Parental care behavior has strong fitness consequences and has been widely studied, but variation in the effort that species invest in warming young has been neglected. Here, we investigate the extent and possible causes of variation in brooding, or warming, effort among 90 species of altricial birds on four continents. We measured parental warming effort based on the percentage of time parents spent warming young for the first approximately 6 h of each day over the entire nestling period of each species. Our measure of warming behaviour was based on 62,249.5 hours of video data over 10,770 video days while studying 33,777 nests. We calculated the initial magnitude of effort at the start of the nestling period and the rate that effort declined with the age of the nestlings. Higher initial magnitude and slower rates of decrease reflected higher effort. We found that brooding behavior varied extensively among species. Brooding effort was greater in species in which both parents shared the effort and those with greater energy reserves from larger body size, indicating effort is under energetic limitation. Brooding effort was substantially lower in enclosed-nesting species than in those using open-cup or cavity nests, consistent with predictions that enclosed nests better retain heat and reduce parental energy demand of warming. Effort was not simply related to thermal demands. Brooding effort increased with the number of young in the brood, opposite to expectations based on greater thermal inertia. Instead, the increase in warming effort with the number of young in the brood fits with life history predictions of those species being under selection for greater reproductive effort. Understanding how the costs and benefits of brooding effort interact with other evolved traits is essential for advancing parental investment and life history theory.