Cowbirds always use others’ nests. The cowbirds here break the eggs already laid in the nest and lay one egg of their own (a white one with brown freckles.) They then expect the host bird to raise the cowbird’s young. I have watched it under my deck with a phoebe’s nest.
“The Brown-headed Cowbird (Molothrus ater) is North America’s most notorious brood parasite. Instead of building their own nests, incubating their own eggs and raising their own nestlings, Brown-headed Cowbirds have a different breeding strategy. Cowbird females use other bird species as hosts—laying their eggs in the nests of other bird species and relying on these hosts to incubate and raise their chicks.
Scientists have now recorded that Brown-headed Cowbirds have parasitized over 220 host species, ranging from the Black-capped Vireo and Wood Thrush to the Blue-winged Teal and Red-headed Woodpecker. While not all hosts make good foster parents—a number of species reject cowbird eggs—cowbird chicks have been successfully reared by over 150 host species, with songbirds comprising the majority of hosts.”