The Cornell Lab of Ornithology studied the variation in eggshell patterns and color by examining house sparrow eggs, which are quite varied.
The researchers note that eggshell coloration and pattern may vary with available calcium, sunlight patterns, or habitat quality, and are expected to differ seasonally and geographically as well.
House sparrows aren’t native to this continent and are a detriment to our native species. They (and European starlings) are not legally protected as are our native birds, so they provide a good opportunity to study variations in eggs without harming native birds.
Since we routinely remove house sparrow eggs from our nest boxes anyway, this was a good opportunity to make the best of a bad situation. Although we don’t enjoy disturbing any bird’s nest, we believe we have an ethical responsibility to help native birds by decreasing the house sparrow population.
The Lab asked people to send in photos of the eggs on white paper, noting the date and place, including a coin in the photo for size comparison.
After sending in the photo, I was contacted by Cornell. They noticed that the egg in the middle of the top row was actually a cowbird egg! Couldn’t have happened to a “nicer guy”!
Reflections
The problem for nest-box monitors—and native birds—is that House Sparrows also breed in nest boxes, and their small 6 1/4-inch frame can easily fit inside a box intended for native songbirds. Male House Sparrows that defend territories early in the season often prevent later-arriving migratory species from nesting. They aggressively defend nest boxes, sometimes evicting other species that attempt to nest in their territory, destroying eggs, killing nestlings, or killing females that are incubating eggs. House Sparrows are also prolific breeders, able to raise up to four broods per season, with each brood averaging between four and five eggs.
~ Cornell Lab of Ornithology, The Trouble with House Sparrows