>Look at first generation immigrants from third world countries. They tend to have a ton more kids than the existing populace, while at the same time being much poorer than average.

The data below indicates otherwise.

I assume it is because contraception is far more available.

See figure 3, showing foreign born vs native born statistics for US. Both show big drops in fertility rate between 1990 and 2019:

https://www.census.gov/library/stories/2022/04/fertility-rat...

This website has graphs showing steep declines in birthrates for immigrants:

https://cis.org/Report/Fertility-Among-Immigrants-and-Native...

>In 2019, immigrant fertility dropped below 2.1 children on average, which is the level necessary to replace the existing population.