If it was a bird who killed the bluebird offspring it was probably a house sparrow. There are some ideas on discouraging them, such as cleaning out the birdhouse if they have built a nest after evicting the bluebirds and then move the nesting box to another location. Since these sparrows are not native birds they are not protected, bluebirds are protected. If you have a bird feeder nearby stop using it or relocate it farther away as seed feeders attract house sparrows. You can also purchase wider, slit opening bluebird houses that deter the sparrows who prefer a round hole. You may need a wire mesh guard over the wider opening as animals may have increased access to the babies. If no house sparrow builds a nest after this calamity maybe it was another creature. Usually a house sparrow throws out the bluebirds to build its own nest. Good luck! Keep trying.
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