Temporal and age-related variation in the laying rate of the parasitic brown-headed cowbird in the eastern Sierra Nevada, California

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Abstract
Spatial, temporal, and age-related variation in the laying rate of female brown-headed cowbirds in Mono Co., California was assessed by determining the proportions of samples of live-trapped females that had an oviducal egg. Females were considered to have an oviducal egg if their abdomens felt distended. The technique was tested for reliability by predicting the laying condition of 36 females and then holding them overnight to obtain eggs the following morning. The predictions were upheld for 92% of the cases. The highest laying rates for the 1982 and 1984 seasons were 0.79 and 0.77 eggs per day, respectively. During the middle part of the 1984 breeding season (22 May – 18 June) laying rates averaged 0.64 ± 0.12 eggs per day. A seasonal average of 30.5 eggs per female was calculated from the daily laying rates. Results from 1982 began in mid-season, but showed that laying rates were higher late in the season and declined more rapidly than in the 1984 season. A total of 75 examinations of 54 females of known age during 1984 revealed that the laying rates of yearling females were significantly lower than those of adults. Dividing these samples into three time periods revealed that the reduced laying rate is caused by a delay in the initiation of reproduction by yearlings. Results suggest that the delay may be due to a later arrival date of yearlings onto the summer range.