An improved understanding of life history responses to current environmental variability is required to predict species-specific responses to anthopogenic climate change. Previous research has suggested that cooperation in social groups may buffer individuals against some of the negative effects of unpredictable climates. We use a 15-year dataset on a cooperative-breeding arid-zone bird, the southern pied babbler Turdoides bicolor, to test i) whether environmental conditions and group size correlate with survival of young during three development stages (egg, nestling, fledgling), and ii) whether group size mitigates the impacts of adverse environmental conditions on reproductive success. Exposure to high mean daily maximum temperatures (mean T_max) during early development was associated with reduced survival probabilities of young in all three development stages. No young survived when mean T_max > 38°C across all group sizes. Low reproductive success at high temperatures has broad implications for recruitment and population persistence in avian communities given the rapid pace of advancing climate change. That impacts of high temperatures were not moderated by group size, a somewhat unexpected result given prevailing theories around the influence of environmental uncertainty on the evolution of cooperation, suggests that cooperative breeding strategies are unlikely to be advantageous in the face of rapid anthropogenic climate change. An improved understanding of life history responses to current environmental variability is required to predict species-specific responses to anthopogenic climate change. Previous research has suggested that cooperation in social groups may buffer individuals against some of the negative effects of unpredictable climates. We use a 15-year dataset on a cooperative-breeding arid-zone bird, the southern pied babbler Turdoides bicolor, to test i) whether environmental conditions and group size correlate with survival of young during three development stages (egg, nestling, fledgling), and ii) whether group size mitigates the impacts of adverse environmental conditions on reproductive success. Exposure to high mean daily maximum temperatures (mean T_max) during early development was associated with reduced survival probabilities of young in all three development stages. No young survived when mean T_max > 38°C across all group sizes. Low reproductive success at high temperatures has broad implications for recruitment and population persistence in avian communities given the rapid pace of advancing climate change. That impacts of high temperatures were not moderated by group size, a somewhat unexpected result given prevailing theories around the influence of environmental uncertainty on the evolution of cooperation, suggests that cooperative breeding strategies are unlikely to be advantageous in the face of rapid anthropogenic climate change.

Study site and system

Fieldwork was conducted at the Kuruman River Reserve (33 km², KRR; 26°58’S, 21°49’E) in the southern Kalahari. Mean summer daily maximum temperatures at the study site, from 1995-2015, averaged 34.7 ± 9.7°C and mean annual precipitation averaged 186.2 ± 87.5mm [49]. The Kalahari region is characterised by hot summers and periodic droughts [50], with extremely variable rainfall between years [51] and increases in both the frequency and severity of high temperature extremes over the last 20 years [52].  Pied babblers are medium-sized (60–90 g), cooperatively-breeding passerines endemic to the Kalahari where they live in territorial groups ranging in size from 3–15 adults [53]. They breed during the austral summer, from September to March [54]. Pied babbler groups consist of a single breeding pair with subordinate helpers [55], and all adult group members (individuals > 1 year old) engage in cooperative behaviours, including territory defence and parental care [48,54]. Previous research has shown that high temperatures and drought negatively affect many aspects of this species’ ecology, including foraging efficiency, body mass maintenance, and provisioning of young [56–58].

Birds in the study population are marked as nestlings with a unique combination of metal and colour rings for individual identification, and are habituated to observation at distances of 1–5 m [48]. Habituated groups are visited weekly during the breeding season to check group composition and record life history events, including breeding activity.

Data collection

Data were collected for each austral summer breeding season from September 2005–February 2019 (14 breeding seasons in total).

Nest life history data

Nest monitoring (location of nests, determination of incubation, hatch, and fledge or failure dates, records of group size and brood size followed Ridley & van den Heuvel [33]. Nests were located by observing nest-building, and incubation start, hatch and fledge dates were determined by checking nests every two to three days. Breeding attempts were considered to have failed when nests were no longer attended, or when dependent fledglings were not seen on two consecutive visits. Failure dates were calculated as the midpoint between the date of the last pre-fail nest/group check and the date when the nest was no longer attended or the fledgling was missing. In most cases, it was not possible to determine the proximate cause of nest failure or death, although common causes of nest failure in this species include predation, abandonment, and nestling starvation [53,59].

Group size (number of adults present in the group; range: 2–10, mean = 4.2 ± 1.5) was recorded for each nest incubated. Brood size was recorded 11 days after hatching (range: 1–5 nestlings, mean = 2.7 ± 0.8), when nestlings were ringed. We defined early development as the period between initiation of incubation and nutritional independence at 90 days of age [48]. Average time from initiation of incubation to hatching is 14 ± 1.2 days. Average time between hatching and fledging is 15.4 ± 1.7 days. Pied babbler are nutrionally independent (receiving < 1 feed per hour) by 90 days of age [48].

Sexing & nestling mass

Pied babblers are sexually monomorphic (Ridley, 2016) and molecular sexing was used to determine the sex of individuals (sensu Fridolfsson & Ellegren 1999). Blood samples were collected by brachial venipuncture and stored in Longmire’s lysis buffer.  Nestlings were ringed, blood sampled, and weighed to 0.1 g on a top-pan scale 11 days post-hatching (Mass₁₁).

Temperature and rainfall

Daily maximum temperature (°C) and rainfall (mm) data were collected from an on-site weather station (Vantage Pro2, Davis Instruments, Hayward, USA). Missing weather data from 2009, 2010, and 2011 were sourced from a nearby South African Weather Services weather station (Van Zylsrus, 28 km away), which produces significantly repeatable temperature measurements (Lin’s concordance correlation coefficient r_c = 0.957, 95 % CI: 0.951–0.962), and moderately repeatable rainfall measurements (r_c = 0.517, 95 % CI: 0.465–0.566) in comparison with the on-site weather station. Absolute differences in measured rainfall were small (average difference = 0.045 ± 3.075 mm, 95 % CI = -5.981–6.072 mm), suggesting that both weather stations adequately detected wet vs. dry periods.

Daily minimum (T_min) and maxium (T_max) temperatures, daily temperature variation (T_max - T_min), were averaged for each development stage: incubation (mean T_minInc, mean T_maxInc, mean T_varInc), nestling (mean T_minBrood, mean T_maxBrood, mean T_varBrood), and fledgling (mean T_min90, mean T_max90, mean T_var90). Rainfall was summed for the 60 days prior to initiation of incubation (Rain₆₀), and for the period between fledging and independence (Rain₉₀).

There are many missing values - we always used only the columns required for our analyses in order to reduce the effect of missing data on samples sizes.