What do you mean by "redbird"? Do you mean a cardinal? Sounds more like a predator has eaten the other two and may come back for the last one. Or if the birds are fully feathered, the other two may have fledged (left the nest on their own) and the last one may be leaving soon. Birds as hardy as cardinals will usually survive right along side a cowbird chick if an egg was laid in their nest. It is the smaller birds such as warblers and vireo that would likely not survive along with a cowbird.
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What do you mean by "redbird"? Do you mean a cardinal? Sounds more like a predator has eaten the other two and may come back for the last one. Or if the birds are fully feathered, the other two may have fledged (left the nest on their own) and the last one may be leaving soon. Birds as hardy as cardinals will usually survive right along side a cowbird chick if an egg was laid in their nest. It is the smaller birds such as warblers and vireo that would likely not survive along with a cowbird.