So apparently songbirds that breed in tropical regions have fewer youngsters, and those youngsters get their wings faster than songbirds in more temperate reasons, despite growing more slowly in general.
Their late growth possibly gives them an edge in escaping being eaten, and with more parental care per baby bird, that means smaller family size, and “slow” growth, isn’t a bad thing. Fewer nestlings doesn’t necessarily mean poorer overall reproductive strategy, or whatever we’re calling it, it’s just a different distribution of the same pro-survival tactics.
Here. An article. Plus photos of cute birds.