Parental effort in warming young: A neglected component of life history strategies influenced by nest structure, brood size, body mass, and bi-parental care

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.