Opened the bluebird house to find each of the eggs penetrated with a jagged hole and swarming with ants and small flies. Classic sign of a House Sparrow raid. Sadly, while death is part of nature, this is not nature's way. House Sparrows are an invasive, non-native species, introduced (both intentionally and accidentally) by European immigrants.
While many birders use glue traps or even electricity, we can't stomach killing them, and have attempted more passive measures, such as controlling what we put in the feeders and pairing bluebird houses in the attempts to draw Tree Swallows, a native species that can coexist with the bluebirds, but often repel House Sparrows. Will have to try monofilaments to frighten them away the next time a Bluebird makes its home in the box.
If you ever see this in one of your bluebird houses (from what I've heard, it can be a lot more gruesome than this), clean the house out, and keep an eye on the house to make sure the House Sparrows do not move in. If House Sparrows do enter a nesting box, it is perfectly legal and advisable to remove the nest immediately.
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