Among most bird speciess, males show aggression. They sing to mark territory, attract mates, and warn competition. When bullies test the limits, bad things happen. For instance, competing male goldfinches easily come to blows, going at it beak and toenail, battling to the ground until one retreats or worse. The bully doesn't necessarily win.
Or consider the cheats. Male red-winged blackbirds maintain a harem, keeping vigil and marking territorial boundaries by song. Females, however, occasionally slip out and rendezvous with an especially alluring neighbor. Cheat though she may, in reality strengthening the gene pool, her harem's kingpin likely doesn't notice. After all, he's keeping eye on an entire entourage, and she's sly.
Unfortunately, a few terrorists work their evil deeds in the secrecy of nest cavities. House sparrows, non-native birds whose numbers have exploded, compete for cavities with native birds, including bluebirds. When a house sparrow deems a given cavity should be his, he will attack the bluebird on the nest, kill her, and construct his nest atop her carcass. That's about as murderous as an avian species can get.
Oh, I know, hawks murder, too. But let's be honest: A hawk kills to eat or feed young. A house sparrow kills to get its way in the housing market. Pure evil.
The biggest spies in the avian world could do the Secret Service proud. A female brown-headed cowbird knows everything happening in the neighborhood. She wears camouflage gray and, from a lofty perch, spies on nesters, sneaking about until she catches a female leaving its nest. Then zip! She slips in, lays her egg, and is gone before the host discovers the trespass.
When the cowbird carries out her dirty act of espionage, she's confident that because her eggs hatch more quickly than other birds' eggs, her baby crowds out the host's. So a surrogate mom raises the cowbird kids to maturity while the cowbird continues spying on other nests, laying more eggs.
Fortunately, at least one model family guy adds a sense of goodness to balance the rottenness. A male bluebird loyally presents his mate with nice juicy caterpillars while she incubates their eggs. He takes a few shifts on the nest to give her time to stretch. And he feeds the nestlings with the same tender care as she does--a sweet twist to some otherwise dirty plots