Biparentally incubating <i>Aegithalos concinnus</i> (Black-throated Tit) provide a more stable nest thermal environment than uniparentally incubating <i>A. glaucogularis</i> (Silver-throated Tit)

Abstract Different parental care modes in birds have been thought to be adaptations to different environmental conditions, but their adaptability to environments remains underexplored. We predict that the biparentally incubating species have better adaptability to environments than the uniparentally incubating species. By comparing two congenic species (the uniparentally incubating Aegithalos glaucogularis [Silver-throated Tit] and the biparentally incubating A. concinnus [Black-throated Tit]) from their sympatric populations in central China, we found that A. glaucogularis had shorter on- and off-bouts (i.e., shorter time on and off the nest) than A. concinnus. When ambient temperature increased, both species decreased on-bout duration and increased off-bout duration, but the decrease of on-bout duration in A. concinnus was to a greater degree. Moreover, only A. concinnus extended off-bouts after longer on-bouts, although both species extended on-bout durations after longer off-bouts. In addition, when ambient temperature increased, their daytime cumulative nest temperatures increased, whereas nest temperature fluctuations decreased. However, despite that their daytime cumulative nest temperatures were similar, A. glaucogularis nests experienced more temperature fluctuations. These patterns imply that biparentally incubating A. concinnus have more flexible strategies in egg care and a better ability to maintain a stable nest thermal environment than uniparentally incubating A. glaucogularis. The findings highlight different adaptive potentials between biparentally and uniparentally incubating birds, shedding light on the evolution of different incubation modes in birds.