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 a better adaptability to environments than the uniparentally incubating species. By comparing two congenic species (the uniparentally incubating Silver-throated Tits Aegithalos glaucogularis and the biparentally incubating Black-throated Tits A. concinnus) from their sympatric populations in central China, we found that Silver-throated Tits had shorter on- and off-bouts (i.e., shorter time on and off the nest) than Black-throated Tits. When ambient temperature increased, both species decreased on-bout duration and increased off-bout duration, but the decrease of on-bout duration in Black-throated Tits was to a greater degree. Moreover, only Black-throated Tits 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, Silver-throated Tit nests experienced more temperature fluctuations. These patterns imply that biparentally incubating Black-throated Tits have more flexible strategies in egg care and a better ability to maintain a stable nest thermal environment than uniparentally incubating Silver-throated Tits. The findings highlight different adaptive potentials between biparentally and uniparentally incubating birds, shedding light on the evolution of different incubation modes in birds.

On-bout duration and off-bout duration

Whenever accessible, nests at the incubation stage were filmed to observe parental incubation behaviors. For the nests involved in this study, they were filmed 1-4 times according to field workload and need in the years between 2009 and 2016, and were filmed once or twice on the 4th/5th day and/or the 9th/10th day of incubation during the years between 2017 and 2019. Cameras were mounted on tripods placed at least 0.5 m from the nests. Each video recording lasted for an average of 125 ± 45 min (mean ± SD, n = 252 video records).

Data of on- and off-bout durations (i.e., time on and off the nest) of both species were obtained from the videos, including a total of 2551.2 h from 245 ST nests and 2059.8 h from 213 BT nests. Through the videos, we recorded the incubating bird’s identity (by color rings) and calculated the duration of each on- and off-bout. The final data set contained 343 on-bouts (n = 115 nests) and 344 off-bouts (n = 115 nests) of STs, and 103 on-bouts (n = 52 nests) and 103 off-bouts (n = 52 nests) of BTs. We did not use the temperature logger data for this analysis because although using temperature change to identify the on- and off-bouts was feasible for the uniparentally incubating ST, it was not for biparentally incubating BT as the time interval between one parent’s leaving and another’s entering the nest was often short and could not be accurately identified from the temperature logger data.

Nest temperature

We monitored the nest temperatures with the temperature logger Lexiang GSP958 (accuracy ± 0.5 °C; Lexiang Electronics Co. Ltd, China) in 2019, 2021, and 2022. Temperature loggers were usually (in 60 out of 99 nests) installed between the day when at least three eggs were laid and the first day of incubation and were collected until nest failure or after nestling hatching. The mean (± SD) number of days that the loggers were left in the nests during the incubation stage were 7.75 (± 2.45) days (range 2 ~ 13 days). The thermistor was wrapped in an artificial egg that was made of paraffin and similar in size to the natural eggs of the two species before being placed into the nests. The recording intervals of the temperature loggers were set as 1 min. Due to unknown technical reasons of the loggers, the temperature data occasionally showed abnormal values at certain specific temperatures (e.g., ~11.9 °C and ~31.8 °C), which can be identified through an abrupt drop in temperature that differed from a normal curve of temperature drop during an off-bout. In such cases, we replaced the abnormal values using values calculated by averaging the temperatures 1 min before and after the abnormal values.

As the two species’ incubation behaviors mainly differ during the daytime, we focused our analyses on the effects of daytime ambient temperature on nest temperature. According to the nest temperature data, we found that among all ST nests, the latest time when the birds left nests after nighttime incubation in the morning and the earliest time that they returned to nests for nighttime incubation in the afternoon were 7:50 and 16:44, respectively. The latest time when the birds left nests in the morning (7:50) was determined by a criterion of an off-bout’s continuous temperature drop of ≥ 1 °C lasting for ≥ 9 min, which was obtained by matching the temperature logger data to corresponding video data at ST nests where they were both available (n = 15 nests and 95.1 h). The earliest time that the birds returned to nests for nighttime incubation in the afternoon (16:44) was determined by a continuous temperature increase following an off-bout, after which the temperature remained relatively stable and no off-bout could be identified based on the off-bout criterion. We thus defined daytime for STs as 8:00–16:30 to enable comparison of temperature across nests, because by choosing this period, all birds in the analyses were during the period of daytime incubation. As it was hard to accurately identify BTs’ off-bouts from the temperature logger data, we were unable to determine their latest time when they left nests in the morning and the earliest time that they returned to nests for nighttime incubation. We therefore assumed their daytime period to be the same as STs (i.e., 8:00–16:30).

In total, nest temperature data from 298 nest days of 35 ST nests and 469 nest days of 64 BT nests were available for the analyses after excluding those that could not be used (i.e., data did not reflect incubation rhythm but fluctuations of ambient temperature) and when there were knowable disturbances (e.g., checking nest, filming nest and capturing parents). The effects of ambient temperature on daytime nest temperature were analyzed in two aspects, including cumulative temperature and temperature fluctuations. Cumulative nest temperature was calculated as daytime average nest temperature × 8.5 h (8:00–16:30). Nest temperature fluctuations, which reflected the stability of nest temperature, were calculated as the number of times when nest temperature continuously dropped by ≥ 1 °C (a minimum temperature drop of an off-bout confirmed by videos), followed by an increase of ≥ 1 °C.