DOI: 10.5061/dryad.7pvmcvdqf
title: High temperatures drive offspring mortality in a cooperatively breeding bird

description: 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 Tmax) during early
 development was associated with reduced survival probabilities of young in
 all three development stages. No young survived when mean Tmax &gt;
 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 Tmax) during early development was associated with
 reduced survival probabilities of young in all three development stages.
 No young survived when mean Tmax &gt; 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.
description: <b>Study
 site and system</b> Fieldwork was
 conducted at the Kuruman River Reserve (33 km<sup>2</sup>,
 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 &gt; 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. <b>Data
 collection</b> Data were collected for each austral
 summer breeding season from September 2005–February 2019 (14 breeding
 seasons in total). <i>Nest life
 history data </i> 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 &lt; 1 feed per hour) by 90 days of
 age [48]. <i>Sexing & nestling mass</i> Pied babblers are sexually monomorphic (Ridley, 2016) and molecular sexing was used to determine the sex of individuals (<i>sensu </i>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 (Mass<sub>11</sub>). <i>Temperature and rainfall</i> 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<i> r<sub>c</sub></i> = 0.957, 95 % CI: 0.951–0.962), and moderately repeatable rainfall measurements (<i>r<sub>c</sub></i> = 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<sub>min</sub>) and maxium (T<sub>max</sub>) temperatures, daily temperature variation (T<sub>max</sub> - T<sub>min</sub>), were averaged for each development stage: incubation (mean T<sub>minInc</sub>, mean T<sub>maxInc</sub>, mean T<sub>varInc</sub>), nestling (mean T<sub>minBrood</sub>, mean T<sub>maxBrood</sub>, mean T<sub>varBrood</sub>), and fledgling (mean T<sub>min90, </sub>mean T<sub>max90, </sub>mean T<sub>var90</sub>). Rainfall was summed for the 60 days prior to initiation of incubation (Rain<sub>60</sub>), and for the period between fledging and independence (Rain<sub>90</sub>).  
description: 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.