How did people brood chicks before electricity? Lots of ways, and a few are still useful today.
Homesteading.news just posted an article about how to do January(!) brooding without electricity, using a heavily insulated brooder with bubble-wrap insulation to reflect the baby chicks’ body heat, allowing them to do well without supplemental heat.
The article kindly credits my insulated pasture hover page, which discusses these topics.
Generally speaking, the body-heat-only technique has been around for 150 years or so, and has been used with several variations: