DOI: 10.5061/dryad.v15dv4216
title: Data from: Helpers don’t help when it’s hot in a cooperatively breeding bird, the Southern Pied Babbler

description: 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 (&gt;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.
description: 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
 km<sup>2</sup>, 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 &lt; 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).