Reproductive Success of Red-Winged Blackbirds in North Central Oklahoma

During the 1965 breeding season the reproductive success of Red-winged Blackbirds was studied on eight ponds and two small lakes located in two north-central Oklahoma counties. A few males were already on territory when observations were initiated on 8 April. Density of males on the study areas generally varied inversely with the size of the cattail nesting cover. Four females were known to be paired by 17 April. The nesting peak occurred during the third week of May. Cattails furnished sites for over 90 per cent of the nests. Generally, redwings built a new nest when they attempted to renest. However, on five occasions, a nest was used for two nesting attempts. This occurred early in the nesting season and only once on any pond. Eight hundred and twenty-one eggs were laid in 243 active nests for an average clutch size of 3.4. Hatching success was 38.9 and young were fledged from 24 per cent of the eggs. Sixty-five nests fledged one or more young, giving a nesting success of 26.7 per cent. Egg or young removal and nests tipping over were the major reasons for nest failure. Egg loss exceeded mortality of the young. The high nest densities in a limited area would make it possible for a single factor to destroy a large number of nests. Nesting success was higher for nests initiated prior to 1 June. Average clutch size was greater in early nests. Nests in old cattails were more successful. Over 50 per cent of the nests were constructed less than two feet above the ground or water. Nesting success was higher for nests in lower situations and decreased as nest height increased. Approximately two-thirds of the nests were constructed above ground or above water that was less than 10 inches deep. Nesting success was greater as the depth of the water below the nest increased.