It may come as a surprise, but very few birds are monogamous. There's a lot of hanky-panky going on in the trees, wetlands and shrubs of north Texas. Recent DNA research shines a light on the "playing around".
Of course, birds' primary concerm when mating is passing on good genes; those that enable better flying, more endurance, better hunting for food etc. In a typical nest of four nestlings, only about two have the genes of the parents raising them. The other two have a different mother or father or both (usually it's the father that's different - for logistical reasons).
In other words, it's not uncommon at all for a bird to copulate with a bird that's not its mate; sometimes birds will even incubate an egg laid by another bird. It's all for the sake of the genes!
|Passenger Pigeon (extinct)|
It was hunted to death, and its natural habitat (and food sources) was going away. The last one died in a zoo in 1914.
Which reminds us, especially during a cold spell like north Texas is having now, to make sure your birdfeeders are full, and (when the weather's better) to plant more "birdscape" plants to provide fresh food for our birds.