The study looked at why some birds mate with relatives even though it can harm their offspring. By studying song sparrows, researchers found that while inbreeding can be bad for baby birds, the selection against mating with relatives is not very strong. Female sparrows showed a slight preference for mating with relatives, while male sparrows that did mate with relatives had better overall fitness. This suggests that even though inbreeding can be harmful, the birds don't always avoid it, leading to a better understanding of this puzzling behavior.