Cooperative breeding, where more than two individuals invest in rearing a single brood, occurs in many bird species globally and often contributes to improved breeding outcomes. However, high temperatures are associated with poor breeding outcomes in many species, including cooperative species. We used data collected over three austral summer breeding seasons to investigate the contribution that helpers make to daytime incubation in a cooperatively breeding species, the Southern Pied Babbler Turdoides bicolor, and the ways in which their contribution is influenced by temperature. Helpers spent a significantly higher percentage of their time foraging (41.8 ± 13.7%) and a significantly lower percentage of their time incubating (18.5 ± 18.8%) than members of the breeding pair (31.3 ± 11% foraging & 37.4 ± 15.7% incubating). In groups with only one helper, the helper’s contribution to incubation was similar to that of breeders. However, helpers in larger groups contributed less to incubation, individually, with some individuals investing no time in incubation on a given observation day. Helpers significantly decrease their investment in incubation on hot days (>35.5°C), while breeders tend to maintain incubation effort as temperatures increase. Our results demonstrate that pied babblers share the workload of incubation unequally between breeders and helpers, and this inequity is more pronounced during hot weather. These results may help to explain why recent studies have found that larger group size does not buffer against the impacts of high temperatures in this and other cooperatively breeding species.

Data were collected for each austral summer breeding season between September 2016 and February 2019 (three breeding seasons in total) at the Kuruman River Reserve (33 km², KRR; 26°58’S, 21°49’E) in the southern Kalahari.

To understand the different contributions that breeders and helpers make to incubation, we collected data on the duration of all daytime incubation bouts per individual by waiting near the nest at first light, observing the first bird to replace the breeding female in the morning (05h00 – 06h28, four minutes before sunrise on average [range 36 minutes before to 26 minutes after sunrise]) and remaining with the group all day until 19h00 (46 observation days at 35 nests, also see Bourne, Ridley, McKechnie, et al. 2021b). We recorded the start and end time of each incubation bout and the identity of each incubating bird. Incubation was defined as the observed bird settling onto the nest, covering the clutch with their body. Incubation bouts that lasted less than 60 seconds were not included. Nest attendance behaviours that involved perching on the edge of the nest or on branches next to the nest were not considered incubation. For most nests (n=28), observations were collected once (whole-day observation) during the incubation period and on two or more days (up to a maximum of four days) for the remaining seven nests, depending on the number of active nests being monitored at the time. The limited number of breeding attempts for which we were able to collect multiple days of observations meant that each observation day was treated as independent in the analyses, and the repeatability of observed patterns of individual incubation behaviour could not be tested. Incubation bout data were used to calculate the proportion of time per day that each group member incubated the clutch. Our focus on daytime incubation effort enabled consideration of the effects of high temperatures on incubation behaviour. However, as the breeding female also incubates overnight, our approach necessarily underestimates the breeding female’s total contribution to incubation.

We additionally conducted 20-minute continuous time-activity focal behaviour observations (Altmann 1974) on up to three different adult birds per day. Each bird was observed during a minimum of six and a maximum of 27 focal sessions per observation day (Bourne, Ridley, Spottiswoode, et al. 2021c). We collected focal behaviour data from 44 different individual birds in 15 different groups incubating clutches at 40 different nests. While pied babblers invest in a single nest at a time, we observed multiple breeding attempts by the same groups over multiple breeding seasons. Group sizes ranged from three to six adults, including the breeding pair and one to four adult helpers. The number of adults present remained constant within all breeding attempts but varied between breeding attempts and/or breeding seasons in seven of the 15 groups. We therefore collected data from seven groups with one helper, eight groups with two helpers, four groups with three helpers, and five groups with four helpers over three breeding seasons. Sub-adult individuals (from recent, dependent fledglings up to 1 year of age) were present on approximately half of the observation days (n = 21). Of the groups with sub-adults present, sub-adults represented 42% of non-breeding group members on average (range 16-75%). Sub-adults rarely invest in helping behaviour and thus, for the purposes of this study, were not considered helpers and were not included in the analyses relating to the number of helpers in a group.

Focal sessions (involving multiple 20-min focal observations) lasted two hours each, with the first starting at 07h00 and the last starting at 17h00, as described in detail by Bourne et al. (2021c). We collected 1,291 focal behaviour observations (mean focal length = 19.4 ± 0.99 min) over 64 focal days (mean daily observation length over a focal day = 411 ± 101 min) during three austral summer breeding seasons (n = 29 days between 14 Oct 2016 and 18 Mar 2017, n = 16 days between 22 Sep 2017 and 7 Jan 2018, n = 19 days between 14 Oct 2018 and 18 Dec 2019). Behaviour observations were collected across a range of naturally occurring daily maximum air temperatures, on 33 hotter focal days (Tmax ≥ 35.5°C, range 35.6 to 40.8°C) and 31 cooler focal days (Tmax < 35.5°C, range 20.7 to 35.2°C). All group sizes were sampled across a range of temperatures. The threshold temperature, 35.5°C, is taken from previous studies which identified a change in foraging efficiency (du Plessis et al. 2012), provisioning effort (Wiley and Ridley 2016), and hatching success (Bourne et al. 2020c) at Tmax ≥ 35.5°C in this species. On the 64 observations days, we observed 31 males and 33 females, 32 breeders and 32 helpers. All birds for which we recorded fewer than six focal observations on an observation day were removed from the analyses (n = 2 individuals). Behaviour data were captured on a Mobicel smartphone using Prim8 software (McDonald and Johnson 2014), in which the duration of each observed behaviour can be recorded to the nearest second.

For analyses of time budgets, we summed the amount of time each individual was observed foraging (foraging effort, including searching for and handling prey), attending the nest (incubation as described above; shading and incubating were not distinguished due to limited visibility of birds once settled into their nests), resting (preening, standing and perching) and engaging in other activities (e.g. hopping along the ground, flying, being vigilant, interacting with neighbouring groups) and calculated the proportion of time individual birds allocated to each set of activities across all focals per individual per observation day, following Bourne et al. (2021c).