Words mean things: Altricial vs. Precocial
So this is basically a baby-having style. Parents who have altricial babies have to spend a lot of time taking care of their young, and parents who have precocial young need to spend a lot less time taking care of their young. Like, if you completely fucked off for parts unknown three days after hatching, would your baby be a) not very hosed at all or b) super-hosed?
If you answered “not very hosed at all” then congratulations, you’ve got very precocial young. Yay! The extreme end of the “born ready” spectrum is called “superprecocial,” and they’re fucking literally born ready. I mean, seriously, we are talking about birds that go from hatching out to flying in under 24 hours. It’s pretty hardcore.
Altricial babies can’t go very far and are dependent on their parents for any nourishment, but that can range from babies that are completely and utterly undeveloped and spend the next nine months developing the ability to have legs and unable to leave mom’s pouch lest they die to babies that just can’t feed themselves or go anywhere.
Above: Actual baby kangaroo.
Science has yet to see an advantage to distinguishing between these two sections of altricial young, presumably out of common courtesy.
And you might be like “Why wouldn’t everybody just have babies that can take care of themselves if you decide to pop off for where-the-fuck-ever because it’s Tuesday? Or get eaten? Or die of cholera? Or whatever, because nature?” And that is an excellent question, because seriously, nature. Babies who stand a good chance on their own if mom gets eaten by an owl five days after they’re born are going to continue mom’s genetic line a lot better than babies who have to hope that some kind, enterprising, or very confused animal with a similar idea of what “food” is adopts them before they freeze, starve, or get eaten to death.
But here’s the deal: those babies also require a lot more up-front investment on the part of the parents. Huge eggs, long incubation periods, long pregnancies, big babies, etc. all cost, both in terms of initial nutrient investment and in terms of missed reproductive opportunities. If you dump all these resources into a huge fuck-off egg with a massive fucking yolk and then something comes along and eats it when it’s 90% done incubating? You’re out a lot of effort.
Conversely, if you put like one caterpillar’s worth of energy into laying ten eggs and then a cowbird comes along and smashes your nest, you can just go build another one without too much trouble. There’s not a lot to be lost by bowing out at any stage of the game and just trying again, which also means you can sort of literally not put all your eggs in one basket, because we have yet to find a songbird that won’t engage in low-level brood parasitism given the opportunity, because their teeny tiny little eggs cost them nothing. The effort comes in once the babies have proven to be somewhat viable, and there are plenty of opportunities at any point in time during that long nurturing phase to cut your losses by abandoning or cannibalizing the weakest of the offspring.
(Cannibalizing precocial young is much more difficult, because they can run the hell away.)
Bonus round: Another way of judging how precocial a baby is is by whether or not it’s nidifugous. “Nidifugous” is a word which here means “the most ridiculous way someone could come up with to describe an animal which leaves the nest early.” It’s a fugitive from the nest. Like, you see a bunch of baby ducks waddling past with tiny duckling bindles and tiny duckling suitcases, all fleeing their parents’ nest. Latin: Making the best stupid-sounding words since forever.