Anyone who has read Anna Karenina knows the famous line: all happy families are alike; unhappy families are all unhappy in their own way.
For birds, this is not true. Family conflict in birds is generalizable.
There are three main categories for family conflict in birds (and all animals, really): sexual conflict, parent-offspring conflict, and sibling competition. All of them derive from the same basic principle: selection favors those who reproduce their genes most in the next generation. In a family—which, for simplicity’s sake, we’ll call a male bird, a female bird, and their genetic offspring; I’ll talk about some variations on this at the end—many members share some of their genes with each other, but not all. Chicks share half of their genes with Mom and half with Dad; Mom and Dad share no genes. (Unless they’re inbred, which is a whole different topic…)
You might expect, from this, that Mom and Dad are likely to come into conflict, and you’re right. Sexual conflict over parental care happens because Mom and Dad get their fitness benefit as long as the chicks survive, regardless of how much work they personally do; so both Mom and Dad would really prefer if the other parent did all the caring for the chicks (which takes time and energy and can be dangerous) while Mom/Dad relaxes, or maybe has even more chicks with someone else. This can result in a sort of parenting-as-a-game-of-chicken phenomenon, where each parent tries to do the absolute minimum that they can without killing the chicks, in the hopes of goading the other parent into compensating and providing more parental care.
Parent-offspring conflict happens because each parent shares just half of their genes with each chick. That means that while Junior’s highest priority is his own well-being, his parents’ highest priority is the cumulative well-being of all of their chicks—including chicks that they might potentially have in the future! If you have ever seen fledgling birds begging for food from a parent who ignores them, you’ve seen parent-offspring conflict. Fledglings do best when their parents continue to feed them for a long time. In many species, however, parents can have multiple broods per season, so parents want to stop feeding their fledglings early so that they can start that second brood.
You can do math with this, if you’re so inclined: say an additional week of parental care increases each fledgling’s survival chances from 60% to 70%. Starting the second brood a week earlier, however, increases that brood’s chances of survival from 33% to 50%. For parents with four fledglings, and a potential second brood of three chicks, what should they do?
Solution: If the parents care for the fledglings for an additional week, they will end the season with (0.7 * 4)+(0.33 * 3) = 3.8 chicks. If they don’t, they will end the season with (0.6 * 4)+(0.5 * 3) = 3.9 chicks. So, ceasing to care for the fledglings early is better for the parents, even though it’s worse for the fledglings.
———OKAY, THE MATH IS GONE————Sibling-sibling competition arises because siblings share, on average, just one-half of their genes with each other. So it is definitely in Sister’s best interest to promote her own well-being at the expense of Brother’s well-being, and vice versa. And since siblings generally share the same limited resources—their parents’ care—they end up competing with each other. And this is just the beginning. Birds often cheat on their mates, so Dad may not share any genes with Junior. Some birds breed cooperatively, laying their eggs all in the same nest and not knowing which chicks are theirs. Some birds are polygynous, meaning that one male raises chicks with several females; a few birds are even polyandrous, meaning one female raises chicks with several males. One bird, the wonderful crazy Dunnock, can be monogamous (1 male + 1 female), polygynous, polyandrous, or polygynandrous (lots of males + lots of females all breeding with each other). All this makes things very complicated—which is good for us behavioral ecologists who get to study it!