibit some individuals in the population, who will await more optimal nesting conditions. Other individuals will initiate nests anyway, and the resulting heterogeneity will be reflected in a longer span of nesting.

(2) That precipitation later in the nesting season will induce renesting among unsuccessful hens, thereby prolonging the nesting season. Conversely, dry weather at that time will tend to curtail further nesting.

(3) That a late nesting season, regardless of the cause, will shorten the span of nesting, because hens tend not to initiate nests after a certain date. This hypothesis was investigated by comparing the span with the peak hatching date.

Mallard

No appreciable association occurred between mallard span of nesting and either mean temperatures during earlier weeks or the peak hatching date.

Gadwall

Blue-winged Teal

The relation between nesting span and peak hatching date was significant (P=0.03, Fig. 4b). The average span decreased 0.31 day for each day delay in the peak. The relation between teal nesting span and precipitation during 23 April-3 June was suggestive (Fig. 4c), with a coefficient of 0.36 day per centimeter of precipitation (P=0.12).

Redhead

General

We detected no effects on the nesting span from mean temperature during the nesting season. Some authors (Dane 1966; Dzubin and Gollop 1972) noted that brief periods of cold weather interrupted nesting activities and occasionally caused bimodal nesting curves, which can lead to elongated nesting spans. Sorensen's (1978) data for mallards, however, do not suggest shorter nesting spans in warmer seasons but, like ours, are based on average temperatures for long periods of time, which may not be sensitive to short periods of cold weather than can interrupt nesting and extend the span.

The suggestion that precipitation will encourage continued nesting and renesting was buttressed by evidence from the mallard and blue-winged teal. Both species had longer nesting spans during years of above-average precipitation during 23 April-3 June, the mallard by 0.72 day per centimeter of rain, and the blue-winged teal by 0.36 day.

For the blue-winged teal and redhead, nesting seasons that were later than usual resulted in shorter nesting spans. For each day the peak hatch date was delayed, the nesting span was shortened by 0.31 day in blue-winged teal and 0.41 day in the redhead. Keith (1961) and Raveling (1978) also observed that nesting seasons were more compressed in late seasons than in normal ones.

The duration of the nesting season thus seems to be affected by several variables whose interactions made interpretation difficult. Cold periods have been shown to interrupt breeding and prolong the nesting period. Precipitation that occurs during the breeding season presumably maintains wetlands in good condition and prolongs nesting. The analysis presented here for blue-winged teal and redheads supports other observations of shorter breeding seasons in years with late nest initiations.