We documented the time of day of laying and the total time spent on the nest during laying (laying bout) in 21 species of passerine birds and the brood-parasitic Brown-headed Cowbird (Molothrus ater) at Delta Marsh, Manitoba, between 1989 and 1995. Brown-headed Cowbirds laid on average 31.2 min before sunrise, whereas mean laying times of the other species ranged from 11.3 min after sunrise through early afternoon. Species that laid later in the day tended to spend more time on their nests during laying. Smaller species tended to lay earlier in the day and had significantly shorter laying bouts than larger species. Future studies accounting for variation in foraging and reproductive behavior and predation risk between taxonomic groups may explain additional variation in laying time and duration.
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Vol. 75 • No. 2