DOI: 10.5061/dryad.mkkwh712k
title: Coordination of care by breeders and helpers in the cooperatively breeding long-tailed tit, Aegithalos caudatus

description: In species with biparental and cooperative brood care, multiple carers
 cooperate by contributing costly investment to raise a shared brood.
 However, shared benefits and individual costs also give rise to conflict
 among carers over investment. Coordination of provisioning visits has been
 hypothesized to facilitate the resolution of this conflict, preventing
 exploitation, and ensuring collective investment in the shared brood. We
 used a 26-year study of long-tailed tits, Aegithalos caudatus, a
 facultative cooperative breeder, to investigate whether care by parents
 and helpers is coordinated, whether there are consistent differences in
 coordination between individuals and reproductive roles, and whether
 coordination varies with helper relatedness to breeders. Coordination
 takes the form of turn-taking (alternation) or feeding within a short time
 interval of another carer (synchrony), and both behaviors were observed to
 occur more than expected by chance, i.e. ‘active’ coordination. First, we
 found that active alternation decreased with group size while active
 synchrony occurred at all group sizes. Secondly, we show that alternation
 was repeatable between observations at the same nest, while synchrony was
 repeatable between observations of the same individual. Active synchrony
 varied with reproductive status, with helpers synchronizing visits more
 than breeders, although active alternation did not vary with reproductive
 status. Finally, we found no significant effect of relatedness on either
 alternation or synchrony exhibited by helpers. In conclusion, we
 demonstrate active coordination of provisioning by carers and conclude
 that coordination is a socially plastic behavior depending on reproductive
 status and the number of carers raising the brood.
description: <strong>Study system and
 data collection</strong> We used data from a long-term study of a
 population of long-tailed tits in the Rivelin Valley, Sheffield, UK
 (53°23′N, 1°34′W) from 1994 to 2019. The field site is
 ~3km<sup>2</sup> with a population of 25-72 breeding pairs
 (Hatchwell 2016). Each year ~95% of adult birds were marked (under British
 Trust for Ornithology license) with a unique combination of two color
 rings on one leg and a BTO metal ring on the other. The adult annual
 mortality rate is ~50% (Meade and Hatchwell 2010), and ~20% of new
 recruits into the adult population were ringed as nestlings in the study
 site, while the remaining ~80% of new recruits were unringed adult
 immigrants that dispersed into the population. Unringed birds were
 captured in mist-nests during the nest-building period and DNA samples
 were collected (under Home Office license) for genotyping and social
 pedigree reconstruction. Nests were found by following adults and once
 located, were monitored every 2-3 days, with daily visits around the
 expected hatch date. Median clutch size is 10 eggs (range: 4-12), which
 are incubated for ~15 days (Hatchwell 2016). Hatching is extremely
 synchronous within clutches, with all chicks typically hatching within 24
 hours of the first. Initial hatch date was recorded as day 0, and chicks
 were ringed and counted on day 11. Protocols for provisioning watches
 (hereafter ‘watches’) were broadly consistent throughout the study. In
 most cases, watches of duration ~60 minutes were carried out every other
 day, starting on day 2, either by direct field observation or by video
 camera, for later review (69% of watches were between 45 and 65 minutes).
 Watches were carried out between 04:00 and 18:00, with 89% starting
 between 06:00 and 14:00. Watches were performed until a nest was predated,
 abandoned or chicks fledged, typically on day
 16-18. For ~5 days post-hatching nestlings are brooded
 regularly by their mothers, who provision offspring only occasionally,
 while fathers either feed the offspring directly or give food to the
 mother, who then feeds the chicks. We restricted our analysis, therefore,
 to watches at day 6 and older, when both parents provision offspring
 directly. Long-tailed tits exhibit facultative cooperative breeding (Lack
 and Lack 1958, Hatchwell 2016), meaning nests may be uniparental (1 carer,
 in the rare event of a parent dying), biparental (2 carers) or cooperative
 (&gt;2 carers). For this study we restricted analysis to watches of
 biparental and cooperative nests with up to 5 carers (i.e. social parents
 and up to 3 helpers). Our dataset contained 65% (516) of watches from
 biparental nests and 21% (171), 11% (88) and 3% (20) from nests with 3, 4
 and 5 carers, respectively. Before starting a watch, ~10 minutes was
 usually allowed for birds to recover from observer disturbance and we
 restricted analysis to watches of total duration ≥ 30.0 minutes and ≤
 180.0 minutes, with duration defined as the time between first and last
 observed feeds. Mean watch duration (± SD) was 54.8 ± 14.4 minutes (range
 30-118 minutes, N = 795 watches). We omitted watches where the identity of
 any provisioning visit was unknown, and from nests that were manipulated
 for other behavioral studies (e.g. Meade et al. 2011). Watches were used
 from 24 years between 1994 and 2019, with 2007 and 2009 excluded because
 experiments conducted in those years meant that they contained no watches
 matching our criteria. In total, our dataset included 795 watches
 performed at 250 unique nests, involving 192 different breeding males, 203
 breeding females and 144 helpers. 
 <strong>Calculating
 coordination</strong> We analyzed alternation and synchrony as the
 absolute number of alternated and synchronized feeding visits in a
 provisioning watch, respectively. We defined an alternated visit as any
 non-consecutive provisioning visit (i.e. a visit occurring after the
 provisioning visit of any carer other than itself) and a synchronized
 visit as an alternated visit occurring within 2-minutes of the previous
 feed (Figure 1). We chose an interval of 2-minutes in accordance with
 previous studies (Mariette and Griffith 2015, Bebbington and Hatchwell
 2016, Ihle et al. 2019a), and further analyses revealed that number of
 synchronized visits was highly correlated for 1, 2 and 3-minute intervals
 (Pearson correlations: 1 v. 2 min, r = 0.97, df = 793, P &lt; 0.001; 2
 v. 3 min, r = 0.97, df = 793, P &lt; 0.001; 1 v. 3 min, r = 0.94, df =
 793, P &lt; 0.001), and analyses of synchrony with different intervals
 produced qualitatively the same results. 
 We calculated observed
 alternation and synchrony directly from visit sequences and times recorded
 through field observation, generating coordination measures per watch and
 for each individual carer present in each watch (Figure 1). We generated
 expected data by null model randomization of observed data, with the
 binary factor ‘Data type’ specifying whether data were observed or
 expected. In accordance with the most conservative method of calculating
 expected alternation and synchrony recommended by Ihle et al. (2019b), our
 null models used a within-watch, within-individual randomization procedure
 in which the order of provisioning visits within a watch was randomized in
 a manner that preserved the length and identity of each period between
 feeding visits (inter-visit intervals) (Figure S1; supplementary
 material). We calculated expected numbers of alternated and synchronized
 visits, both for group total and for individual carers, from the median of
 1000 iterations of the null model applied to each provisioning watch. We
 used median values to preserve integer values for subsequent analysis in
 Poisson-distributed linear models; mean and median values were highly
 positively correlated (Pearson correlations: alternated visits, r = 0.99,
 df = 793, P &lt; 0.001; synchronized visits, r = 0.99, df = 793, P
 &lt; 0.001). <strong>Calculating
 kinship</strong> To calculate pairwise values of pedigree
 relatedness of helpers to parents we constructed an additive relationship
 matrix using the R package NADIV (Wolak 2012), partially reconstructed
 using molecular genetic data from up to 17 microsatellite loci to perform
 offspring-parent reconstruction on CERVUS v. 3.0.7 (Kalinowski et al.
 2007) and sibling-sibling reconstruction on KINGROUP v.2 (Konovalov et al.
 2004). Building on the social pedigree and protocol used in Leedale et al.
 (2018, 2020) we expanded the pedigree to include 2018 and 2019 data. Our
 study population is open, so even after reconstruction the social pedigree
 remained incomplete; therefore, where necessary we omitted data with
 incomplete pairwise relatedness metrics to either social
 parent. <strong>Statistical
 analysis</strong> All statistical analysis was performed in R
 version 4.0.2 (R core development team, 2020). All models were built and
 analyzed using the lme4 package (Bates et al. 2015) and lmerTest
 (Kuznetsova et al. 2017), except for our repeatability models which were
 built and analyzed using the rptR package (Stoffel et al.
 2017). <strong>Collective coordination models (Alt-C and
 Sync-C)</strong> To investigate collective alternation and
 synchrony performed by all carers at a nest we defined two
 Poisson-distributed generalized linear mixed effects models (GLMM) named
 ‘Alt-C’ and ‘Sync-C’, respectively. The response variables to these models
 were the number of alternated visits (collective) and synchronized visits
 (collective) by all carers at each watch, respectively. To control for
 observation and population structure, these models were built with the
 following random effects: ‘Year’, ‘Nest ID’, ‘Watch ID’, ‘Male ID’,
 ‘Female ID’, ‘Helper1 ID’, ‘Helper2 ID’, ‘Helper3 ID’ and ‘Row reference’
 (see Table 1 for explanation). The fixed effects tested were as follows:
 ‘Data type’ (observed vs. expected values of alternation and synchrony),
 ‘Provisioning rate (collective)’, ‘Carer number’, ‘Watch duration’, ‘Brood
 size’, ‘Time of day’, ‘Brood age’, ‘Hatch date’ and ‘AMax (or SMax)’
 (Table 1). We focused our analysis on ‘Data type’ and 2-way interactions
 with other fixed effect terms, as a disparity between observed and
 expected data represents the level of active coordination
 performed. <strong>Individual coordination models (Alt-I and
 Sync-I)</strong> To investigate the effect of carer status on
 alternation and synchrony performed by a given carer, we built two
 Poisson-distributed GLMMs named ‘Alt-I’ and ‘Sync-I’, respectively. The
 response variables to these models were the number of alternated visits
 (individual) and synchronized visits (individual), respectively. These
 models were built with the following random effects: ‘Year’, ‘Nest ID’,
 ‘Watch ID’, ‘Carer ID’ and ‘Row reference’ (Table 1). The fixed effects
 tested were as follows: ‘Data type’, ‘Carer status’, ‘Provisioning rate
 (individual)’, ‘Carer number’, ‘Watch duration’, ‘Brood size’, ‘Time of
 day’, ‘Brood age’, ‘Hatch date’ and ‘Amax’ or ‘SMax’ (Table 1). In this
 analysis, the focus was on the interaction of ‘Data type’ with ‘Carer
 status’ because this term represents the disparity in active coordination
 between carers of different breeding status.
 <strong>Repeatability
 models (Alt-R and Sync-R)</strong>
 To investigate the
 repeatability of active alternation and synchrony within nests and within
 individuals we constructed two Gaussian-distributed GLMMs named ‘Alt-R’
 and ‘Sync-R’, respectively. In these models, response variables were the
 number of actively alternated (individual) and actively synchronized
 (individual) visits by an individual during a watch, respectively (active
 alternation range: -3 to 6; active synchrony range: -7 to 9). We used
 these metrics because repeatability analyses required active coordination
 to be the response variable, rather than using interaction terms with
 ‘Data type’ as in our other models. To control for the effect of
 confounding factors on active coordination we included all fixed effects
 previously found to significantly influence either individual alternation
 or synchrony (Alt-I, Sync-I) and, using the rptR function, ran models with
 1000 bootstrapped simulations and 1000 permutations. We investigated both
 within-nest repeatability (‘Nest ID’) and within-individual repeatability
 (‘Carer ID’) in the same models. Additionally, we included ‘Year’ as a
 random effect to account for between-year variation. As active
 coordination was the response variable and a Gaussian error distribution
 was used, ‘Watch ID’ and the ‘Row reference’ random effects were not
 required for these models. We present our repeatability results as values
 of R and extracted 2.5% and 97.5% confidence intervals (CIs) in addition
 to P-values. In our dataset many individuals were observed
 provisioning at only one nest, potentially confounding repeatability of an
 individual’s behavior with the potential effect of common nest factors.
 Therefore, we ran the repeatability analysis on a subset of data,
 restricted to carers observed provisioning at two or more nests (Table S2,
 supplementary material). Results from these models were qualitatively the
 same as those for the full dataset for both within-nest and
 within-individual repeatability for both alternation and synchrony
 models. <strong>Kinship models (Alt-K and
 Sync-K)</strong> To investigate the effect of kinship to the
 breeding pair on alternation and synchrony performed by helpers we
 constructed two Poisson-distributed GLMMs named ‘Alt-K’ and ‘Sync-K’,
 respectively. Just as with ‘Alt-I’ and ‘Sync-I’, the response variables to
 these models were the number of alternated visits (individual) and
 synchronized visits (individual) performed by an individual during a
 watch, respectively, however analysis was restricted to helpers whose
 pedigree kinship with breeders was known. These models were built with the
 same random and fixed effects as ‘Alt-I’ and ‘Sync-I’ but with the
 addition of three fixed effects: ‘Sex’, ‘Kinship with father’ and ‘Kinship
 with mother’ (Table 1). We focused our analysis on the interactions of
 ‘Data type’ with our kinship terms as these represent the relationship
 between the level of active coordination and
 relatedness.
description: Raw provisioning watch data are available as it was recorded
 directly from field observation prior to processing.
 Collective (and individual) level processed data are available.
 These are the values per watch (and per individual) for terms of interest
 e.g. number of alternated visits for observed (from field observation) and
 expected (from null model randomization). These also contain nest-specific
 information e.g. brood size, hatch date. Relatedness
 data are available in the form of each individual's known dam and
 sire, either from known brood associations or via reconstruction from
 genetic loci analysis.