Abstract
Despite experimental reduction of the value of parental effort, male Brewer's blackbirds did not reallocate resources into mating effort; they did not attempt to mate polygynously. Although it was selectively advantageous for members of this population to do so, apparently no strong environmental potential for polygyny existed. Male yellow-headed blackbirds, when faced with a reduction of the marginal value of their 10 broods, reallocated investment toward their 20 broods but would not redirect feeding effort toward their 30 broods. Allocation of parental investment by these yellowheads was based on clutch size, nestling age and possible nestling sex.
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